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

Igor Mammedov
2015-06-17T07:34:12Z

On Wed, 17 Jun 2015 08:31:23 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> 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@redhat.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?
I hasn't been able to measure immediate delta between function
start/end with precision more than 10ns, perhaps used method
(system tap) is to blame.
But it's still possible to measure indirectly like 2% from 5/5.

>
> > 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.
ok, will do.

>
> > > > 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