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

Sorry I didn't understood this part at first read.

In the linux guest can you monitor:
address@hidden:~$ cat /sys/class/block/xyz/inflight ?

This would give us a faily precise number of the requests actually in flight 
between the guest and qemu.

Best regards

Benoît

> >>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...
> 
> Chris
> 
>