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NFC: sizeclass: differentiate minimum step size and minimum allocation sizes #651

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merged 9 commits into from
May 24, 2024

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@nwf-msr nwf-msr commented Jan 4, 2024

No functional change intended yet. Just going through the exercise of separating MIN_ALLOC_SIZE's two hats, but leaving the two at the same value.

So why?

We think this might be useful, letting us use less space in #637 (by leaving MIN_ALLOC_STEP_SIZE at 16 but raising MIN_ALLOC_SIZE to 32, meaning that sizeclasses 0 and 1 would not be allocable), but it might be more generally useful. It's also probably easier to review without all the rest of that mess at the same time. Speaking of review, as per usual, this might be best read in commit order rather than all at once, and the punchline is in the last commit.

@nwf-msr nwf-msr force-pushed the 202401-sizeclass-min-step-vs-alloc branch 2 times, most recently from 7d0b53e to b04b7ab Compare January 4, 2024 21:19
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LGTM

I wonder if we should make STEP or SIZE configurable from CMake, so we can experiment with configurations.

It would be interesting to reduce step to 8, which would get us 16, 24, 32, 40, 48, 56, 64, 80, ...

@nwf-msr nwf-msr force-pushed the 202401-sizeclass-min-step-vs-alloc branch from b04b7ab to ab87e2b Compare January 5, 2024 17:48
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nwf-msr commented Jan 5, 2024

Like that?

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mjp41 commented Jan 5, 2024

Like that?

Is there a static_assert that the main is at least two pointers?

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nwf-msr commented Jan 5, 2024

Is there a static_assert that the main is at least two pointers?

Yes, two, sort of. Once directly by the configuration:

https://github.com/nwf-msr/snmalloc/blob/ab87e2b49b7371a1fa93a96f131703a87925fcd3/src/snmalloc/ds/allocconfig.h#L104-L106

And once by the freelist::Object that actually imposes the requirement:

https://github.com/nwf-msr/snmalloc/blob/ab87e2b49b7371a1fa93a96f131703a87925fcd3/src/snmalloc/mem/freelist.h#L429-L431

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LGTM

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nwf-msr commented Jan 6, 2024

Out of curiosity, I did a little parameter sweeping with mimalloc-bench. aNN is the minimum size, sNN is the step size. (That is, a16-s16 is the current policy on amd64.) Don't take these results too seriously, they're from my laptop and the machine was not particularly quiesced at the time.

image

It could be more interesting to do an analogous sweep on CHERI, where a32-s8 or a32-s16 might be of interest, or with #637, where a64-s16 or a64-s32 might be.

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mjp41 commented Jan 6, 2024

Looks pretty noisy, but I can run some experiments on Monday

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mjp41 commented Jan 8, 2024

@nwf-msr would it make sense to add something like:

  if (SNMALLOC_BENCHMARK_INDIVIDUAL_CONFIGS)
    foreach (STEP 8 16 32)
      foreach (MIN 16 32 64)
        if ((${STEP} EQUAL 8) AND (${MIN} EQUAL 64))
          continue()
        endif()
        set(DEFINES "-DSNMALLOC_MIN_ALLOC_STEP_SIZE=${STEP}" "-DSNMALLOC_MIN_ALLOC_SIZE=${MIN}")
        set(NAME "step-${STEP}-min-${MIN}")
        add_shim(snmallocshim-${NAME} SHARED ${SHIM_FILES})
        target_compile_definitions(snmallocshim-${NAME} PRIVATE ${DEFINES})
        if (SNMALLOC_BUILD_TESTING)
          make_tests(${NAME} ${DEFINES})
        endif()
      endforeach()
    endforeach()
  endif() # SNMALLOC_BENCHMARK_INDIVIDUAL_CONFIGS

So that we can rebuild the libraries for the test?

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mjp41 commented Jan 9, 2024

Here is running on my perf VM in Azure:

memory_nwf
time_nwf

I dropped a couple of the noisier ones, but overall it doesn't have much change.

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mjp41 commented Jan 9, 2024

memory_nwf
time_nwf

The sizeclass was already testing most of this, so just add the missing bits.
Forgo some tests whose failure would have implied earlier failures.

This moves the last dynamic call of size_to_sizeclass_const into tests
(and so, too, to_exp_mant_const).  sizeclasstable.h still contains a static
call to compute NUM_SMALL_SIZECLASSES from MAX_SMALL_SIZECLASS_SIZE.
Only its _const sibling is used, and little at that, now that almost everything
to do with sizes and size classes is table-driven.
This just means I don't need to remember to set a breakpoint on exit
@nwf-msr nwf-msr force-pushed the 202401-sizeclass-min-step-vs-alloc branch from ab87e2b to 6c89829 Compare May 24, 2024 03:08
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Lgtm

src/snmalloc/mem/sizeclasstable.h Outdated Show resolved Hide resolved
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@nwf-msr nwf-msr force-pushed the 202401-sizeclass-min-step-vs-alloc branch from 6c89829 to d5d9d40 Compare May 24, 2024 17:38
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nwf-msr commented May 24, 2024

Taking those two suggestions; thanks! Will merge as soon as CI's happy.

@nwf-msr nwf-msr merged commit 846a926 into microsoft:main May 24, 2024
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@nwf-msr nwf-msr deleted the 202401-sizeclass-min-step-vs-alloc branch May 24, 2024 17:49
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2 participants