Parallel Computing¶

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...)return2*rand(dims...)endjulia>rand2(2,2)2×2Array{Float64,2}:0.1537560.3685141.151190.918912julia>fetch(@spawnrand2(2,2))ERROR:Onworker2:function rand2notdefinedonprocess2

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:

moduleDummyModuleexportMyType,ftype MyTypea::Intendf(x)=x^2+1println("loaded")end

Starting Julia with julia-p2, you can use this to verify the following:

• include("DummyModule.jl") loads the file on just a single process (whichever one executes the statement).

• usingDummyModule causes the module to be loaded on all processes; however, the module is brought into scope only on the one executing the statement.

• As long as DummyModule is loaded on process 2, commands like

  rr=RemoteChannel(2)put!(rr,MyType(7))

  allow you to store an object of type MyType on process 2 even if DummyModule is not in scope on process 2.

You can force a command to run on all processes using the @everywhere macro. For example, @everywhere can also be used to directly define a function on all processes:

julia>@everywhereid=myid()julia>remotecall_fetch(()->id,2)2

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

julia-p<n>-Lfile1.jl-Lfile2.jldriver.jl

Each process has an associated identifier. The process providing the interactive Julia prompt always has an id equal to 1, as would the Julia process running the driver script in the example above. 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.

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 --machinefile option. This uses a passwordless ssh login to start Julia worker processes (from the same path as the current host) on the specified machines.

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

Note that workers do not run a .juliarc.jl startup script, nor do they synchronize their global state (such as global variables, new method definitions, and loaded modules) with any of the other running 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 parallel 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 parallel programming constructs.

fetch() can be considered an explicit data movement operation, since it directly asks that an object be moved to the local machine. @spawn (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 1A=rand(1000,1000)Bref=@spawnA^2...fetch(Bref)# method 2Bref=@spawnrand(1000,1000)^2...fetch(Bref)

The difference seems trivial, but in fact is quite significant due to the behavior of @spawn. 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 @spawn 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 @spawn statement just for this step.

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 @spawn to flip coins on two processes. First, write the following function in count_heads.jl:

function count_heads(n)c::Int=0fori=1:nc+=rand(Bool)endcend

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:

@everywhereinclude("count_heads.jl")a=@spawncount_heads(100000000)b=@spawncount_heads(100000000)fetch(a)+fetch(b)

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 @spawn statements, which limits the parallelism to two processes. To run on any number of processes, we can use a parallel for loop, which can be written in Julia like this:

nheads=@parallel(+)fori=1:200000000Int(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)@parallelfori=1:100000a[i]=iend

However, 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:

a=SharedArray(Float64,10)@parallelfori=1:10a[i]=iend

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

a=randn(1000)@parallel(+)fori=1:100000f(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@parallelfor.

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:

M=Matrix{Float64}[rand(1000,1000)fori=1:10]pmap(svd,M)

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

Synchronization With Remote References¶

Scheduling¶

Julia’s parallel programming platform uses Tasks (aka Coroutines) to switch among multiple computations. Whenever code performs a communication operation like fetch() or wait(), the current task is suspended and a scheduler picks another task to run. A task is restarted when the event it is waiting for completes.

For many problems, it is not necessary to think about tasks directly. However, they can be used to wait for multiple events at the same time, which provides for dynamic scheduling. In dynamic scheduling, a program decides what to compute or where to compute it based on when other jobs finish. This is needed for unpredictable or unbalanced workloads, where we want to assign more work to processes only when they finish their current tasks.

As an example, consider computing the singular values of matrices of different sizes:

M=Matrix{Float64}[rand(800,800),rand(600,600),rand(800,800),rand(600,600)]pmap(svd,M)

If one process handles both 800×800 matrices and another handles both 600×600 matrices, we will not get as much scalability as we could. The solution is to make a local task to “feed” work to each process when it completes its current task. For example, consider a simple pmap() implementation:

function pmap(f,lst)np=nprocs()# determine the number of processes availablen=length(lst)results=Vector{Any}(n)i=1# function to produce the next work item from the queue.# in this case it's just an index.nextidx()=(idx=i;i+=1;idx)@syncbeginforp=1:npifp!=myid()||np==1@asyncbeginwhiletrueidx=nextidx()ifidx>nbreakendresults[idx]=remotecall_fetch(f,p,lst[idx])endendendendendresultsend

@async is similar to @spawn, 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 indexes. 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. The feeder tasks are able to share state via nextidx() because they all run on the same process. No locking is required, since the threads are scheduled cooperatively and not preemptively. This means context switches only occur at well-defined points: in this case, when remotecall_fetch() is called.

