Re: [PATCH v4 3/3] mm/free_pcppages_bulk: prefetch buddy while not holding lock

[Aaron Lu]

On Thu, Mar 01, 2018 at 03:00:44PM +0100, Michal Hocko wrote:
> On Thu 01-03-18 14:28:45, Aaron Lu wrote:
> > When a page is freed back to the global pool, its buddy will be checked
> > to see if it's possible to do a merge. This requires accessing buddy's
> > page structure and that access could take a long time if it's cache cold.
> > 
> > This patch adds a prefetch to the to-be-freed page's buddy outside of
> > zone->lock in hope of accessing buddy's page structure later under
> > zone->lock will be faster. Since we *always* do buddy merging and check
> > an order-0 page's buddy to try to merge it when it goes into the main
> > allocator, the cacheline will always come in, i.e. the prefetched data
> > will never be unused.
> > 
> > In the meantime, there are two concerns:
> > 1 the prefetch could potentially evict existing cachelines, especially
> >   for L1D cache since it is not huge;
> > 2 there is some additional instruction overhead, namely calculating
> >   buddy pfn twice.
> > 
> > For 1, it's hard to say, this microbenchmark though shows good result but
> > the actual benefit of this patch will be workload/CPU dependant;
> > For 2, since the calculation is a XOR on two local variables, it's expected
> > in many cases that cycles spent will be offset by reduced memory latency
> > later. This is especially true for NUMA machines where multiple CPUs are
> > contending on zone->lock and the most time consuming part under zone->lock
> > is the wait of 'struct page' cacheline of the to-be-freed pages and their
> > buddies.
> > 
> > Test with will-it-scale/page_fault1 full load:
> > 
> > kernel      Broadwell(2S)  Skylake(2S)   Broadwell(4S)  Skylake(4S)
> > v4.16-rc2+  9034215        7971818       13667135       15677465
> > patch2/3    9536374 +5.6%  8314710 +4.3% 14070408 +3.0% 16675866 +6.4%
> > this patch 10338868 +8.4%  8544477 +2.8% 14839808 +5.5% 17155464 +2.9%
> > Note: this patch's performance improvement percent is against patch2/3.
> 
> I am really surprised that this has such a big impact.  Is this a win on
> other architectures as well?

For NUMA machines, I guess so. But I didn't test other archs so can't
say for sure.

>  
> > [changelog stole from Dave Hansen and Mel Gorman's comments]
> > https://lkml.org/lkml/2018/1/24/551
> 
> Please use http://lkml.kernel.org/r/<msg-id> for references because
> lkml.org is quite unstable. It would be
> http://lkml.kernel.org/r/148a42d8-8306-2f2f-7f7c-86bc118f8ccd@xxxxxxxxx
> here.

Good to know this, thanks!