linux-kernel - Re: [PATCH 0/5] vhost: support upto 509 memory regions

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Date:	Wed, 17 Jun 2015 08:31:23 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Igor Mammedov <imammedo@...hat.com>
Cc:	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	pbonzini@...hat.com
Subject: Re: [PATCH 0/5] vhost: support upto 509 memory regions

On Wed, Jun 17, 2015 at 12:19:15AM +0200, Igor Mammedov wrote:
> On Tue, 16 Jun 2015 23:16:07 +0200
> "Michael S. Tsirkin" <mst@...hat.com> wrote:
> 
> > On Tue, Jun 16, 2015 at 06:33:34PM +0200, Igor Mammedov wrote:
> > > Series extends vhost to support upto 509 memory regions,
> > > and adds some vhost:translate_desc() performance improvemnts
> > > so it won't regress when memslots are increased to 509.
> > > 
> > > It fixes running VM crashing during memory hotplug due
> > > to vhost refusing accepting more than 64 memory regions.
> > > 
> > > It's only host kernel side fix to make it work with QEMU
> > > versions that support memory hotplug. But I'll continue
> > > to work on QEMU side solution to reduce amount of memory
> > > regions to make things even better.
> > 
> > I'm concerned userspace work will be harder, in particular,
> > performance gains will be harder to measure.
> it appears so, so far.
> 
> > How about a flag to disable caching?
> I've tried to measure cost of cache miss but without much luck,
> difference between version with cache and with caching removed
> was within margin of error (±10ns) (i.e. not mensurable on my
> 5min/10*10^6 test workload).

Confused. I thought it was very much measureable.
So why add a cache if you can't measure its effect?

> Also I'm concerned about adding extra fetch+branch for flag
> checking will make things worse for likely path of cache hit,
> so I'd avoid it if possible.
> 
> Or do you mean a simple global per module flag to disable it and
> wrap thing in static key so that it will be cheap jump to skip
> cache?

Something like this, yes.

> > > Performance wise for guest with (in my case 3 memory regions)
> > > and netperf's UDP_RR workload translate_desc() execution
> > > time from total workload takes:
> > > 
> > > Memory      |1G RAM|cached|non cached
> > > regions #   |  3   |  53  |  53
> > > ------------------------------------
> > > upstream    | 0.3% |  -   | 3.5%
> > > ------------------------------------
> > > this series | 0.2% | 0.5% | 0.7%
> > > 
> > > where "non cached" column reflects trashing wokload
> > > with constant cache miss. More details on timing in
> > > respective patches.
> > > 
> > > Igor Mammedov (5):
> > >   vhost: use binary search instead of linear in find_region()
> > >   vhost: extend memory regions allocation to vmalloc
> > >   vhost: support upto 509 memory regions
> > >   vhost: add per VQ memory region caching
> > >   vhost: translate_desc: optimization for desc.len < region size
> > > 
> > >  drivers/vhost/vhost.c | 95
> > > +++++++++++++++++++++++++++++++++++++--------------
> > > drivers/vhost/vhost.h |  1 + 2 files changed, 71 insertions(+), 25
> > > deletions(-)
> > > 
> > > -- 
> > > 1.8.3.1
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