[libvirt] blkio cgroup

[Dominik Klein]

<snip>

> I was also running "vmstat 2" when throttled machine did IO and noticed
> that number of blocked processes went up, around 25-35. I am assuming
> these all are qemu IO threads blocked waiting for throttled IO to finish.
> I am not sure if blocked processes also contribute towards load average.
> 
> While googling a bit, I found one wiki page which says following.
> 
> "Most UNIX systems count only processes in the running (on CPU) or runnable
> (waiting for CPU) states. However, Linux also includes processes in
> uninterruptible sleep states (usually waiting for disk activity), which
> can lead to markedly different results if many processes remain blocked in
> I/O due to a busy or stalled I/O system."
> 
> If this is true, that explains high system load in your testing.
> Throttling is working and we have around 30-35 IO threads/processes
> per qemu instance. You have 8 qemu instance running and roughly 240-280
> processes blocked waiting for IO to finish and that will explain high
> load. But that is expected given the fact we are throttling IO?
> 
> I also tried direct IO in virtual machine and that seems to be forking
> only 1 IO thread.
> 
> # time dd if=/dev/zero of=/mnt/vdb/testfile bs=1M count=1500 oflag=direct
> 1500+0 records in
> 1500+0 records out
> 1572864000 bytes (1.6 GB) copied, 31.4301 s, 50.0 MB/s
> 
> real	0m31.664s
> user	0m0.003s
> sys	0m0.819s
> 
> While running this I noticed number of processes blocked was 1 all the
> time and hence low load average. Try oflag=direct option in your tests.

I can second that.

oflag=direct in the vm shows 1 blocked process most of the time on the
host. No oflag shows 1-16.

Regards
Dominik