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

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Date:	Mon, 02 Jul 2012 09:24:27 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	Asias He <asias@...hat.com>, Sasha Levin <levinsasha928@...il.com>
Cc:	dlaor@...hat.com, 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 Tue, 19 Jun 2012 10:51:18 +0800, Asias He <asias@...hat.com> 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.

Confused.  So, without merging we get 6k exits (per second?)  How many
do we get when we use the request-based IO path?

If your device is slow, then you won't be able to make many requests per
second: why worry about exit costs?  If your device is fast (eg. ram),
you've already shown that your patch is a win, right?

Cheers,
Rusty.
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