Re: [resent PATCH] Re: very slow parallel read performance

Daniel Phillips
2001-08-24T22:22:34Z

On August 24, 2001 09:02 pm, Rik van Riel wrote:
> On Fri, 24 Aug 2001, Roger Larsson wrote:
>
> > Not having the patch gives you another effect - disk arm is
> > moving from track to track in a furiously tempo...
>
> Fully agreed, but remember that when you reach the point
> where the readahead windows are pushing each other out
> you'll be off even worse.
>
> I guess in the long run we should have automatic collapse
> of the readahead window when we find that readahead window
> thrashing is going on, in the short term I think it is
> enough to have the maximum readahead size tunable in /proc,
> like what is happening in the -ac kernels.

Yes, and the most effective way to detect that the readahead window is too
high is by keeping a history of recently evicted pages. When we find
ourselves re-reading pages that were evicted before ever being used we know
exactly what the problem is.

--
Daniel