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

[Benoît Canet]

The Monday 21 Jul 2014 à 09:35:29 (-0600), Chris Friesen wrote :
> On 07/21/2014 09:15 AM, Benoît Canet wrote:
> >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.
> 
> Those numbers don't line up with what we were seeing...we saw a 120MB+ jump
> when running dbench, which would map to more like 2MB per operation assuming
> a limit of VIRTIO_PCI_QUEUE_MAX (i.e. 64).
> 
> Unless there are other buffers involved that we haven't factored in...

What VIRTIO_PCI_QUEUE_MAX would take into account would be the in flight 
requests between the guest and qemu.
What's left to study is if the Linux guest could have by himself some 
additional in flight request.
I guess so.

Best regards

Benoît

> 
> Chris
> 
>