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

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Message-ID: <5006CC26.20203@redhat.com>
Date:	Wed, 18 Jul 2012 22:45:58 +0800
From:	Asias He <asias@...hat.com>
To:	Jeff Moyer <jmoyer@...hat.com>
CC:	linux-kernel@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>, kvm@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RESEND 5/5] vhost-blk: Add vhost-blk support

On 07/18/2012 10:31 PM, Jeff Moyer wrote:
> Asias He <asias@...hat.com> writes:
>
>> On 07/18/2012 03:10 AM, Jeff Moyer wrote:
>>> Asias He <asias@...hat.com> writes:
>>>
>>>> vhost-blk is a in kernel virito-blk device accelerator.
>>>>
>>>> This patch is based on Liu Yuan's implementation with various
>>>> improvements and bug fixes. Notably, this patch makes guest notify and
>>>> host completion processing in parallel which gives about 60% performance
>>>> improvement compared to Liu Yuan's implementation.
>>>
>>> So, first off, some basic questions.  Is it correct to assume that you
>>> tested this with buffered I/O (files opened *without* O_DIRECT)?
>>>   I'm pretty sure that if you used O_DIRECT, you'd run into problems (which
>>> are solved by the patch set posted by Shaggy, based on Zach Brown's work
>>> of many moons ago).  Note that, with buffered I/O, the submission path
>>> is NOT asynchronous.  So, any speedups you've reported are extremely
>>> suspect.  ;-)
>>
>> I always used O_DIRECT to test this patchset. And I mostly used raw
>> block device as guest image. Is this the reason why I did not hit the
>> problem you mentioned. Btw, I do have run this patchset on image based
>> file. I still do not see problems like IO hangs.
>
> Hmm, so do the iovec's passed in point to buffers in userspace?  I
> thought they were kernel buffers, which would have blown up in
> get_user_pages_fast.

Yes. The iovec's passed in point to userspace buffers. ;-)

-- 
Asias


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