Re: [PATCH] Reduce vm_stat cacheline contention in__vm_enough_memory

[Dimitri Sivanich]

On Fri, Oct 14, 2011 at 07:25:06AM -0500, Dimitri Sivanich wrote:
> On Thu, Oct 13, 2011 at 02:24:34PM -0700, Andrew Morton wrote:
> > On Thu, 13 Oct 2011 16:02:58 -0500 (CDT)
> > Christoph Lameter <cl@xxxxxxxxxx> wrote:
> > 
> > > On Thu, 13 Oct 2011, Andrew Morton wrote:
> > > 
> > > > > If there are no updates occurring for a while (due to increased deltas
> > > > > and/or vmstat updates) then the vm_stat cacheline should be able to stay
> > > > > in shared mode in multiple processors and the performance should increase.
> > > > >
> > > >
> > > > We could cacheline align vm_stat[].  But the thing is pretty small - we
> > > > couild put each entry in its own cacheline.
> > > 
> > > Which in turn would increase the cache footprint of some key kernel
> > > functions (because they need multiple vm_stat entries) and cause eviction
> > > of other cachelines that then reduce overall system performance again.
> > 
> > Sure, but we gain performance by not having different CPUs treading on
> > each other when they update different vmstat fields.  Sometimes one
> > effect will win and other times the other effect will win.  Some
> > engineering is needed..
> 
> I think the first step is to determine the role (if any) that false sharing may be playing in this, since that's a simpler fix (cacheline align and pad the array).
>

Testing on a smaller machine with 46 writer threads in parallel (my original
test used 120).

Looks as though cache-aligning and padding the end of the vm_stat array
results in a ~150 MB/sec speedup.  This is a nice improvement for only 46
writer threads, though it's not the full ~250 MB/sec speedup I get from
setting OVERCOMMIT_NEVER.
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