Multi-processing and Distributed Computing

An implementation of distributed memory parallel computing is provided by module Distributed as part of the standard library shipped with Julia.

Most modern computers possess more than one CPU, and several computers can be combined together in a cluster. Harnessing the power of these multiple CPUs allows many computations to be completed more quickly. There are two major factors that influence performance: the speed of the CPUs themselves, and the speed of their access to memory. In a cluster, it's fairly obvious that a given CPU will have fastest access to the RAM within the same computer (node). Perhaps more surprisingly, similar issues are relevant on a typical multicore laptop, due to differences in the speed of main memory and the cache. Consequently, a good multiprocessing environment should allow control over the "ownership" of a chunk of memory by a particular CPU. Julia provides a multiprocessing environment based on message passing to allow programs to run on multiple processes in separate memory domains at once.

Julia's implementation of message passing is different from other environments such as MPI^[1]. Communication in Julia is generally "one-sided", meaning that the programmer needs to explicitly manage only one process in a two-process operation. Furthermore, these operations typically do not look like "message send" and "message receive" but rather resemble higher-level operations like calls to user functions.

Distributed programming in Julia is built on two primitives: remote references and remote calls. A remote reference is an object that can be used from any process to refer to an object stored on a particular process. A remote call is a request by one process to call a certain function on certain arguments on another (possibly the same) process.

Remote references come in two flavors: Future and RemoteChannel.

A remote call returns a Future to its result. Remote calls return immediately; the process that made the call proceeds to its next operation while the remote call happens somewhere else. You can wait for a remote call to finish by calling wait on the returned Future, and you can obtain the full value of the result using fetch.

On the other hand, RemoteChannel s are rewritable. For example, multiple processes can coordinate their processing by referencing the same remote Channel.

Each process has an associated identifier. The process providing the interactive Julia prompt always has an id equal to 1. The processes used by default for parallel operations are referred to as "workers". When there is only one process, process 1 is considered a worker. Otherwise, workers are considered to be all processes other than process 1. As a result, adding 2 or more processes is required to gain benefits from parallel processing methods like pmap. Adding a single process is beneficial if you just wish to do other things in the main process while a long computation is running on the worker.

Let's try this out. Starting with julia -p n provides n worker processes on the local machine. Generally it makes sense for n to equal the number of CPU threads (logical cores) on the machine. Note that the -p argument implicitly loads module Distributed.

$ julia -p 2

julia> r = remotecall(rand, 2, 2, 2)
Future(2, 1, 4, nothing)

julia> s = @spawnat 2 1 .+ fetch(r)
Future(2, 1, 5, nothing)

julia> fetch(s)
2×2 Array{Float64,2}:
 1.18526  1.50912
 1.16296  1.60607

The first argument to remotecall is the function to call. Most parallel programming in Julia does not reference specific processes or the number of processes available, but remotecall is considered a low-level interface providing finer control. The second argument to remotecall is the id of the process that will do the work, and the remaining arguments will be passed to the function being called.

As you can see, in the first line we asked process 2 to construct a 2-by-2 random matrix, and in the second line we asked it to add 1 to it. The result of both calculations is available in the two futures, r and s. The @spawnat macro evaluates the expression in the second argument on the process specified by the first argument.

Occasionally you might want a remotely-computed value immediately. This typically happens when you read from a remote object to obtain data needed by the next local operation. The function remotecall_fetch exists for this purpose. It is equivalent to fetch(remotecall(...)) but is more efficient.

julia> remotecall_fetch(r-> fetch(r)[1, 1], 2, r)
0.18526337335308085

This fetches the array on worker 2 and returns the first value. Note, that fetch doesn't move any data in this case, since it's executed on the worker that owns the array. One can also write:

julia> remotecall_fetch(getindex, 2, r, 1, 1)
0.10824216411304866

Remember that getindex(r,1,1) is equivalent to r[1,1], so this call fetches the first element of the future r.

To make things easier, the symbol :any can be passed to @spawnat, which picks where to do the operation for you:

julia> r = @spawnat :any rand(2,2)
Future(2, 1, 4, nothing)

julia> s = @spawnat :any 1 .+ fetch(r)
Future(3, 1, 5, nothing)

julia> fetch(s)
2×2 Array{Float64,2}:
 1.38854  1.9098
 1.20939  1.57158

Note that we used 1 .+ fetch(r) instead of 1 .+ r. This is because we do not know where the code will run, so in general a fetch might be required to move r to the process doing the addition. In this case, @spawnat is smart enough to perform the computation on the process that owns r, so the fetch will be a no-op (no work is done).

(It is worth noting that @spawnat is not built-in but defined in Julia as a macro. It is possible to define your own such constructs.)

An important thing to remember is that, once fetched, a Future will cache its value locally. Further fetch calls do not entail a network hop. Once all referencing Futures have fetched, the remote stored value is deleted.

Threads.@spawn is similar to @spawnat, but only runs tasks on the local process. We use it to create a "feeder" task for each process. Each task picks the next index that needs to be computed, then waits for its process to finish, then repeats until we run out of indices. Note that the feeder tasks do not begin to execute until the main task reaches the end of the @sync block, at which point it surrenders control and waits for all the local tasks to complete before returning from the function. As for v0.7 and beyond, the feeder tasks are able to share state via nextidx because they all run on the same process. Even if Tasks are scheduled cooperatively, locking may still be required in some contexts, as in asynchronous I/O. This means context switches only occur at well-defined points: in this case, when remotecall_fetch is called. This is the current state of implementation and it may change for future Julia versions, as it is intended to make it possible to run up to N Tasks on M Process, aka M:N Threading. Then a lock acquiring\releasing model for nextidx will be needed, as it is not safe to let multiple processes read-write a resource at the same time.

Code Availability and Loading Packages

Your code must be available on any process that runs it. For example, type the following into the Julia prompt:

julia> function rand2(dims...)
           return 2*rand(dims...)
       end

julia> rand2(2,2)
2×2 Array{Float64,2}:
 0.153756  0.368514
 1.15119   0.918912

julia> fetch(@spawnat :any rand2(2,2))
ERROR: RemoteException(2, CapturedException(UndefVarError(Symbol("#rand2"))))
Stacktrace:
[...]

