linux-kernel - Re: [PATCH] vhost-net: utilize PUBLISH_USED

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Message-ID: <20100519222718.GB4111@redhat.com>
Date:	Thu, 20 May 2010 01:27:18 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	davem@...emloft.net, Juan Quintela <quintela@...hat.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Arnd Bergmann <arnd@...db.de>, kvm@...r.kernel.org,
	virtualization@...ts.osdl.org, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org, alex.williamson@...hat.com,
	amit.shah@...hat.com
Subject: Re: [PATCH] vhost-net: utilize PUBLISH_USED_IDX feature

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@...hat.com>
>> ---
>>
>> 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?

> 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.

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