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

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Date:	Thu, 21 Jun 2012 12:49:12 +0300
From:	Dor Laor <dlaor@...hat.com>
To:	Asias He <asias@...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 06/20/2012 07:46 AM, Asias He wrote:
> 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?
>

Does it merge 20 request _on top_ of what event idx does? Of course if 
that's the case, we should keep that.

Dor

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