Process 1 knew about the function rand2, but process 2 did not.

Most commonly you'll be loading code from files or packages, and you have a considerable amount of flexibility in controlling which processes load code. Consider a file, DummyModule.jl, containing the following code:

module DummyModule

export MyType, f

mutable struct MyType
    a::Int
end

f(x) = x^2+1

println("loaded")

end

In order to refer to MyType across all processes, DummyModule.jl needs to be loaded on every process. Calling include("DummyModule.jl") loads it only on a single process. To load it on every process, use the @everywhere macro (starting Julia with julia -p 2):

julia> @everywhere include("DummyModule.jl")
loaded
      From worker 3:    loaded
      From worker 2:    loaded

As usual, this does not bring DummyModule into scope on any of the process, which requires using or import. Moreover, when DummyModule is brought into scope on one process, it is not on any other:

julia> using .DummyModule

julia> MyType(7)
MyType(7)

julia> fetch(@spawnat 2 MyType(7))
ERROR: On worker 2:
UndefVarError: `MyType` not defined in `Main`
⋮

julia> fetch(@spawnat 2 DummyModule.MyType(7))
MyType(7)

However, it's still possible, for instance, to send a MyType to a process which has loaded DummyModule even if it's not in scope:

julia> put!(RemoteChannel(2), MyType(7))
RemoteChannel{Channel{Any}}(2, 1, 13)

A file can also be preloaded on multiple processes at startup with the -L flag, and a driver script can be used to drive the computation:

julia -p <n> -L file1.jl -L file2.jl driver.jl

The Julia process running the driver script in the example above has an id equal to 1, just like a process providing an interactive prompt.

Finally, if DummyModule.jl is not a standalone file but a package, then using DummyModule will load DummyModule.jl on all processes, but only bring it into scope on the process where using was called.

Starting and managing worker processes

The base Julia installation has in-built support for two types of clusters:

• A local cluster specified with the -p option as shown above.
• A cluster spanning machines using the --machine-file option. This uses a passwordless ssh login to start Julia worker processes (from the same path as the current host) on the specified machines. Each machine definition takes the form [count*][user@]host[:port] [bind_addr[:port]]. user defaults to current user, port to the standard ssh port. count is the number of workers to spawn on the node, and defaults to 1. The optional bind-to bind_addr[:port] specifies the IP address and port that other workers should use to connect to this worker.

Functions addprocs, rmprocs, workers, and others are available as a programmatic means of adding, removing and querying the processes in a cluster.

julia> using Distributed

julia> addprocs(2)
2-element Array{Int64,1}:
 2
 3

Module Distributed must be explicitly loaded on the master process before invoking addprocs. It is automatically made available on the worker processes.

Other types of clusters can be supported by writing your own custom ClusterManager, as described below in the ClusterManagers section.

Data Movement

Sending messages and moving data constitute most of the overhead in a distributed program. Reducing the number of messages and the amount of data sent is critical to achieving performance and scalability. To this end, it is important to understand the data movement performed by Julia's various distributed programming constructs.

fetch can be considered an explicit data movement operation, since it directly asks that an object be moved to the local machine. @spawnat (and a few related constructs) also moves data, but this is not as obvious, hence it can be called an implicit data movement operation. Consider these two approaches to constructing and squaring a random matrix:

Method 1:

julia> A = rand(1000,1000);

julia> Bref = @spawnat :any A^2;

[...]

julia> fetch(Bref);

Method 2:

julia> Bref = @spawnat :any rand(1000,1000)^2;

[...]

julia> fetch(Bref);

The difference seems trivial, but in fact is quite significant due to the behavior of @spawnat. In the first method, a random matrix is constructed locally, then sent to another process where it is squared. In the second method, a random matrix is both constructed and squared on another process. Therefore the second method sends much less data than the first.

In this toy example, the two methods are easy to distinguish and choose from. However, in a real program designing data movement might require more thought and likely some measurement. For example, if the first process needs matrix A then the first method might be better. Or, if computing A is expensive and only the current process has it, then moving it to another process might be unavoidable. Or, if the current process has very little to do between the @spawnat and fetch(Bref), it might be better to eliminate the parallelism altogether. Or imagine rand(1000,1000) is replaced with a more expensive operation. Then it might make sense to add another @spawnat statement just for this step.

Global variables

Expressions executed remotely via @spawnat, or closures specified for remote execution using remotecall may refer to global variables. Global bindings under module Main are treated a little differently compared to global bindings in other modules. Consider the following code snippet:

A = rand(10,10)
remotecall_fetch(()->sum(A), 2)

In this case sum MUST be defined in the remote process. Note that A is a global variable defined in the local workspace. Worker 2 does not have a variable called A under Main. The act of shipping the closure ()->sum(A) to worker 2 results in Main.A being defined on 2. Main.A continues to exist on worker 2 even after the call remotecall_fetch returns. Remote calls with embedded global references (under Main module only) manage globals as follows:

• New global bindings are created on destination workers if they are referenced as part of a remote call.

• Global constants are declared as constants on remote nodes too.

• Globals are re-sent to a destination worker only in the context of a remote call, and then only if its value has changed. Also, the cluster does not synchronize global bindings across nodes. For example:

  A = rand(10,10)
  remotecall_fetch(()->sum(A), 2) # worker 2
  A = rand(10,10)
  remotecall_fetch(()->sum(A), 3) # worker 3
  A = nothing

  Executing the above snippet results in Main.A on worker 2 having a different value from Main.A on worker 3, while the value of Main.A on node 1 is set to nothing.

As you may have realized, while memory associated with globals may be collected when they are reassigned on the master, no such action is taken on the workers as the bindings continue to be valid. clear! can be used to manually reassign specific globals on remote nodes to nothing once they are no longer required. This will release any memory associated with them as part of a regular garbage collection cycle.

Thus programs should be careful referencing globals in remote calls. In fact, it is preferable to avoid them altogether if possible. If you must reference globals, consider using let blocks to localize global variables.

