jemalloc performance for very small allocations

Jason Evans jasone at
Tue Apr 30 22:52:24 PDT 2013

On Apr 26, 2013, at 9:39 AM, Rajat Garg <garg_rajat at> wrote:
>  I am using jemalloc 3.1.0, compiled with gcc 4.1.2 on CentOS release 5.6 for a compute intensive multithreaded application where bulk of allocations falls in 8-byte, 16-byte (stats below). We are seeing very high runtime in tcache_alloc_small_hard(), arena_tcache_fill_small() and arena_bin_malloc_hard().  The 8 and 16 bytes allocations happen in very large number. We probably want the allocations to come form tcache_alloc_easy() to not hit the locks and take less runtime.  The allocations are mostly temporary in nature -- especially 8-byte ones -- so some number of 8-byte allocations are done, then they are freed and then alloc/free process repeated. 
> The runs use 8 threads (so 48 arenas and all other jemalloc settings are default) and are on an Intel Xeon server with 12-cores (no hyperthreading and no other user so no contention with other user; the peak memory in application is ~6.5GB and server has 128GB memory so no memory shortage/swapping etc.); the jemalloc output at the end of run is given below. As we can see, pretty much 8 and 16 byte bins are overloaded.

Only 8 of the 48 arenas are being used in your test case.  That said, the extra 40 are lazily initialized, so there's no need to change settings.

> A run with TCACHE_NSLOTS_SMALL_MAX=10000, LG_TCACHE_MAXCLASS_DEFAULT=10, changing small bins to hold only upto 224 bytes (NBINS=15),  and hardcoding 
> tcache_bin_info[i].ncached_max = TCACHE_NSLOTS_SMALL_MAX in tcache.c file improves runtime by about 16% (for overall application runtime) but increases peak memory from 6.5GB to 6.7GB. We want the runtime to at least improve by another 10% without preferably any increase in peak memory (so even 6.5GB to 6.7GB is not desirable). 
> Any suggestions on what changes to jemalloc settings to try?

I don't think the TCACHE_NSLOTS_SMALL_MAX setting is having as much effect for your test case as you might expect.  Even with the setting of 10000, tcache is limited to 1002 and 504 regions for 8- and 16-byte regions, respectively.  If you want to further increase the tcache region count, you will need to either increase the logical page size, or modify bin_info_run_size_calc() to use a larger initial min_run_size.

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