linux-kernel - Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

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Date:	Wed, 20 Jun 2012 12:46:43 +0800
From:	Asias He <asias@...hat.com>
To:	dlaor@...hat.com
CC:	Sasha Levin <levinsasha928@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>, kvm@...r.kernel.org,
	"Michael S. Tsirkin" <mst@...hat.com>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3/3] virtio-blk: Add bio-based IO path for virtio-blk

On 06/19/2012 02:21 PM, Dor Laor wrote:
> On 06/19/2012 05:51 AM, Asias He wrote:
>> On 06/18/2012 07:39 PM, Sasha Levin wrote:
>>> On Mon, 2012-06-18 at 14:14 +0300, Dor Laor wrote:
>>>> On 06/18/2012 01:05 PM, Rusty Russell wrote:
>>>>> On Mon, 18 Jun 2012 16:03:23 +0800, Asias He<asias@...hat.com> wrote:
>>>>>> On 06/18/2012 03:46 PM, Rusty Russell wrote:
>>>>>>> On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@...hat.com>
>>>>>>> wrote:
>>>>>>>> This patch introduces bio-based IO path for virtio-blk.
>>>>>>>
>>>>>>> Why make it optional?
>>>>>>
>>>>>> request-based IO path is useful for users who do not want to bypass
>>>>>> the
>>>>>> IO scheduler in guest kernel, e.g. users using spinning disk. For
>>>>>> users
>>>>>> using fast disk device, e.g. SSD device, they can use bio-based IO
>>>>>> path.
>>>>>
>>>>> Users using a spinning disk still get IO scheduling in the host
>>>>> though.
>>>>> What benefit is there in doing it in the guest as well?
>>>>
>>>> The io scheduler waits for requests to merge and thus batch IOs
>>>> together. It's not important w.r.t spinning disks since the host can do
>>>> it but it causes much less vmexits which is the key issue for VMs.
>>>
>>> Is the amount of exits caused by virtio-blk significant at all with
>>> EVENT_IDX?
>>
>> Yes. EVENT_IDX saves the number of notify and interrupt. Let's take the
>> interrupt as an example, The guest fires 200K request to host, the
>> number of interrupt is about 6K thanks to EVENT_IDX. The ratio is 200K /
>> 6K = 33. The ratio of merging is 40000K / 200K = 20.
>>
>
> In this case, why don't you always recommend bio over request based?

This case shows that IO scheduler's merging in guest saves a lot of 
requests to host side. Why should I recommend bio over request based here?

-- 
Asias


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