Use Case: Heap Profiling

Jason Evans edited this page Jan 16, 2017 · 9 revisions

In addition to leak checking at exit, jemalloc can be told dump a profile:

  • Approximately every so many bytes of allocation. See the prof_leak and lg_prof_interval MALLOC_CONF options.
  • Every time the total virtual memory in use reaches a new high. See the prof_gdump MALLOC_CONF option.
  • Manually, via the "prof.dump" mallctl. Use function calls something like the following (with proper error checking, of course).

    • Dump to specified filename. Set prof to true in your MALLOC_CONF:

      export MALLOC_CONF="prof:true,prof_prefix:jeprof.out"

      Then execute code similar to the following C code:

      const char *fileName = "heap_info.out";
      mallctl("prof.dump", NULL, NULL, &fileName, sizeof(const char *));
    • Use automatic filename generation. Set prof_prefix in your MALLOC_CONF in addition to prof:

      export MALLOC_CONF="prof:true,prof_prefix:jeprof.out"

      Then execute code similar to the following C code:

      mallctl("prof.dump", NULL, NULL, NULL, 0);

jeprof can compare any two of the resulting series of profile dumps, and show what allocation activity occurred during the intervening time. Use the --base=<base> flag to specify the first profile.

It is possible to start an application with profiling enabled but inactive, by specifying MALLOC_CONF=prof_active:false. This is only useful if the application manually activates/deactivates profiling via the "prof.active" mallctl during execution. Use cases include:

  • Activate profiling after initialization is complete, so that profiles only show objects allocated during steady-state execution.
  • Dump a profile, activate profiling for 30 seconds, wait 30 seconds after deactivating profiling, then dump another profile and use jeprof to compare the two dumps. This will focus on objects that were allocated during steady-state execution, but are long-lived. These objects are prime candidates for explaining memory growth over time.

Walking the call stack to capture a backtrace is typically quite computationally intensive. Therefore it is infeasible to use precise leak checking for long-lived, heavily loaded applications. Statistical sampling of allocations makes it possible to keep the computational overhead low, yet get a general idea of how the application utilizes memory. See the lg_prof_sample MALLOC_CONF option for information on controlling sampling interval.