linux-kernel - Re: [RFC][PATCH 9/9] make kvm mmu shrinker more aggressive

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Message-Id: <1276876156.6437.23323.camel@nimitz>
Date:	Fri, 18 Jun 2010 08:49:16 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC][PATCH 9/9] make kvm mmu shrinker more aggressive

On Wed, 2010-06-16 at 08:25 -0700, Dave Hansen wrote:
> On Wed, 2010-06-16 at 12:24 +0300, Avi Kivity wrote:
> > On 06/15/2010 04:55 PM, Dave Hansen wrote:
> > > In a previous patch, we removed the 'nr_to_scan' tracking.
> > > It was not being used to track the number of objects
> > > scanned, so we stopped using it entirely.  Here, we
> > > strart using it again.
> > >
> > > The theory here is simple; if we already have the refcount
> > > and the kvm->mmu_lock, then we should do as much work as
> > > possible under the lock.  The downside is that we're less
> > > fair about the KVM instances from which we reclaim.  Each
> > > call to mmu_shrink() will tend to "pick on" one instance,
> > > after which it gets moved to the end of the list and left
> > > alone for a while.
> > >    
> > 
> > That also increases the latency hit, as well as a potential fault storm, 
> > on that instance.  Spreading out is less efficient, but smoother.
> 
> This is probably something that we need to go back and actually measure.
> My suspicion is that, when memory fills up and this shrinker is getting
> called a lot, it will be naturally fair.  That list gets shuffled around
> enough, and mmu_shrink() called often enough that no VMs get picked on
> too unfairly.
> 
> I'll go back and see if I can quantify this a bit, though.

The shrink _query_ (mmu_shrink() with nr_to_scan=0) code is called
really, really often.  Like 5,000-10,000 times a second during lots of
VM pressure.  But, it's almost never called on to actually shrink
anything.

Over the 20 minutes or so that I tested, I saw about 700k calls to
mmu_shrink().  But, only 6 (yes, six) calls that had a non-zero
nr_to_scan.  I'm not sure whether this is because of the .seeks argument
to the shrinker or what, but the slab code stays far, far away from
making mmu_shrink() do much real work.

That changes a few things.  I bet all the contention we were seeing was
just from nr_to_scan=0 calls and not from actual shrink operations.
Perhaps we should just stop this set after patch 4.

Any thoughts?

-- Dave

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