linux-kernel - Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into ring itself

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Message-ID: <4BF4D0F5.7050003@redhat.com>
Date:	Thu, 20 May 2010 09:04:37 +0300
From:	Avi Kivity <avi@...hat.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
	qemu-devel@...gnu.org
Subject: Re: [Qemu-devel] [PATCH RFC] virtio: put last seen used index into
 ring itself

On 05/20/2010 01:33 AM, Michael S. Tsirkin wrote:
>
>> 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.
>    

It's still suboptimal, we have to bounce both the avail/used rings and 
the descriptor pool, compared to just the descriptor ring with a direct 
design.  Plus we don't need a fancy allocator.

When amortizing cachelines, simple data structures win.

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

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