Re: [PATCH 2/4] mm: Send one IPI per CPU to TLB flush all entries after unmapping pages

Mel Gorman
2015-06-10T09:58:41Z

On Wed, Jun 10, 2015 at 10:26:40AM +0200, Ingo Molnar wrote:
>
> * Mel Gorman <mgorman@suse.de> wrote:
>
> > On a 4-socket machine the results were
> >
> > 4.1.0-rc6 4.1.0-rc6
> > batchdirty-v6 batchunmap-v6
> > Ops lru-file-mmap-read-elapsed 121.27 ( 0.00%) 118.79 ( 2.05%)
> >
> > 4.1.0-rc6 4.1.0-rc6
> > batchdirty-v6 batchunmap-v6
> > User 620.84 608.48
> > System 4245.35 4152.89
> > Elapsed 122.65 120.15
> >
> > In this case the workload completed faster and there was less CPU overhead
> > but as it's a NUMA machine there are a lot of factors at play. It's easier
> > to quantify on a single socket machine;
> >
> > 4.1.0-rc6 4.1.0-rc6
> > batchdirty-v6 batchunmap-v6
> > Ops lru-file-mmap-read-elapsed 20.35 ( 0.00%) 21.52 ( -5.75%)
> >
> > 4.1.0-rc6 4.1.0-rc6
> > batchdirty-v6r5batchunmap-v6r5
> > User 58.02 60.70
> > System 77.57 81.92
> > Elapsed 22.14 23.16
> >
> > That shows the workload takes 5.75% longer to complete with a similar
> > increase in the system CPU usage.
>
> Btw., do you have any stddev noise numbers?
>

4.1.0-rc6 4.1.0-rc6 4.1.0-rc6 4.1.0-rc6
vanilla flushfull-v6r5 batchdirty-v6r5 batchunmap-v6r5
Ops lru-file-mmap-read-elapsed 25.43 ( 0.00%) 20.59 ( 19.03%) 20.35 ( 19.98%) 21.52 ( 15.38%)
Ops lru-file-mmap-read-time_stddv 0.32 ( 0.00%) 0.32 ( -1.30%) 0.39 (-23.00%) 0.45 (-40.91%)

flushfull -- patch 2
batchdirty -- patch 3
batchunmap -- patch 4

So the impact of tracking the PFNs is outside the noise and there is
definite direct cost to it. This was expected for both the PFN tracking
and the individual flushes.

> The batching speedup is brutal enough to not need any noise estimations, it's a
> clear winner.
>

Agreed.

> But this PFN tracking patch is more difficult to judge as the numbers are pretty
> close to each other.
>

It's definitely measurable, no doubt about it and there never was. The
concerns were always the refill costs due to flushing potentially active
TLB entries unnecessarily. From https://lkml.org/lkml/2014/7/31/825, this
is potentially high where it says that a 512 DTLB refill takes 22,000
cycles which is higher than the individual flushes. However, this is an
estimate and it'll always be a case of "it depends". It's been asserted
that the refill costs are really low so lets just go with that, drop
patch 4 and wait and see who complains.

--
Mel Gorman
SUSE Labs