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Date: Tue, 18 May 2010 11:29:17 -0500
From: Anthony Liguori <anthony@codemonkey.ws>
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Subject: Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request
	structure
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To: Alexander Graf <agraf@suse.de>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

On 05/18/2010 11:26 AM, Alexander Graf wrote:
> Jes Sorensen wrote:
>    
>> On 05/14/10 23:52, Stefan Hajnoczi wrote:
>>
>>      
>>> The VirtIOBlockRequest structure is about 40 KB in size.  This patch
>>> avoids zeroing every request by only initializing fields that are read.
>>> The other fields are either written to or may not be used at all.
>>>
>>> Oprofile shows about 10% of CPU samples in memset called by
>>> virtio_blk_alloc_request().  The workload is
>>> dd if=/dev/vda of=/dev/null iflag=direct bs=8k running concurrently 4
>>> times.  This patch makes memset disappear to the bottom of the profile.
>>>
>>> Signed-off-by: Stefan Hajnoczi<stefanha@linux.vnet.ibm.com>
>>>
>>>        
>> Great catch!
>>
>> I ran some benchmarks using a ramdisk passed to the guest as a virtio
>> device and with this patch I saw improvements ranging from 5-20%. I
>> believe the fluctuations are due to not being able to numa bind it due
>> to limited memory.
>>
>> However a win all the way round!
>>
>>      
> It looks like a fairly small change with a huge win. Sounds like a
> perfect candidate for 0.12.5 to me.
>    

I'd prefer to stick to bug fixes for stable releases.  Performance 
improvements are a good motivation for people to upgrade to 0.13 :-)

Regards,

Anthony Liguori

> Alex
>
>
>