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 likejulia. For this reason it is crucial to pass--smc-check=all-non-file tovalgrind, 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 compilejulia with memory pools disabled. The compile-time flagMEMDEBUG disables memory pools in Julia, andMEMDEBUG2 disables memory pools in FemtoLisp. To buildjulia with both flags, add the following line toMake.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 tovalgrind.

Yet another thing to note: if usingvalgrind errors withUnable to find compatible target in system image, try rebuilding the sysimage with targetgeneric or julia withJULIA_CPU_TARGET=generic.

Suppressions

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

The suppressions file can be used from thejulia/ 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 areshipped 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 thejulia/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 tovalgrind 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 thesystem calls that are specific to rr, theRecord and Replay Framework. In particular, a warning about an unhandled1008 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 issuehas been reported to the Valgrind developers as they have requested.

Caveats

Valgrind currentlydoes 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 tovalgrind 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.