Channels¶

Channels provide for a fast means of inter-task communication. A Channel{T}(n::Int) is a shared queue of maximum length n holding objects of type T. Multiple readers can read off the Channel via fetch() and take!(). Multiple writers can add to the Channel via put!(). isready() tests for the presence of any object in the channel, while wait() waits for an object to become available. close() closes a Channel. On a closed Channel, put!() will fail, while take!() and fetch() successfully return any existing values till it is emptied.

A Channel can be used as an iterable object in a for loop, in which case the loop runs as long as the Channel has data or is open. The loop variable takes on all values added to the Channel. An empty, closed Channel causes the for loop to terminate.

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 provided in examples/dictchannel.jl which uses a dictionary as its remote store.

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.

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 fetch()ing 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.

Shared Arrays¶

Shared Arrays use system shared memory to map the same array across many processes. While there are some similarities to a DArray, the behavior of a SharedArray is quite different. In a DArray, each process has local access to just a chunk of the data, and no two processes share the same chunk; in contrast, in a SharedArray each “participating” process has access to the entire array. 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.

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::Type,dims::NTuple;init=false,pids=Int[])

which creates a 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).

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>addprocs(3)3-elementArray{Int64,1}:234julia>S=SharedArray(Int,(3,4),init=S->S[Base.localindexes(S)]=myid())3×4SharedArray{Int64,2}:223423342344julia>S[3,2]=77julia>S3×4SharedArray{Int64,2}:223423342744

Base.localindexes() provides disjoint one-dimensional ranges of indexes, 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,(3,4),init=S->S[indexpids(S):length(procs(S)):length(S)]=myid())3×4SharedArray{Int64,2}:222233334444

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

@syncbeginforpinprocs(S)@asyncbeginremotecall_wait(fill!,p,S,p)endendend

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:

# This function retuns the (irange,jrange) indexes assigned to this worker@everywherefunction myrange(q::SharedArray)idx=indexpids(q)ifidx==0# This worker is not assigned a piecereturn1:0,1:0endnchunks=length(procs(q))splits=[round(Int,s)forsinlinspace(0,size(q,2),nchunks+1)]1:size(q,1),splits[idx]+1:splits[idx+1]end# Here's the kernel@everywherefunction advection_chunk!(q,u,irange,jrange,trange)@show(irange,jrange,trange)# display so we can see what's happeningfortintrange,jinjrange,iinirangeq[i,j,t+1]=q[i,j,t]+u[i,j,t]endqend# Here's a convenience wrapper for a SharedArray implementation@everywhereadvection_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:

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

one that uses @parallel:

function advection_parallel!(q,u)fort=1:size(q,3)-1@sync@parallelforj=1:size(q,2)fori=1:size(q,1)q[i,j,t+1]=q[i,j,t]+u[i,j,t]endendendqend

and one that delegates in chunks:

function advection_shared!(q,u)@syncbeginforpinprocs(q)@asyncremotecall_wait(advection_shared_chunk!,p,q,u)endendqend

If we create SharedArrays and time these functions, we get the following results (with julia-p4):

q=SharedArray(Float64,(500,500,500))u=SharedArray(Float64,(500,500,500))# Run once to JIT-compileadvection_serial!(q,u)advection_parallel!(q,u)advection_shared!(q,u)# Now the real results:julia>@timeadvection_serial!(q,u);(irange,jrange,trange)=(1:500,1:500,1:499)830.220milliseconds(216allocations:13820bytes)julia>@timeadvection_parallel!(q,u);2.495seconds(3999kallocations:289MB,2.09%gctime)julia>@timeadvection_shared!(q,u);Fromworker2:(irange,jrange,trange)=(1:500,1:125,1:499)Fromworker4:(irange,jrange,trange)=(1:500,251:375,1:499)Fromworker3:(irange,jrange,trange)=(1:500,126:250,1:499)Fromworker5:(irange,jrange,trange)=(1:500,376:500,1:499)238.119milliseconds(2264allocations:169KB)

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)...endfunction 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:

immutableLocalManager<:ClusterManagernp::Integerendfunction launch(manager::LocalManager,params::Dict,launched::Array,c::Condition)...endfunction manage(manager::LocalManager,id::Integer,config::WorkerConfig,op::Symbol)...end

