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

["Benoît Canet" <benoit.canet@irqsave.net>]

The Friday 22 Aug 2014 à 18:59:38 (-0600), Chris Friesen wrote :
> On 07/21/2014 10:10 AM, Benoît Canet wrote:
> >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:
> >benoit@Laure:~$ 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.
> 
> 
> After a bit of a break I'm looking at this again.
> 

Strange.

I would use dd with the flag oflag=nocache to make sure the write request
does not do in the guest cache though.

Best regards

Benoît

> While doing "dd if=/dev/zero of=testfile bs=1M count=700" in the guest, I
> got a max "inflight" value of 181.  This seems quite a bit higher than
> VIRTIO_PCI_QUEUE_MAX.
> 
> I've seen throughput as high as ~210 MB/sec, which also kicked the RSS
> numbers up above 200MB.
> 
> I tried dropping VIRTIO_PCI_QUEUE_MAX down to 32 (it didn't seem to work at
> all for values much less than that, though I didn't bother getting an exact
> value) and it didn't really make any difference, I saw inflight values as
> high as 177.
> 
> Chris
>