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

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Date:	Tue, 17 Jul 2012 12:42:13 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	"Michael S. Tsirkin" <mst@...hat.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 12:26 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?

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

Stefan
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