The launch() method takes the following arguments:

• manager::ClusterManager - the cluster manager 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 a all-to-all manner. Specifying the command argument --worker<cookie> results in the launched processes initializing themselves as workers and connections being setup via TCP/IP sockets. Optionally, --bind-tobind_addr[:port] may also be specified to enable other workers to connect to it at the specified bind_addr and 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

type WorkerConfig# Common fields relevant to all cluster managersio::Nullable{IO}host::Nullable{AbstractString}port::Nullable{Integer}# Used when launching additional workers at a hostcount::Nullable{Union{Int,Symbol}}exename::Nullable{AbstractString}exeflags::Nullable{Cmd}# 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::Nullable{Any}# SSHManager / SSH tunnel connections to workerstunnel::Nullable{Bool}bind_addr::Nullable{AbstractString}sshflags::Nullable{Cmd}max_parallel::Nullable{Integer}connect_at::Nullable{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 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 a 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 involves the new implementation to setup 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.

Folder examples/clustermanager/0mq 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 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.

examples/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 on 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 the first free port starting from 9009.

• LocalManager, i.e. addprocs(N), by default binds only to the loopback interface. This means that workers consequently started on remote hosts, or anyone with malicious intentions is unable to connect to the cluster. A addprocs(4) followed by a addprocs(["remote_host"]) will fail. Some users may need to create a cluster comprised of 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. For example, addprocs(4;restrict=false).

• SSHManager, i.e. addprocs(list_of_remote_hosts) launches workers on remote hosts via SSH. It is to be noted that 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=<sshkeysandotherflags>) 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=`-e<keyfile>`.

  Note that worker-worker connections are still plain TCP and the local security policy on the remote cluster must allow for free connections between worker nodes, at least for ports 9009 and above.

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

Cluster cookie¶

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

• Base.cluster_cookie() returns the cookie, while Base.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 must be passed to the workers at startup via argument --worker<cookie>. Custom ClusterManagers can retrieve the cookie on the master by calling Base.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)¶

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

• :all_to_all : is the default, where all workers are connected to each other.
• :master_slave : 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. Fields ident and connect_idents in WorkerConfig are used to specify the same. connect_idents is a list of ClusterManager provided identifiers to workers that worker with identified by ident must connect to.

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.

Multi-threading (Experimental)¶

In addition to tasks, remote calls, and remote references, Julia from v0.5 forwards will natively support multi-threading. Note that this section is experimental and the interfaces may change in the future.

julia>Threads.nthreads()1

exportJULIA_NUM_THREADS=4

julia>Threads.nthreads()4

julia>Threads.threadid()1

julia>a=zeros(10)10-elementArray{Float64,1}:0.00.00.00.00.00.00.00.00.00.0

Threads.@threadsfori=1:10a[i]=Threads.threadid()end

julia>a10-elementArray{Float64,1}:1.01.01.02.02.02.03.03.04.04.0

@threadcall (Experimental)¶

All I/O tasks, timers, REPL commands, etc are multiplexed onto a single OS thread via an event loop. A patched version of libuv (http://docs.libuv.org/en/v1.x/) provides this functionality. Yield points provide for co-operatively scheduling multiple tasks onto the same OS thread. I/O tasks and timers yield implicitly while waiting for the event to occur. Calling yield() explicitly allows for other tasks to be scheduled.

Thus, a task executing a ccall effectively prevents the Julia scheduler from executing any other tasks till the call returns. This is true for all calls into external libraries. Exceptions are calls into custom C code that call back into Julia (which may then yield) or C code that calls jl_yield() (C equivalent of yield()).

Note that while Julia code runs on a single thread (by default), libraries used by Julia may launch their own internal threads. For example, the BLAS library may start as many threads as there are cores on a machine.

The @threadcall macro addresses scenarios where we do not want a ccall to block the main Julia event loop. It schedules a C function for execution in a separate thread. A threadpool with a default size of 4 is used for this. The size of the threadpool is controlled via environment variable UV_THREADPOOL_SIZE. While waiting for a free thread, and during function execution once a thread is available, the requesting task (on the main Julia event loop) yields to other tasks. Note that @threadcall does not return till the execution is complete. From a user point of view, it is therefore a blocking call like other Julia APIs.

It is very important that the called function does not call back into Julia.

@threadcall may be removed/changed in future versions of Julia.

Footnotes