linux-kernel - Re: [PATCH 0/5] Add vhost-blk support

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Message-ID: <20120717115450.GA9796@redhat.com>
Date:	Tue, 17 Jul 2012 14:54:50 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Stefan Hajnoczi <stefanha@...il.com>
Cc:	Asias He <asias@...hat.com>, Paolo Bonzini <pbonzini@...hat.com>,
	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
	Benjamin LaHaise <bcrl@...ck.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 0/5] Add vhost-blk support

On Tue, Jul 17, 2012 at 12:42:13PM +0100, Stefan Hajnoczi wrote:
> On Tue, Jul 17, 2012 at 12:26 PM, Michael S. Tsirkin <mst@...hat.com> wrote:
> > On Tue, Jul 17, 2012 at 12:11:15PM +0100, Stefan Hajnoczi wrote:
> >> On Tue, Jul 17, 2012 at 10:21 AM, Asias He <asias@...hat.com> wrote:
> >> > On 07/17/2012 04:52 PM, Paolo Bonzini wrote:
> >> >>
> >> >> Il 17/07/2012 10:29, Asias He ha scritto:
> >> >>>
> >> >>> So, vhost-blk at least saves ~6 syscalls for us in each request.
> >> >>
> >> >>
> >> >> Are they really 6?  If I/O is coalesced by a factor of 3, for example
> >> >> (i.e. each exit processes 3 requests), it's really 2 syscalls per request.
> >> >
> >> >
> >> > Well. I am counting the number of syscalls in one notify and response
> >> > process. Sure the IO can be coalesced.
> >>
> >> Linux AIO also supports batching in io_submit() and io_getevents().
> >> Depending on the request pattern in the vring when you process it, you
> >> should be able to do better than 1 set of syscalls per host I/O
> >> request.
> >>
> >> Are you taking advantage of that at the moment in your userspace benchmark?
> >>
> >> Stefan
> >
> > Injecting an interrupt directly from kernel bypasses two context switches.
> > Yes some worloads can coalesce interrupts efficiently but others can't.
> > It is not really hard to speculate more.
> >
> > Personally I don't understand where all this speculation leads us.
> > Are you guys disputing the measurements posted? If not would not
> > it be better if discussion focused on the amount of extra code versus
> > measured gain?
> 
> 5-15% is nice.  But what causes the performance advantage?

Well, check the number of interrupts. If it's high then that is
part of it.

> Knowing the answer to that is important before anyone can say whether
> this approach is good or not.
> 
> Stefan

Why is it?

-- 
MST
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