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Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself |
Date: |
Thu, 20 May 2010 01:33:12 +0300 |
User-agent: |
Mutt/1.5.19 (2009-01-05) |
On Wed, May 19, 2010 at 11:06:42AM +0300, Avi Kivity wrote:
> On 05/19/2010 10:39 AM, Rusty Russell wrote:
>>
>> I think we're talking about the last 2 entries of the avail ring. That means
>> the worst case is 1 false bounce every time around the ring.
>
> It's low, but why introduce an inefficiency when you can avoid doing it
> for the same effort?
>
>> I think that's
>> why we're debating it instead of measuring it :)
>>
>
> Measure before optimize is good for code but not for protocols.
> Protocols have to be robust against future changes. Virtio is warty
> enough already, we can't keep doing local optimizations.
>
>> Note that this is a exclusive->shared->exclusive bounce only, too.
>>
>
> A bounce is a bounce.
>
> Virtio is already way too bouncy due to the indirection between the
> avail/used rings and the descriptor pool. A device with out of order
> completion (like virtio-blk) will quickly randomize the unused
> descriptor indexes, so every descriptor fetch will require a bounce.
>
> In contrast, if the rings hold the descriptors themselves instead of
> pointers, we bounce (sizeof(descriptor)/cache_line_size) cache lines for
> every descriptor, amortized.
On the other hand, consider that on fast path we are never using all
of the ring. With a good allocator we might be able to keep
reusing only small part of the ring, instead of wrapping around
all of it all of the time.
> --
> Do not meddle in the internals of kernels, for they are subtle and quick to
> panic.
- [Qemu-devel] Re: [PATCH RFC] virtio: put last seen used index into ring itself, (continued)
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/06
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Rusty Russell, 2010/05/06
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/11
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Michael S. Tsirkin, 2010/05/11
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Rusty Russell, 2010/05/19
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/19
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/20
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Rusty Russell, 2010/05/20
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Rusty Russell, 2010/05/20
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Michael S. Tsirkin, 2010/05/23
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/23
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Michael S. Tsirkin, 2010/05/23
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/23
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Michael S. Tsirkin, 2010/05/23
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Avi Kivity, 2010/05/24
- Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself, Michael S. Tsirkin, 2010/05/24