Using Valgrind with Julia

Valgrind is a tool for memory debugging, memory leak detection, and profiling. This section describes things to keep in mind when using Valgrind to debug memory issues with Julia.

General considerations

By default, Valgrind assumes that there is no self modifying code in the programs it runs. This assumption works fine in most instances but fails miserably for a just-in-time compiler like julia. For this reason it is crucial to pass --smc-check=all-non-file to valgrind, else code may crash or behave unexpectedly (often in subtle ways).

In some cases, to better detect memory errors using Valgrind, it can help to compile julia with memory pools disabled. The compile-time flag MEMDEBUG disables memory pools in Julia, and MEMDEBUG2 disables memory pools in FemtoLisp. To build julia with both flags, add the following line to Make.user:

CFLAGS = -DMEMDEBUG -DMEMDEBUG2

Another thing to note: if your program uses multiple worker processes, it is likely that you want all such worker processes to run under Valgrind, not just the parent process. To do this, pass --trace-children=yes to valgrind.

Yet another thing to note: if using valgrind errors with Unable to find compatible target in system image, try rebuilding the sysimage with target generic or julia with JULIA_CPU_TARGET=generic.

Suppressions

Valgrind will typically display spurious warnings as it runs. To reduce the number of such warnings, it helps to provide a suppressions file to Valgrind. A sample suppressions file is included in the Julia source distribution at contrib/valgrind-julia.supp.

The suppressions file can be used from the julia/ source directory as follows:

$ valgrind --smc-check=all-non-file --suppressions=contrib/valgrind-julia.supp ./julia progname.jl

Any memory errors that are displayed should either be reported as bugs or contributed as additional suppressions. Note that some versions of Valgrind are shipped with insufficient default suppressions, so that may be one thing to consider before submitting any bugs.

Running the Julia test suite under Valgrind

It is possible to run the entire Julia test suite under Valgrind, but it does take quite some time (typically several hours). To do so, run the following command from the julia/test/ directory:

valgrind --smc-check=all-non-file --trace-children=yes --suppressions=$PWD/../contrib/valgrind-julia.supp ../julia runtests.jl all

If you would like to see a report of "definite" memory leaks, pass the flags --leak-check=full --show-leak-kinds=definite to valgrind as well.

Additional spurious warnings

This section covers Valgrind warnings that cannot be added to the suppressions file yet are nonetheless safe to ignore.

Unhandled rr system calls

Valgrind will emit a warning if it encounters any of the system calls that are specific to rr, the Record and Replay Framework. In particular, a warning about an unhandled 1008 syscall will be shown when julia tries to detect whether it is running under rr:

--xxxxxx-- WARNING: unhandled amd64-linux syscall: 1008
--xxxxxx-- You may be able to write your own handler.
--xxxxxx-- Read the file README_MISSING_SYSCALL_OR_IOCTL.
--xxxxxx-- Nevertheless we consider this a bug.  Please report
--xxxxxx-- it at http://valgrind.org/support/bug_reports.html.

This issue has been reported to the Valgrind developers as they have requested.

Caveats

Valgrind currently does not support multiple rounding modes, so code that adjusts the rounding mode will behave differently when run under Valgrind.

In general, if after setting --smc-check=all-non-file you find that your program behaves differently when run under Valgrind, it may help to pass --tool=none to valgrind as you investigate further. This will enable the minimal Valgrind machinery but will also run much faster than when the full memory checker is enabled.