For example:

julia> A = rand(10,10);

julia> remotecall_fetch(()->A, 2);

julia> B = rand(10,10);

julia> let B = B
           remotecall_fetch(()->B, 2)
       end;

julia> @fetchfrom 2 InteractiveUtils.varinfo()
name           size summary
––––––––– ––––––––– ––––––––––––––––––––––
A         800 bytes 10×10 Array{Float64,2}
Base                Module
Core                Module
Main                Module

As can be seen, global variable A is defined on worker 2, but B is captured as a local variable and hence a binding for B does not exist on worker 2.

Parallel Map and Loops

Fortunately, many useful parallel computations do not require data movement. A common example is a Monte Carlo simulation, where multiple processes can handle independent simulation trials simultaneously. We can use @spawnat to flip coins on two processes. First, write the following function in count_heads.jl:

function count_heads(n)
    c::Int = 0
    for i = 1:n
        c += rand(Bool)
    end
    c
end

The function count_heads simply adds together n random bits. Here is how we can perform some trials on two machines, and add together the results:

julia> @everywhere include_string(Main, $(read("count_heads.jl", String)), "count_heads.jl")

julia> a = @spawnat :any count_heads(100000000)
Future(2, 1, 6, nothing)

julia> b = @spawnat :any count_heads(100000000)
Future(3, 1, 7, nothing)

julia> fetch(a)+fetch(b)
100001564

This example demonstrates a powerful and often-used parallel programming pattern. Many iterations run independently over several processes, and then their results are combined using some function. The combination process is called a reduction, since it is generally tensor-rank-reducing: a vector of numbers is reduced to a single number, or a matrix is reduced to a single row or column, etc. In code, this typically looks like the pattern x = f(x,v[i]), where x is the accumulator, f is the reduction function, and the v[i] are the elements being reduced. It is desirable for f to be associative, so that it does not matter what order the operations are performed in.

Notice that our use of this pattern with count_heads can be generalized. We used two explicit @spawnat statements, which limits the parallelism to two processes. To run on any number of processes, we can use a parallel for loop, running in distributed memory, which can be written in Julia using @distributed like this:

nheads = @distributed (+) for i = 1:200000000
    Int(rand(Bool))
end

This construct implements the pattern of assigning iterations to multiple processes, and combining them with a specified reduction (in this case (+)). The result of each iteration is taken as the value of the last expression inside the loop. The whole parallel loop expression itself evaluates to the final answer.

Note that although parallel for loops look like serial for loops, their behavior is dramatically different. In particular, the iterations do not happen in a specified order, and writes to variables or arrays will not be globally visible since iterations run on different processes. Any variables used inside the parallel loop will be copied and broadcast to each process.

For example, the following code will not work as intended:

a = zeros(100000)
@distributed for i = 1:100000
    a[i] = i
end

This code will not initialize all of a, since each process will have a separate copy of it. Parallel for loops like these must be avoided. Fortunately, Shared Arrays can be used to get around this limitation:

using SharedArrays

a = SharedArray{Float64}(10)
@distributed for i = 1:10
    a[i] = i
end

Using "outside" variables in parallel loops is perfectly reasonable if the variables are read-only:

a = randn(1000)
@distributed (+) for i = 1:100000
    f(a[rand(1:end)])
end

Here each iteration applies f to a randomly-chosen sample from a vector a shared by all processes.

As you could see, the reduction operator can be omitted if it is not needed. In that case, the loop executes asynchronously, i.e. it spawns independent tasks on all available workers and returns an array of Future immediately without waiting for completion. The caller can wait for the Future completions at a later point by calling fetch on them, or wait for completion at the end of the loop by prefixing it with @sync, like @sync @distributed for.

In some cases no reduction operator is needed, and we merely wish to apply a function to all integers in some range (or, more generally, to all elements in some collection). This is another useful operation called parallel map, implemented in Julia as the pmap function. For example, we could compute the singular values of several large random matrices in parallel as follows:

julia> M = Matrix{Float64}[rand(1000,1000) for i = 1:10];

julia> pmap(svdvals, M);

Julia's pmap is designed for the case where each function call does a large amount of work. In contrast, @distributed for can handle situations where each iteration is tiny, perhaps merely summing two numbers. Only worker processes are used by both pmap and @distributed for for the parallel computation. In case of @distributed for, the final reduction is done on the calling process.

Remote References and AbstractChannels

Remote references always refer to an implementation of an AbstractChannel.

A concrete implementation of an AbstractChannel (like Channel), is required to implement put!, take!, fetch, isready and wait. The remote object referred to by a Future is stored in a Channel{Any}(1), i.e., a Channel of size 1 capable of holding objects of Any type.

RemoteChannel, which is rewritable, can point to any type and size of channels, or any other implementation of an AbstractChannel.

The constructor RemoteChannel(f::Function, pid)() allows us to construct references to channels holding more than one value of a specific type. f is a function executed on pid and it must return an AbstractChannel.

For example, RemoteChannel(()->Channel{Int}(10), pid), will return a reference to a channel of type Int and size 10. The channel exists on worker pid.

Methods put!, take!, fetch, isready and wait on a RemoteChannel are proxied onto the backing store on the remote process.

RemoteChannel can thus be used to refer to user implemented AbstractChannel objects. A simple example of this is the following DictChannel which uses a dictionary as its remote store:

julia> struct DictChannel{T} <: AbstractChannel{T}
           d::Dict
           cond_take::Threads.Condition    # waiting for data to become available
           DictChannel{T}() where {T} = new(Dict(), Threads.Condition())
           DictChannel() = DictChannel{Any}()
       end

julia> begin
       function Base.put!(D::DictChannel, k, v)
           @lock D.cond_take begin
               D.d[k] = v
               notify(D.cond_take)
           end
           return D
       end
       function Base.take!(D::DictChannel, k)
           @lock D.cond_take begin
               v = fetch(D, k)
               delete!(D.d, k)
               return v
           end
       end
       Base.isready(D::DictChannel) = @lock D.cond_take !isempty(D.d)
       Base.isready(D::DictChannel, k) = @lock D.cond_take haskey(D.d, k)
       function Base.fetch(D::DictChannel, k)
           @lock D.cond_take begin
               wait(D, k)
               return D.d[k]
           end
       end
       function Base.wait(D::DictChannel, k)
           @lock D.cond_take begin
               while !isready(D, k)
                   wait(D.cond_take)
               end
           end
       end
       end;

