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

On Wed, Sep 10, 2014 at 11:18 PM, Paolo Bonzini [off-list ref] wrote:

Il 07/09/2014 12:32, Ming Lei ha scritto:

quoted

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.

quoted

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

quoted

From my test(virtio-blk over null_blk), looks enabling SG merge may cause
throughput a little drop(~3%).

Thanks,
-- 
Ming Lei