Re: regression: 100% io-wait with 2.6.24-rcX

Mel Gorman
2008-01-17T22:12:35Z

On (17/01/08 13:50), Martin Knoblauch didst pronounce:
> > <mail manglement snipped>
>
> The effect is defintely depending on the IO hardware. I performed the same tests
> on a different box with an AACRAID controller and there things look different.

I take it different also means it does not show this odd performance
behaviour and is similar whether the patch is applied or not?

> Basically
> the "offending" commit helps seingle stream performance on that box, while dual/triple
> stream are not affected. So I suspect that the CCISS is just not behaving well.
>
> And yes, the tests are usually done on a freshly booted box. Of course, I repeat them
> a few times. On the CCISS box the numbers are very constant. On the AACRAID box
> they vary quite a bit.
>
> I can certainly stress the box before doing the tests. Please define "many" for the kernel
> compiles :-)
>

With 8GiB of RAM, try making 24 copies of the kernel and compiling them
all simultaneously. Running that for for 20-30 minutes should be enough to
randomise the freelists affecting what color of page is used for the dd test.

> > > OK, the change happened between rc5 and rc6. Just following a
> > > gut feeling, I reverted
> > >
> > > #commit 81eabcbe0b991ddef5216f30ae91c4b226d54b6d
> > > #Author: Mel Gorman
> > > #Date: Mon Dec 17 16:20:05 2007 -0800
> > > #
>
> > >
> > > This has brought back the good results I observed and reported.
> > > I do not know what to make out of this. At least on the systems
> > > I care about (HP/DL380g4, dual CPUs, HT-enabled, 8 GB Memory,
> > > SmartaArray6i controller with 4x72GB SCSI disks as RAID5 (battery
> > > protected writeback cache enabled) and gigabit networking (tg3)) this
> > > optimisation is a dissaster.
> > >
> >
> > That patch was not an optimisation, it was a regression fix
> > against 2.6.23 and I don't believe reverting it is an option. Other IO
> > hardware benefits from having the allocator supply pages in PFN order.
>
> I think this late in the 2.6.24 game we just should leave things as they are. But
> we should try to find a way to make CCISS faster, as it apparently can be faster.
>
> > Your controller would seem to suffer when presented with the same situation
> > but I don't know why that is. I've added James to the cc in case he has seen this
> > sort of situation before.
> >
> > > On the other hand, it is not a regression against 2.6.22/23. Those
> >
> > had bad IO scaling to. It would just be a shame to loose an apparently
> >
> > great performance win.
> >
> > Could you try running your tests again when the system has been
> > stressed with some other workload first?
> >
>
> Will do.
>

Thanks

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab