Re: [PATCH 0/9] re-shrink 'struct page' when SLUB is on.

[Dave Hansen]

On 01/05/2014 08:32 PM, Joonsoo Kim wrote:
> On Fri, Jan 03, 2014 at 02:18:16PM -0800, Andrew Morton wrote:
>> On Fri, 03 Jan 2014 10:01:47 -0800 Dave Hansen <dave@xxxxxxxx> wrote:
>>> SLUB depends on a 16-byte cmpxchg for an optimization which
>>> allows it to not disable interrupts in its fast path.  This
>>> optimization has some small but measurable benefits:
>>>
>>> 	http://lkml.kernel.org/r/52B345A3.6090700@xxxxxxxx
>>
>> So really the only significant benefit from the cmpxchg16 is with
>> cache-cold eight-byte kmalloc/kfree?  8% faster in this case?  But with
>> cache-hot kmalloc/kfree the benefit of cmpxchg16 is precisely zero.
> 
> I guess that cmpxchg16 is not used in this cache-hot kmalloc/kfree test,
> because kfree would be done in free fast-path. In this case,
> this_cpu_cmpxchg_double() would be called, so you cannot find any effect
> of cmpxchg16.

That's a good point.  I also confirmed this theory with the
free_{fast,slow}path slub counters.  So, I ran another round of tests.

One important difference from the last round: I'm now writing to each
allocation.  I originally did this so that I could store the allocations
in a linked-list, but I also realized that it's important.  It's rare in
practice to do an allocation and not write _something_ to it.  This
change adds a bit of cache pressure which changed the results pretty
substantially.

I tested 4 cases, all of these on the "cache-cold kfree()" case.  The
first 3 are with vanilla upstream kernel source.  The 4th is patched
with my new slub code (all single-threaded):

	http://www.sr71.net/~dave/intel/slub/slub-perf-20140109.png

There are a few important takeaways here:
1. The double-cmpxchg optimization has a measurable benefit
2. 64-byte 'struct page' is faster than the 56-byte one independent of
   the cmpxchg optimization.  Maybe because foo/sizeof(struct page) is
   then a simple shift.
3. My new code is probably _slightly_ slower than the existing code,
   but still has the huge space savings
4. All of these deltas are greatly magnified here and are hard or
   impossible to demonstrate in practice.

Why does the double-cmpxchg help?  The extra cache references that it
takes to go out and touch the paravirt structures and task struct to
disable interrupts in the spinlock cases start to show up and hurt our
allocation rates by about 30%.  This advantage starts to evaporate when
there is more noise in the caches, or when we start to run the tests
across more cores.

But the real question here is whether we can shrink 'struct page'.  The
existing (64-byte struct page) slub code wins on allocations under 256b
by as much as 5% (the 32-byte kmalloc()), but my new code wins on
allocations over 1k.  4k allocations just happen to be the most common
on my systems, and they're also very near the "sweet spot" for the new
code.  But, the delta here is _much_ smaller that it was in the spinlock
vs. cmpxchg cases.  This advantage also evaporates when we run things
across more cores or in less synthetic benchmarks.

I also explored that 5% hit that my code caused in the 32-byte
allocation case.  It looked to me to be mostly explained by the code
that I added.  There were more instructions executed and the
cycles-per-instruction went down.  This looks to be mostly due to a ~15%
increase in branch misses, probably from the increased code size and
complexity.

This is the perf stat output for a run doing 16.8M kmalloc(32)/kfree()'s:
vanilla:
>            883,412 LLC-loads                 #    0.296 M/sec                   [39.76%]
>            566,546 LLC-load-misses           #   64.13% of all LL-cache hits    [39.98%]
patched:
>            556,751 LLC-loads                 #    0.186 M/sec                   [39.86%]
>            339,106 LLC-load-misses           #   60.91% of all LL-cache hits    [39.72%]

My best guess is that most of the LLC references are going out and
touching the struct pages for slab management.  It's why we see such a
big change.
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