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

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Message-ID: <50067009.9030508@redhat.com>
Date:	Wed, 18 Jul 2012 16:12:57 +0800
From:	Asias He <asias@...hat.com>
To:	Stefan Hajnoczi <stefanha@...il.com>
CC:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	linux-aio@...ck.org, kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	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 07/17/2012 07:11 PM, 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?

OK. I know that batching in io_submit() and io_getevetns(). There was a 
patch for kvm tool long time ago. Now, both vhost-blk and kvm tool are 
not taking advantage of that atm. There are issues: e.g. How many number 
of request we want to batch? Does this batching hurt latency?

-- 
Asias


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