[Qemu-devel] Re: [RFC] vhost-blk implementation

[Badari Pulavarty]

Michael S. Tsirkin wrote:
> On Mon, Mar 22, 2010 at 05:34:04PM -0700, Badari Pulavarty wrote:
>   
> > Write Results:
> > ==============
> >
> > I see degraded IO performance when doing sequential IO write
> > tests with vhost-blk compared to virtio-blk.
> >
> > # time dd of=/dev/vda if=/dev/zero bs=2M oflag=direct
> >
> > I get ~110MB/sec with virtio-blk, but I get only ~60MB/sec with
> > vhost-blk. Wondering why ?
> >     
>
> Try to look and number of interrupts and/or number of exits.
>   

I checked interrupts and IO exits - there is no major noticeable 
difference between
vhost-blk and virtio-blk scenerios.
> It could also be that you are overrunning some queue.
>
> I don't see any exit mitigation strategy in your patch:
> when there are already lots of requests in a queue, it's usually
> a good idea to disable notifications and poll the
> queue as requests complete. That could help performance.
>   
Do you mean poll eventfd for new requests instead of waiting for new 
notifications ?
Where do you do that in vhost-net code ?

Unlike network socket, since we are dealing with a file, there is no 
->poll support for it.
So I can't poll for the data. And also, Issue I am having is on the 
write() side.

I looked at it some more - I see 512K write requests on the virtio-queue 
in both
vhost-blk and virtio-blk cases. Both qemu or vhost is doing synchronous 
writes
to page cache (there is no write batching in qemu that is affecting this 
case).
I still puzzled on why virtio-blk outperforms vhost-blk.

Thanks,
Badari