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.
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
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
CFLAGS = -DMEMDEBUG -DMEMDEBUG2
Another thing to note: if your program uses multiple workers processes, it is likely that you want all such worker processes to run under Valgrind, not just the parent process. To do this, pass
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
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
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.
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
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.
This section covers Valgrind warnings which cannot be added to the suppressions file yet are nonetheless safe to ignore.
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.
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
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.