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

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Message-ID: <54106BBF.7010907@redhat.com>
Date:	Wed, 10 Sep 2014 17:18:23 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Ming Lei <tom.leiming@...il.com>, Christoph Hellwig <hch@....de>
CC:	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

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.

> 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%).

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