Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Benoît Canet]

The Monday 21 Jul 2014 à 08:59:45 (-0600), Chris Friesen wrote :
> On 07/19/2014 02:45 AM, Benoît Canet wrote:
> 
> >I think in the throttling case the number of in flight operation is limited 
> >by
> >the emulated hardware queue. Else request would pile up and throttling would 
> >be
> >inefective.
> >
> >So this number should be around: #define VIRTIO_PCI_QUEUE_MAX 64 or 
> >something like than that.
> 
> Okay, that makes sense.  Do you know how much data can be written as part of
> a single operation?  We're using 2MB hugepages for the guest memory, and we
> saw the qemu RSS numbers jump from 25-30MB during normal operation up to
> 120-180MB when running dbench.  I'd like to know what the worst-case would
> be.

I think Linux as a limit of 512Kb for io size or something like that.
So the guest would have VIRTIO_PCI_QUEUE_MAX * 512Kb of in flight buffers at 
max.

> 
> Thanks,
> Chris