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

[Igor Mammedov]

On Tue, 16 Jun 2015 23:16:07 +0200
"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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 (Â10ns) (i.e. not mensurable on my
5min/10*10^6 test workload).
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?
 
> > 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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