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Date: Tue, 18 May 2010 18:31:04 +0200
From: Alexander Graf <agraf@suse.de>
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Subject: Re: [Qemu-devel] [PATCH] virtio-blk: Avoid zeroing every request
	structure
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Jes Sorensen <Jes.Sorensen@redhat.com>, Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>, qemu-devel@nongnu.org

Anthony Liguori wrote:
> 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 :-)

In general I agree, but this one looks like a really simple one.

Alex