🔗 Hunting for memory leaks

Memory management is a thorny issue in Squid. Its single-process nature makes it very important no to leak memory in any circumstance, as even a single leaked byte per request can grind a proxy to a halt in a few hours of production useage.

🔗 Valgrind

Valgrind is an advanced runtime profiler that can be very useful for finding memory leaks. Unfortunately, if you are not a Squid developer, you will need to work with one to correctly interpret valgrind results because Squid memory usage often triggers false valgrind leak reports.

Squid can be executed within a valgrind environment “as is”, but to reduce the number of false positives, it is best to ./configure Squid --with-valgrind-debug. ./configure --disable-optimizations helps when interpreting valgrind reports (but slows Squid down). Disabling memory pools (memory_pools off) further reduces the number of false positives. Another essential technique for minimizing false positives, is using valgrind suppressions. The attached suppressions file will probably be out of date when you need to use it (or will not be compatible with your Squid version), so adjust it as needed. Such adjustments require good understanding of Squid code.

Here is one way to start Squid under valgrind control:

valgrind -v \
    --trace-children=yes \
    --num-callers=50 \
    --log-file=valgrind-%p.log \
    --leak-check=full \
    --leak-resolution=high \
    --show-reachable=yes \
    --gen-suppressions=all \
    --suppressions=etc/valgrind.supp \
    sbin/squid ...

With the above command line options, detected problems (including leaks) are detailed throughout each valgrind report (one report for each process started by Squid).

There is a useful summary at the end of a report. For example:

==11905== LEAK SUMMARY:
==11905==    definitely lost: 0 bytes in 0 blocks
==11905==    indirectly lost: 0 bytes in 0 blocks
==11905==      possibly lost: 36 bytes in 1 blocks
==11905==    still reachable: 1,905,982 bytes in 111 blocks
==11905==         suppressed: 1,026,644 bytes in 10,263 blocks

When Squid is ./configured --with-valgrind-debug, the memory usage information reported via the cache manager interface (e.g., squidclient mgr:mem) includes a valgrind report.

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