linux-kernel - Re: [PATCH] virtio_blk: merge S/G list entries by default

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Date:	Wed, 10 Sep 2014 23:21:20 +0800
From:	Ming Lei <tom.leiming@...il.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	Linux Virtualization <virtualization@...ts.linux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH] virtio_blk: merge S/G list entries by default

On Wed, Sep 10, 2014 at 11:18 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> Il 07/09/2014 12:32, Ming Lei ha scritto:
>> It is a good idea to disable SG merge for vq incapable of indirect because
>> there are very limited direct descriptors.
>
> I think you mean _enabling_ SG merge if indirect descriptors are not there.

You are right, sorry for the typo.

>> For vq capable of indirect, it should be better to not do SG merge at default
>> because:
>>
>> - from hypervisor view, no matter how many segments one req has, all are
>> submitted to host kernel by one syscall, such as readv/io_submit
>>
>> - host kernel still need to do the same merge again
>
> Here we agree.
>
> Paolo
>
>> From my test(virtio-blk over null_blk), looks enabling SG merge may cause
>> throughput a little drop(~3%).
>


Thanks,
-- 
Ming Lei
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