julia> d = DictChannel();

julia> isready(d)
false

julia> put!(d, :k, :v);

julia> isready(d, :k)
true

julia> fetch(d, :k)
:v

julia> wait(d, :k)

julia> take!(d, :k)
:v

julia> isready(d, :k)
false

Channels and RemoteChannels

• A Channel is local to a process. Worker 2 cannot directly refer to a Channel on worker 3 and vice-versa. A RemoteChannel, however, can put and take values across workers.
• A RemoteChannel can be thought of as a handle to a Channel.
• The process id, pid, associated with a RemoteChannel identifies the process where the backing store, i.e., the backing Channel exists.
• Any process with a reference to a RemoteChannel can put and take items from the channel. Data is automatically sent to (or retrieved from) the process a RemoteChannel is associated with.
• Serializing a Channel also serializes any data present in the channel. Deserializing it therefore effectively makes a copy of the original object.
• On the other hand, serializing a RemoteChannel only involves the serialization of an identifier that identifies the location and instance of Channel referred to by the handle. A deserialized RemoteChannel object (on any worker), therefore also points to the same backing store as the original.

The channels example from above can be modified for interprocess communication, as shown below.

We start 4 workers to process a single jobs remote channel. Jobs, identified by an id (job_id), are written to the channel. Each remotely executing task in this simulation reads a job_id, waits for a random amount of time and writes back a tuple of job_id, time taken and its own pid to the results channel. Finally all the results are printed out on the master process.

julia> addprocs(4); # add worker processes

julia> const jobs = RemoteChannel(()->Channel{Int}(32));

julia> const results = RemoteChannel(()->Channel{Tuple}(32));

julia> @everywhere function do_work(jobs, results) # define work function everywhere
           while true
               job_id = take!(jobs)
               exec_time = rand()
               sleep(exec_time) # simulates elapsed time doing actual work
               put!(results, (job_id, exec_time, myid()))
           end
       end

julia> function make_jobs(n)
           for i in 1:n
               put!(jobs, i)
           end
       end;

julia> n = 12;

julia> errormonitor(Threads.@spawn make_jobs(n)); # feed the jobs channel with "n" jobs

julia> for p in workers() # start tasks on the workers to process requests in parallel
           remote_do(do_work, p, jobs, results)
       end

julia> @elapsed while n > 0 # print out results
           job_id, exec_time, where = take!(results)
           println("$job_id finished in $(round(exec_time; digits=2)) seconds on worker $where")
           global n = n - 1
       end
1 finished in 0.18 seconds on worker 4
2 finished in 0.26 seconds on worker 5
6 finished in 0.12 seconds on worker 4
7 finished in 0.18 seconds on worker 4
5 finished in 0.35 seconds on worker 5
4 finished in 0.68 seconds on worker 2
3 finished in 0.73 seconds on worker 3
11 finished in 0.01 seconds on worker 3
12 finished in 0.02 seconds on worker 3
9 finished in 0.26 seconds on worker 5
8 finished in 0.57 seconds on worker 4
10 finished in 0.58 seconds on worker 2
0.055971741

Remote References and Distributed Garbage Collection

Objects referred to by remote references can be freed only when all held references in the cluster are deleted.

The node where the value is stored keeps track of which of the workers have a reference to it. Every time a RemoteChannel or a (unfetched) Future is serialized to a worker, the node pointed to by the reference is notified. And every time a RemoteChannel or a (unfetched) Future is garbage collected locally, the node owning the value is again notified. This is implemented in an internal cluster aware serializer. Remote references are only valid in the context of a running cluster. Serializing and deserializing references to and from regular IO objects is not supported.

The notifications are done via sending of "tracking" messages–an "add reference" message when a reference is serialized to a different process and a "delete reference" message when a reference is locally garbage collected.

Since Futures are write-once and cached locally, the act of fetching a Future also updates reference tracking information on the node owning the value.

The node which owns the value frees it once all references to it are cleared.

With Futures, serializing an already fetched Future to a different node also sends the value since the original remote store may have collected the value by this time.

It is important to note that when an object is locally garbage collected depends on the size of the object and the current memory pressure in the system.

In case of remote references, the size of the local reference object is quite small, while the value stored on the remote node may be quite large. Since the local object may not be collected immediately, it is a good practice to explicitly call finalize on local instances of a RemoteChannel, or on unfetched Futures. Since calling fetch on a Future also removes its reference from the remote store, this is not required on fetched Futures. Explicitly calling finalize results in an immediate message sent to the remote node to go ahead and remove its reference to the value.

Once finalized, a reference becomes invalid and cannot be used in any further calls.

Local invocations

Data is necessarily copied over to the remote node for execution. This is the case for both remotecalls and when data is stored to a RemoteChannel / Future on a different node. As expected, this results in a copy of the serialized objects on the remote node. However, when the destination node is the local node, i.e. the calling process id is the same as the remote node id, it is executed as a local call. It is usually (not always) executed in a different task - but there is no serialization/deserialization of data. Consequently, the call refers to the same object instances as passed - no copies are created. This behavior is highlighted below:

julia> using Distributed;

julia> rc = RemoteChannel(()->Channel(3));   # RemoteChannel created on local node

julia> v = [0];

julia> for i in 1:3
           v[1] = i                          # Reusing `v`
           put!(rc, v)
       end;

julia> result = [take!(rc) for _ in 1:3];

julia> println(result);
Array{Int64,1}[[3], [3], [3]]

julia> println("Num Unique objects : ", length(unique(map(objectid, result))));
Num Unique objects : 1

julia> addprocs(1);

julia> rc = RemoteChannel(()->Channel(3), workers()[1]);   # RemoteChannel created on remote node

julia> v = [0];

julia> for i in 1:3
           v[1] = i
           put!(rc, v)
       end;

julia> result = [take!(rc) for _ in 1:3];

julia> println(result);
Array{Int64,1}[[1], [2], [3]]

julia> println("Num Unique objects : ", length(unique(map(objectid, result))));
Num Unique objects : 3

As can be seen, put! on a locally owned RemoteChannel with the same object v modified between calls results in the same single object instance stored. As opposed to copies of v being created when the node owning rc is a different node.

