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

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 10 Jan 2014 12:52:32 -0800 Dave Hansen <dave@sr71.net> wrote:

> 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@sr71.net> 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@sr71.net
> >>
> >> 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

So we're converging on the most complex option.  argh.

> 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%.

So all this testing was performed in a VM?  If so, how much is that
likely to have impacted the results?

>  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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