[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
[Qemu-devel] Re: Re: [RFC][PATCH] performance improvement for windows gu
From: |
Michael S. Tsirkin |
Subject: |
[Qemu-devel] Re: Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device |
Date: |
Mon, 11 Jan 2010 20:20:48 +0200 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Mon, Jan 11, 2010 at 08:37:10AM -0600, Anthony Liguori wrote:
> On 01/11/2010 08:29 AM, Avi Kivity wrote:
>> On 01/11/2010 03:49 PM, Anthony Liguori wrote:
>>>> So instead of disabling notify while requests are active we might want
>>>> to only disable it while we are inside virtio_blk_handle_output.
>>>> Something like the following minimally tested patch:
>>>
>>>
>>> I'd suggest that we get even more aggressive and install an idle
>>> bottom half that checks the queue for newly submitted requests. If
>>> we keep getting requests submitted before a new one completes, we'll
>>> never take an I/O exit.
>>>
>>
>> That has the downside of bouncing a cache line on unrelated exits.
>
> The read and write sides of the ring are widely separated in physical
> memory specifically to avoid cache line bouncing.
>
>> It probably doesn't matter with qemu as it is now, since it will
>> bounce qemu_mutex, but it will hurt with large guests (especially if
>> they have many rings).
>>
>> IMO we should get things to work well without riding on unrelated
>> exits, especially as we're trying to reduce those exits.
>
> A block I/O request can potentially be very, very long lived. By
> serializing requests like this, there's a high likelihood that it's
> going to kill performance with anything capable of processing multiple
> requests.
>
> OTOH, if we aggressively poll the ring when we have an opportunity to,
> there's very little down side to that and it addresses the serialization
> problem.
>
>>> The same approach is probably a good idea for virtio-net.
>>
>> With vhost-net you don't see exits.
>
> The point is, when we've disabled notification, we should poll on the
> ring for additional requests instead of waiting for one to complete
> before looking at another one.
>
> Even with vhost-net, this logic is still applicable although potentially
> harder to achieve.
>
> Regards,
>
> Anthony Liguori
>
vhost net does this already: it has a mode where it poll when skbs have
left send queue: for tap this is when they have crossed the bridge, for packet
socket this is when they have been transmitted.
--
MST
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, (continued)
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Anthony Liguori, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Avi Kivity, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Anthony Liguori, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Avi Kivity, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Anthony Liguori, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Avi Kivity, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Anthony Liguori, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Avi Kivity, 2010/01/11
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Anthony Liguori, 2010/01/11
- [Qemu-devel] Re: Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Michael S. Tsirkin, 2010/01/11
- [Qemu-devel] Re: Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device,
Michael S. Tsirkin <=
- Re: [Qemu-devel] Re: [RFC][PATCH] performance improvement for windows guests, running on top of virtio block device, Avi Kivity, 2010/01/11