Re: [PATCH] vhost-net: utilize PUBLISH_USED_IDX feature

From: Avi Kivity
Date: Thu May 20 2010 - 02:06:43 EST

On 05/20/2010 01:27 AM, Michael S. Tsirkin wrote:
On Wed, May 19, 2010 at 08:04:51PM +0300, Avi Kivity wrote:
On 05/18/2010 04:19 AM, Michael S. Tsirkin wrote:
With PUBLISH_USED_IDX, guest tells us which used entries
it has consumed. This can be used to reduce the number
of interrupts: after we write a used entry, if the guest has not yet
consumed the previous entry, or if the guest has already consumed the
new entry, we do not need to interrupt.
This imporves bandwidth by 30% under some workflows.

Signed-off-by: Michael S. Tsirkin<mst@xxxxxxxxxx>

Rusty, Dave, this patch depends on the patch
"virtio: put last seen used index into ring itself"
which is currently destined at Rusty's tree.
Rusty, if you are taking that one for 2.6.35, please
take this one as well.
Dave, any objections?

I object: I think the index should have its own cacheline,
The issue here is that host/guest do not know each
other's cache line size. I guess we could just put it
at offset 128 or something like that ... Rusty?

Not so pretty, but ok.

and that it should be documented before merging.
I think you meant to object to the virtio patch, not this one. This
patch does not introduce new layout, just implements host support.
virtio spec patch will follow: it is not part of linux tree so
there is no patch dependency.

There is a reviewer dependency.

Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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