Re: [RFC] Block IO Controller V2 - some results

Alan D. Brunelle
2009-11-16T21:32:10Z

On Mon, 2009-11-16 at 16:14 -0500, Vivek Goyal wrote:
> On Mon, Nov 16, 2009 at 03:51:00PM -0500, Alan D. Brunelle wrote:
> > Hi Vivek:
> >
> > I'm finding some things that don't quite seem right - executive
> > summary:
>
> Hi Alan,
>
> Thanks a lot for such an extensive testing and test results. I am still
> digesting the results but I thought I will make a quick note about writes.
> This patchset works only for sync IO. If you are performing buffered
> writes then you will not see any service differentiation. Providing
> support for buffered write path is in TODO list.

Ah, I thought you meant sync I/O versus async I/O. So do you mean that
the testing should use _direct_ I/O (bypassing the cache)?

>
> >
> > o I think the apportionment algorithm doesn't work consistently well
> > for writes.
> >
> > o I think there are problems with significant performance loss when
> > doing random I/Os.
>
> This concerns me. I had a quick look and as per your results, even with
> group_idle=0 you are seeing this regression. I guess this might be coming
> from the fact that we idle on sync-noidle workload per group and that
> idling becomes significant as number of groups increase.
>
> Thanks
> Vivek