It is to be noted that this is generally not an issue. It is something to be factored in only if the object is both being stored locally and modified post the call. In such cases it may be appropriate to store a deepcopy of the object.

This is also true for remotecalls on the local node as seen in the following example:

julia> using Distributed; addprocs(1);

julia> v = [0];

julia> v2 = remotecall_fetch(x->(x[1] = 1; x), myid(), v);     # Executed on local node

julia> println("v=$v, v2=$v2, ", v === v2);
v=[1], v2=[1], true

julia> v = [0];

julia> v2 = remotecall_fetch(x->(x[1] = 1; x), workers()[1], v); # Executed on remote node

julia> println("v=$v, v2=$v2, ", v === v2);
v=[0], v2=[1], false

As can be seen once again, a remote call onto the local node behaves just like a direct invocation. The call modifies local objects passed as arguments. In the remote invocation, it operates on a copy of the arguments.

To repeat, in general this is not an issue. If the local node is also being used as a compute node, and the arguments used post the call, this behavior needs to be factored in and if required deep copies of arguments must be passed to the call invoked on the local node. Calls on remote nodes will always operate on copies of arguments.

Shared Arrays

Shared Arrays use system shared memory to map the same array across many processes. A SharedArray is a good choice when you want to have a large amount of data jointly accessible to two or more processes on the same machine. Shared Array support is available via the module SharedArrays, which must be explicitly loaded on all participating workers.

A complementary data structure is provided by the external package DistributedArrays.jl in the form of a DArray. While there are some similarities to a SharedArray, the behavior of a DArray is quite different. In a SharedArray, each "participating" process has access to the entire array; in contrast, in a DArray, each process has local access to just a chunk of the data, and no two processes share the same chunk.

SharedArray indexing (assignment and accessing values) works just as with regular arrays, and is efficient because the underlying memory is available to the local process. Therefore, most algorithms work naturally on SharedArrays, albeit in single-process mode. In cases where an algorithm insists on an Array input, the underlying array can be retrieved from a SharedArray by calling sdata. For other AbstractArray types, sdata just returns the object itself, so it's safe to use sdata on any Array-type object.

The constructor for a shared array is of the form:

SharedArray{T,N}(dims::NTuple; init=false, pids=Int[])

which creates an N-dimensional shared array of a bits type T and size dims across the processes specified by pids. Unlike distributed arrays, a shared array is accessible only from those participating workers specified by the pids named argument (and the creating process too, if it is on the same host). Note that only elements that are isbits are supported in a SharedArray.

If an init function, of signature initfn(S::SharedArray), is specified, it is called on all the participating workers. You can specify that each worker runs the init function on a distinct portion of the array, thereby parallelizing initialization.

Here's a brief example:

julia> using Distributed

julia> addprocs(3)
3-element Array{Int64,1}:
 2
 3
 4

julia> @everywhere using SharedArrays

julia> S = SharedArray{Int,2}((3,4), init = S -> S[localindices(S)] = repeat([myid()], length(localindices(S))))
3×4 SharedArray{Int64,2}:
 2  2  3  4
 2  3  3  4
 2  3  4  4

julia> S[3,2] = 7
7

julia> S
3×4 SharedArray{Int64,2}:
 2  2  3  4
 2  3  3  4
 2  7  4  4

SharedArrays.localindices provides disjoint one-dimensional ranges of indices, and is sometimes convenient for splitting up tasks among processes. You can, of course, divide the work any way you wish:

julia> S = SharedArray{Int,2}((3,4), init = S -> S[indexpids(S):length(procs(S)):length(S)] = repeat([myid()], length( indexpids(S):length(procs(S)):length(S))))
3×4 SharedArray{Int64,2}:
 2  2  2  2
 3  3  3  3
 4  4  4  4

Since all processes have access to the underlying data, you do have to be careful not to set up conflicts. For example:

@sync begin
    for p in procs(S)
        Threads.@spawn begin
            remotecall_wait(fill!, p, S, p)
        end
    end
end

would result in undefined behavior. Because each process fills the entire array with its own pid, whichever process is the last to execute (for any particular element of S) will have its pid retained.

As a more extended and complex example, consider running the following "kernel" in parallel:

q[i,j,t+1] = q[i,j,t] + u[i,j,t]

In this case, if we try to split up the work using a one-dimensional index, we are likely to run into trouble: if q[i,j,t] is near the end of the block assigned to one worker and q[i,j,t+1] is near the beginning of the block assigned to another, it's very likely that q[i,j,t] will not be ready at the time it's needed for computing q[i,j,t+1]. In such cases, one is better off chunking the array manually. Let's split along the second dimension. Define a function that returns the (irange, jrange) indices assigned to this worker:

julia> @everywhere function myrange(q::SharedArray)
           idx = indexpids(q)
           if idx == 0 # This worker is not assigned a piece
               return 1:0, 1:0
           end
           nchunks = length(procs(q))
           splits = [round(Int, s) for s in range(0, stop=size(q,2), length=nchunks+1)]
           1:size(q,1), splits[idx]+1:splits[idx+1]
       end

Next, define the kernel:

julia> @everywhere function advection_chunk!(q, u, irange, jrange, trange)
           @show (irange, jrange, trange)  # display so we can see what's happening
           for t in trange, j in jrange, i in irange
               q[i,j,t+1] = q[i,j,t] + u[i,j,t]
           end
           q
       end

We also define a convenience wrapper for a SharedArray implementation

julia> @everywhere advection_shared_chunk!(q, u) =
           advection_chunk!(q, u, myrange(q)..., 1:size(q,3)-1)

