Re: Virtio Block Device Queue Depth

[Vadim Rozenfeld <vrozenfe@redhat.com>]

On Friday, March 09, 2012 11:56:36 AM you wrote:
> On Thu, Mar 8, 2012 at 5:48 PM, George Bottas <gbottas@juniper.net> wrote:
> > I have a question regarding changing the queue size that is set in
> > virtio_blk_init(). The current value is 128, which results in setting
> > the queue depth in the Windows guest device to 8. Does anyone know if
Actually, it's 6 128/(16+1) - 1
> > changing this value to defined maximum (1024) going to result in any
In my experience, enalarging the queueu size doen't leas to any 
significant performance improvement 
> > issues? NB, we have also increased the thread pool accordingly. Our
> > testing so far has produced a situation where initially we are seeing
> > very high read request rates (1500+), and then at some point the reads
> > from the Windows guest become serialized, i.e. the Windows guest
> > synchronously sends one READ to the host, waits for it to complete,
> > sends the next one, etc.  Has this ever been seen anywhere else?
On Win2K3 and higher, viostor operats in full-duplex mode. Which means
that it must be able to process several requests simulteniousle. But
if you have 6 requests pending (processing), storport driver will throttle
incomming requestss. 
> > 
> > Any help on this would be appreciated.
> 
> If you are really limited to 8 requests then the issue is not the
> number of vring descriptors since recent virtio-blk has the indirect
> vring feature enabled.  The indirect vring feature allows the guest to
> send 128 parallel requests - much higher than the 8 you mentioned from
> Windows.
It's true. However, I cannot say that indirect buffers will automaticaly
give you a better performance. Inderect buffers should work faster for
peretty big data chanks (128K..256K), while for the normal load (4K..64K)
they give you almost the same numbers. 
Vadim.
> 
> CCing Vadim who maintains the Windows virtio-blk driver.
> 
> Stefan