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

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Message-ID: <4BF4D15E.4040908@redhat.com>
Date:	Thu, 20 May 2010 09:06:22 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...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 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@...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?
>    

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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