Now let's compare three different versions, one that runs in a single process:

julia> advection_serial!(q, u) = advection_chunk!(q, u, 1:size(q,1), 1:size(q,2), 1:size(q,3)-1);

one that uses @distributed:

julia> function advection_parallel!(q, u)
           for t = 1:size(q,3)-1
               @sync @distributed for j = 1:size(q,2)
                   for i = 1:size(q,1)
                       q[i,j,t+1]= q[i,j,t] + u[i,j,t]
                   end
               end
           end
           q
       end;

and one that delegates in chunks:

julia> function advection_shared!(q, u)
           @sync begin
               for p in procs(q)
                   Threads.@spawn remotecall_wait(advection_shared_chunk!, p, q, u)
               end
           end
           q
       end;

If we create SharedArrays and time these functions, we get the following results (with julia -p 4):

julia> q = SharedArray{Float64,3}((500,500,500));

julia> u = SharedArray{Float64,3}((500,500,500));

Run the functions once to JIT-compile and @time them on the second run:

julia> @time advection_serial!(q, u);
(irange,jrange,trange) = (1:500,1:500,1:499)
 830.220 milliseconds (216 allocations: 13820 bytes)

julia> @time advection_parallel!(q, u);
   2.495 seconds      (3999 k allocations: 289 MB, 2.09% gc time)

julia> @time advection_shared!(q,u);
        From worker 2:       (irange,jrange,trange) = (1:500,1:125,1:499)
        From worker 4:       (irange,jrange,trange) = (1:500,251:375,1:499)
        From worker 3:       (irange,jrange,trange) = (1:500,126:250,1:499)
        From worker 5:       (irange,jrange,trange) = (1:500,376:500,1:499)
 238.119 milliseconds (2264 allocations: 169 KB)

The biggest advantage of advection_shared! is that it minimizes traffic among the workers, allowing each to compute for an extended time on the assigned piece.

Shared Arrays and Distributed Garbage Collection

Like remote references, shared arrays are also dependent on garbage collection on the creating node to release references from all participating workers. Code which creates many short lived shared array objects would benefit from explicitly finalizing these objects as soon as possible. This results in both memory and file handles mapping the shared segment being released sooner.

ClusterManagers

The launching, management and networking of Julia processes into a logical cluster is done via cluster managers. A ClusterManager is responsible for

• launching worker processes in a cluster environment
• managing events during the lifetime of each worker
• optionally, providing data transport

A Julia cluster has the following characteristics:

• The initial Julia process, also called the master, is special and has an id of 1.
• Only the master process can add or remove worker processes.
• All processes can directly communicate with each other.

Connections between workers (using the in-built TCP/IP transport) is established in the following manner:

• addprocs is called on the master process with a ClusterManager object.
• addprocs calls the appropriate launch method which spawns required number of worker processes on appropriate machines.
• Each worker starts listening on a free port and writes out its host and port information to stdout.
• The cluster manager captures the stdout of each worker and makes it available to the master process.
• The master process parses this information and sets up TCP/IP connections to each worker.
• Every worker is also notified of other workers in the cluster.
• Each worker connects to all workers whose id is less than the worker's own id.
• In this way a mesh network is established, wherein every worker is directly connected with every other worker.

While the default transport layer uses plain TCPSocket, it is possible for a Julia cluster to provide its own transport.

Julia provides two in-built cluster managers:

• LocalManager, used when addprocs() or addprocs(np::Integer) are called
• SSHManager, used when addprocs(hostnames::Array) is called with a list of hostnames

LocalManager is used to launch additional workers on the same host, thereby leveraging multi-core and multi-processor hardware.

Thus, a minimal cluster manager would need to:

• be a subtype of the abstract ClusterManager
• implement launch, a method responsible for launching new workers
• implement manage, which is called at various events during a worker's lifetime (for example, sending an interrupt signal)

addprocs(manager::FooManager) requires FooManager to implement:

function launch(manager::FooManager, params::Dict, launched::Array, c::Condition)
    [...]
end

function manage(manager::FooManager, id::Integer, config::WorkerConfig, op::Symbol)
    [...]
end

As an example let us see how the LocalManager, the manager responsible for starting workers on the same host, is implemented:

struct LocalManager <: ClusterManager
    np::Integer
end

function launch(manager::LocalManager, params::Dict, launched::Array, c::Condition)
    [...]
end

function manage(manager::LocalManager, id::Integer, config::WorkerConfig, op::Symbol)
    [...]
end

The launch method takes the following arguments:

• manager::ClusterManager: the cluster manager that addprocs is called with
• params::Dict: all the keyword arguments passed to addprocs
• launched::Array: the array to append one or more WorkerConfig objects to
• c::Condition: the condition variable to be notified as and when workers are launched

The launch method is called asynchronously in a separate task. The termination of this task signals that all requested workers have been launched. Hence the launch function MUST exit as soon as all the requested workers have been launched.

Newly launched workers are connected to each other and the master process in an all-to-all manner. Specifying the command line argument --worker[=<cookie>] results in the launched processes initializing themselves as workers and connections being set up via TCP/IP sockets.

All workers in a cluster share the same cookie as the master. When the cookie is unspecified, i.e, with the --worker option, the worker tries to read it from its standard input. LocalManager and SSHManager both pass the cookie to newly launched workers via their standard inputs.

By default a worker will listen on a free port at the address returned by a call to getipaddr(). A specific address to listen on may be specified by optional argument --bind-to bind_addr[:port]. This is useful for multi-homed hosts.

As an example of a non-TCP/IP transport, an implementation may choose to use MPI, in which case --worker must NOT be specified. Instead, newly launched workers should call init_worker(cookie) before using any of the parallel constructs.

For every worker launched, the launch method must add a WorkerConfig object (with appropriate fields initialized) to launched

mutable struct WorkerConfig
    # Common fields relevant to all cluster managers
    io::Union{IO, Nothing}
    host::Union{AbstractString, Nothing}
    port::Union{Integer, Nothing}

    # Used when launching additional workers at a host
    count::Union{Int, Symbol, Nothing}
    exename::Union{AbstractString, Cmd, Nothing}
    exeflags::Union{Cmd, Nothing}

    # External cluster managers can use this to store information at a per-worker level
    # Can be a dict if multiple fields need to be stored.
    userdata::Any

    # SSHManager / SSH tunnel connections to workers
    tunnel::Union{Bool, Nothing}
    bind_addr::Union{AbstractString, Nothing}
    sshflags::Union{Cmd, Nothing}
    max_parallel::Union{Integer, Nothing}

    # Used by Local/SSH managers
    connect_at::Any

    [...]
end

Most of the fields in WorkerConfig are used by the inbuilt managers. Custom cluster managers would typically specify only io or host / port:

• If io is specified, it is used to read host/port information. A Julia worker prints out its bind address and port at startup. This allows Julia workers to listen on any free port available instead of requiring worker ports to be configured manually.

