Linux Virtualization - Sep 2014 - [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%).

On Wed, Sep 10, 2014 at 11:18 PM, Paolo Bonzini <pbonzini at redhat.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