Messages in this thread | | | Date | Fri, 29 Oct 2010 11:25:41 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 1/2] mm: page allocator: Adjust the per-cpu counter threshold when memory is low |
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On Fri, 29 Oct 2010 09:58:25 -0500 (CDT) Christoph Lameter <cl@linux.com> wrote:
> On Thu, 28 Oct 2010, Andrew Morton wrote: > > > > To ensure that kswapd wakes up, a safe version of zone_watermark_ok() > > > is introduced that takes a more accurate reading of NR_FREE_PAGES when > > > called from wakeup_kswapd, when deciding whether it is really safe to go > > > back to sleep in sleeping_prematurely() and when deciding if a zone is > > > really balanced or not in balance_pgdat(). We are still using an expensive > > > function but limiting how often it is called. > > > > Here I go again. I have a feeling that I already said this, but I > > can't find versions 2 or 3 in the archives.. > > > > Did you evaluate using plain on percpu_counters for this? They won't > > solve the performance problem as they're basically the same thing as > > these open-coded counters. But they'd reduce the amount of noise and > > custom-coded boilerplate in mm/. > > The zone counters are done using the ZVCs in vmstat.c to save space
well, they actually waste space because of that threshold thing.
> and to > be in the same cacheline as other hot data necessary for allocation and > free.
Yes, that'll save some misses.
> > > > > + threshold = max(1, (int)(watermark_distance / num_online_cpus())); > > > + > > > + /* > > > + * Maximum threshold is 125 > > > > Reasoning? > > Differentials are stored in 8 bit signed ints. > > > > + put_online_cpus(); > > > +} > > > > Given that ->stat_threshold is the same for each CPU, why store it for > > each CPU at all? Why not put it in the zone and eliminate the inner > > loop? > > Doing that caused cache misses in the past and reduced the performance of > the ZVCs. This way the threshold is in the same cacheline as the > differentials.
This sounds wrong. As long as that threshold isn't stored in a cacheline which other CPUs are modifying, all CPUs should be able to happily cache it. Maybe it needed a bit of padding inside the zone struct.
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