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SubjectRe: [PATCH 3/4] mm, page_allocator: Only use per-cpu allocator for irq-safe requests
On Wed, Jan 11, 2017 at 02:27:12PM +0100, Jesper Dangaard Brouer wrote:
> On Wed, 11 Jan 2017 13:44:20 +0100
> Jesper Dangaard Brouer <brouer@redhat.com> wrote:
>
> > On Mon, 9 Jan 2017 16:35:17 +0000 Mel Gorman <mgorman@techsingularity.net> wrote:
> >
> > > The following is results from a page allocator micro-benchmark. Only
> > > order-0 is interesting as higher orders do not use the per-cpu allocator
> >
> > Micro-benchmarked with [1] page_bench02:
> > modprobe page_bench02 page_order=0 run_flags=$((2#010)) loops=$((10**8)); \
> > rmmod page_bench02 ; dmesg --notime | tail -n 4
> >
> > Compared to baseline: 213 cycles(tsc) 53.417 ns
> > - against this : 184 cycles(tsc) 46.056 ns
> > - Saving : -29 cycles
> > - Very close to expected 27 cycles saving [see below [2]]
>
> When perf benchmarking I noticed that the "summed" children perf
> overhead from calling alloc_pages_current() is 65.05%. Compared to
> "free-path" of summed 28.28% of calls "under" __free_pages().
>
> This is caused by CONFIG_NUMA=y, as call path is long with NUMA
> (and other helpers are also non-inlined calls):
>
> alloc_pages
> -> alloc_pages_current
> -> __alloc_pages_nodemask
> -> get_page_from_freelist
>
> Without NUMA the call levels gets compacted by inlining to:
>
> __alloc_pages_nodemask
> -> get_page_from_freelist
>
> After disabling NUMA, the split between alloc(48.80%) vs. free(42.67%)
> side is more balanced.
>
> Saving by disabling CONFIG_NUMA of:
> - CONFIG_NUMA=y : 184 cycles(tsc) 46.056 ns
> - CONFIG_NUMA=n : 143 cycles(tsc) 35.913 ns
> - Saving: : 41 cycles (approx 22%)
>
> I would conclude, there is room for improvements with CONFIG_NUMA code
> path case. Lets followup on that in a later patch series...
>

Potentially. The NUMA paths do memory policy work and has more
complexity in the statistics path. It may be possible to side-step some
of it. There were not many safe options when I last looked but that was
a long time ago. Most of the focus has been on the core allocator
itself and not the wrappers around it.

--
Mel Gorman
SUSE Labs

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