• If io is not specified, host and port are used to connect.

• count, exename and exeflags are relevant for launching additional workers from a worker. For example, a cluster manager may launch a single worker per node, and use that to launch additional workers.

  • count with an integer value n will launch a total of n workers.
  • count with a value of :auto will launch as many workers as the number of CPU threads (logical cores) on that machine.
  • exename is the name of the julia executable including the full path.
  • exeflags should be set to the required command line arguments for new workers.

• tunnel, bind_addr, sshflags and max_parallel are used when a ssh tunnel is required to connect to the workers from the master process.

• userdata is provided for custom cluster managers to store their own worker-specific information.

manage(manager::FooManager, id::Integer, config::WorkerConfig, op::Symbol) is called at different times during the worker's lifetime with appropriate op values:

• with :register/:deregister when a worker is added / removed from the Julia worker pool.
• with :interrupt when interrupt(workers) is called. The ClusterManager should signal the appropriate worker with an interrupt signal.
• with :finalize for cleanup purposes.

Cluster Managers with Custom Transports

Replacing the default TCP/IP all-to-all socket connections with a custom transport layer is a little more involved. Each Julia process has as many communication tasks as the workers it is connected to. For example, consider a Julia cluster of 32 processes in an all-to-all mesh network:

• Each Julia process thus has 31 communication tasks.
• Each task handles all incoming messages from a single remote worker in a message-processing loop.
• The message-processing loop waits on an IO object (for example, a TCPSocket in the default implementation), reads an entire message, processes it and waits for the next one.
• Sending messages to a process is done directly from any Julia task–not just communication tasks–again, via the appropriate IO object.

Replacing the default transport requires the new implementation to set up connections to remote workers and to provide appropriate IO objects that the message-processing loops can wait on. The manager-specific callbacks to be implemented are:

connect(manager::FooManager, pid::Integer, config::WorkerConfig)
kill(manager::FooManager, pid::Int, config::WorkerConfig)

The default implementation (which uses TCP/IP sockets) is implemented as connect(manager::ClusterManager, pid::Integer, config::WorkerConfig).

connect should return a pair of IO objects, one for reading data sent from worker pid, and the other to write data that needs to be sent to worker pid. Custom cluster managers can use an in-memory BufferStream as the plumbing to proxy data between the custom, possibly non-IO transport and Julia's in-built parallel infrastructure.

A BufferStream is an in-memory IOBuffer which behaves like an IO–it is a stream which can be handled asynchronously.

The folder clustermanager/0mq in the Examples repository contains an example of using ZeroMQ to connect Julia workers in a star topology with a 0MQ broker in the middle. Note: The Julia processes are still all logically connected to each other–any worker can message any other worker directly without any awareness of 0MQ being used as the transport layer.

When using custom transports:

• Julia workers must NOT be started with --worker. Starting with --worker will result in the newly launched workers defaulting to the TCP/IP socket transport implementation.
• For every incoming logical connection with a worker, Base.process_messages(rd::IO, wr::IO)() must be called. This launches a new task that handles reading and writing of messages from/to the worker represented by the IO objects.
• init_worker(cookie, manager::FooManager) must be called as part of worker process initialization.
• Field connect_at::Any in WorkerConfig can be set by the cluster manager when launch is called. The value of this field is passed in all connect callbacks. Typically, it carries information on how to connect to a worker. For example, the TCP/IP socket transport uses this field to specify the (host, port) tuple at which to connect to a worker.

kill(manager, pid, config) is called to remove a worker from the cluster. On the master process, the corresponding IO objects must be closed by the implementation to ensure proper cleanup. The default implementation simply executes an exit() call on the specified remote worker.

The Examples folder clustermanager/simple is an example that shows a simple implementation using UNIX domain sockets for cluster setup.

Network Requirements for LocalManager and SSHManager

Julia clusters are designed to be executed on already secured environments on infrastructure such as local laptops, departmental clusters, or even the cloud. This section covers network security requirements for the inbuilt LocalManager and SSHManager:

• The master process does not listen on any port. It only connects out to the workers.

• Each worker binds to only one of the local interfaces and listens on an ephemeral port number assigned by the OS.

• LocalManager, used by addprocs(N), by default binds only to the loopback interface. This means that workers started later on remote hosts (or by anyone with malicious intentions) are unable to connect to the cluster. An addprocs(4) followed by an addprocs(["remote_host"]) will fail. Some users may need to create a cluster comprising their local system and a few remote systems. This can be done by explicitly requesting LocalManager to bind to an external network interface via the restrict keyword argument: addprocs(4; restrict=false).

• SSHManager, used by addprocs(list_of_remote_hosts), launches workers on remote hosts via SSH. By default SSH is only used to launch Julia workers. Subsequent master-worker and worker-worker connections use plain, unencrypted TCP/IP sockets. The remote hosts must have passwordless login enabled. Additional SSH flags or credentials may be specified via keyword argument sshflags.

• addprocs(list_of_remote_hosts; tunnel=true, sshflags=<ssh keys and other flags>) is useful when we wish to use SSH connections for master-worker too. A typical scenario for this is a local laptop running the Julia REPL (i.e., the master) with the rest of the cluster on the cloud, say on Amazon EC2. In this case only port 22 needs to be opened at the remote cluster coupled with SSH client authenticated via public key infrastructure (PKI). Authentication credentials can be supplied via sshflags, for example sshflags=`-i <keyfile>`.

  In an all-to-all topology (the default), all workers connect to each other via plain TCP sockets. The security policy on the cluster nodes must thus ensure free connectivity between workers for the ephemeral port range (varies by OS).

  Securing and encrypting all worker-worker traffic (via SSH) or encrypting individual messages can be done via a custom ClusterManager.

