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skavi • today at 7:45 PM • 3 replies • view on HN

We evaluated a few allocators for some of our Linux apps and found (modern) tcmalloc to consistently win in time and space. Our applications are primarily written in Rust and the allocators were linked in statically (except for glibc). Unfortunately I didn't capture much context on the allocation patterns. I think in general the apps allocate and deallocate at a higher rate than most Rust apps (or more than I'd like at least).

Our results from July 2025:

rows are <allocator>: <RSS>, <time spent for allocator operations>

  app1:
  glibc: 215,580 KB, 133 ms
  mimalloc 2.1.7: 144,092 KB, 91 ms
  mimalloc 2.2.4: 173,240 KB, 280 ms
  tcmalloc: 138,496 KB, 96 ms
  jemalloc: 147,408 KB, 92 ms

  app2, bench1
  glibc: 1,165,000 KB, 1.4 s
  mimalloc 2.1.7: 1,072,000 KB, 5.1 s
  mimalloc 2.2.4:
  tcmalloc: 1,023,000 KB, 530 ms

  app2, bench2
  glibc: 1,190,224 KB, 1.5 s
  mimalloc 2.1.7: 1,128,328 KB, 5.3 s
  mimalloc 2.2.4: 1,657,600 KB, 3.7 s
  tcmalloc: 1,045,968 KB, 640 ms
  jemalloc: 1,210,000 KB, 1.1 s

  app3
  glibc: 284,616 KB, 440 ms
  mimalloc 2.1.7: 246,216 KB, 250 ms
  mimalloc 2.2.4: 325,184 KB, 290 ms
  tcmalloc: 178,688 KB, 200 ms
  jemalloc: 264,688 KB, 230 ms
tcmalloc was from github.com/google/tcmalloc/tree/24b3f29.

i don't recall which jemalloc was tested.

Replies

hedora • today at 7:54 PM

I’m surprised (unless they replaced the core tcmalloc algorithm but kept the name).

tcmalloc (thread caching malloc) assumes memory allocations have good thread locality. This is often a double win (less false sharing of cache lines, and most allocations hit thread-local data structures in the allocator).

Multithreaded async systems destroy that locality, so it constantly has to run through the exception case: A allocated a buffer, went async, the request wakes up on thread B, which frees the buffer, and has to synchronize with A to give it back.

Are you using async rust, or sync rust?

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ComputerGuru • today at 7:55 PM

That’s a considerable regression for mimalloc between 2.1 and 2.2 – did you track it down or report it upstream?

Edit: I see mimalloc v3 is out – I missed that! That probably moots this discussion altogether.

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codexon • today at 9:20 PM

This is similar to what I experienced when I tested mimalloc many years ago. If it was faster, it wasn't faster by much, and had pretty bad worst cases.