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

[Dor Laor]

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@xxxxxxxxxx> wrote:
> > > > > > > On 06/18/2012 03:46 PM, Rusty Russell wrote:
> > > > > > > > On Mon, 18 Jun 2012 14:53:10 +0800, Asias He<asias@xxxxxxxxxx>
> > > > > > > > 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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