• If you specify multiplex=true as an option to addprocs, SSH multiplexing is used to create a tunnel between the master and workers. If you have configured SSH multiplexing on your own and the connection has already been established, SSH multiplexing is used regardless of multiplex option. If multiplexing is enabled, forwarding is set by using the existing connection (-O forward option in ssh). This is beneficial if your servers require password authentication; you can avoid authentication in Julia by logging in to the server ahead of addprocs. The control socket will be located at ~/.ssh/julia-%r@%h:%p during the session unless the existing multiplexing connection is used. Note that bandwidth may be limited if you create multiple processes on a node and enable multiplexing, because in that case processes share a single multiplexing TCP connection.

Cluster Cookie

All processes in a cluster share the same cookie which, by default, is a randomly generated string on the master process:

• cluster_cookie() returns the cookie, while cluster_cookie(cookie)() sets it and returns the new cookie.
• All connections are authenticated on both sides to ensure that only workers started by the master are allowed to connect to each other.
• The cookie may be passed to the workers at startup via argument --worker=<cookie>. If argument --worker is specified without the cookie, the worker tries to read the cookie from its standard input (stdin). The stdin is closed immediately after the cookie is retrieved.
• ClusterManagers can retrieve the cookie on the master by calling cluster_cookie(). Cluster managers not using the default TCP/IP transport (and hence not specifying --worker) must call init_worker(cookie, manager) with the same cookie as on the master.

Note that environments requiring higher levels of security can implement this via a custom ClusterManager. For example, cookies can be pre-shared and hence not specified as a startup argument.

Specifying Network Topology (Experimental)

The keyword argument topology passed to addprocs is used to specify how the workers must be connected to each other:

• :all_to_all, the default: all workers are connected to each other.
• :master_worker: only the driver process, i.e. pid 1, has connections to the workers.
• :custom: the launch method of the cluster manager specifies the connection topology via the fields ident and connect_idents in WorkerConfig. A worker with a cluster-manager-provided identity ident will connect to all workers specified in connect_idents.

Keyword argument lazy=true|false only affects topology option :all_to_all. If true, the cluster starts off with the master connected to all workers. Specific worker-worker connections are established at the first remote invocation between two workers. This helps in reducing initial resources allocated for intra-cluster communication. Connections are setup depending on the runtime requirements of a parallel program. Default value for lazy is true.

Currently, sending a message between unconnected workers results in an error. This behaviour, as with the functionality and interface, should be considered experimental in nature and may change in future releases.

Noteworthy external packages

Outside of Julia parallelism there are plenty of external packages that should be mentioned. For example, MPI.jl is a Julia wrapper for the MPI protocol, Dagger.jl provides functionality similar to Python's Dask, and DistributedArrays.jl provides array operations distributed across workers, as outlined above.

A mention must be made of Julia's GPU programming ecosystem, which includes:

1. CUDA.jl wraps the various CUDA libraries and supports compiling Julia kernels for Nvidia GPUs.

2. oneAPI.jl wraps the oneAPI unified programming model, and supports executing Julia kernels on supported accelerators. Currently only Linux is supported.

3. AMDGPU.jl wraps the AMD ROCm libraries and supports compiling Julia kernels for AMD GPUs. Currently only Linux is supported.

4. High-level libraries like KernelAbstractions.jl, Tullio.jl and ArrayFire.jl.

In the following example we will use both DistributedArrays.jl and CUDA.jl to distribute an array across multiple processes by first casting it through distribute() and CuArray().

Remember when importing DistributedArrays.jl to import it across all processes using @everywhere

$ ./julia -p 4

julia> addprocs()

julia> @everywhere using DistributedArrays

julia> using CUDA

julia> B = ones(10_000) ./ 2;

julia> A = ones(10_000) .* π;

julia> C = 2 .* A ./ B;

julia> all(C .≈ 4*π)
true

julia> typeof(C)
Array{Float64,1}

julia> dB = distribute(B);

julia> dA = distribute(A);

julia> dC = 2 .* dA ./ dB;

julia> all(dC .≈ 4*π)
true

julia> typeof(dC)
DistributedArrays.DArray{Float64,1,Array{Float64,1}}

julia> cuB = CuArray(B);

julia> cuA = CuArray(A);

julia> cuC = 2 .* cuA ./ cuB;

julia> all(cuC .≈ 4*π);
true

julia> typeof(cuC)
CuArray{Float64,1}

In the following example we will use both DistributedArrays.jl and CUDA.jl to distribute an array across multiple processes and call a generic function on it.

function power_method(M, v)
    for i in 1:100
        v = M*v
        v /= norm(v)
    end

    return v, norm(M*v) / norm(v)  # or  (M*v) ./ v
end

power_method repeatedly creates a new vector and normalizes it. We have not specified any type signature in function declaration, let's see if it works with the aforementioned datatypes:

julia> M = [2. 1; 1 1];

julia> v = rand(2)
2-element Array{Float64,1}:
0.40395
0.445877

julia> power_method(M,v)
([0.850651, 0.525731], 2.618033988749895)

julia> cuM = CuArray(M);

julia> cuv = CuArray(v);

julia> curesult = power_method(cuM, cuv);

julia> typeof(curesult)
CuArray{Float64,1}

julia> dM = distribute(M);

julia> dv = distribute(v);

julia> dC = power_method(dM, dv);

julia> typeof(dC)
Tuple{DistributedArrays.DArray{Float64,1,Array{Float64,1}},Float64}

To end this short exposure to external packages, we can consider MPI.jl, a Julia wrapper of the MPI protocol. As it would take too long to consider every inner function, it would be better to simply appreciate the approach used to implement the protocol.

Consider this toy script which simply calls each subprocess, instantiate its rank and when the master process is reached, performs the ranks' sum

import MPI

MPI.Init()

comm = MPI.COMM_WORLD
MPI.Barrier(comm)

root = 0
r = MPI.Comm_rank(comm)

sr = MPI.Reduce(r, MPI.SUM, root, comm)

if(MPI.Comm_rank(comm) == root)
   @printf("sum of ranks: %s\n", sr)
end

MPI.Finalize()

mpirun -np 4 ./julia example.jl