Re: Errors in the VM - detailed

[Jens Axboe]

On Fri, Feb 01 2002, Roger Larsson wrote:
> On Fridayen den 1 February 2002 17.11, Roy Sigurd Karlsbakk wrote:
> > It does not seem to be possible to reproduce the error with apache2. But
> > this may be because Apache2's i/o handling doesn't impress much. With Tux,
> > I keep getting up to 40 megs per sec, but with Apache the average is
> > ~15MB/s.
> >
> > Btw ... It looks like your patch (against rmap12a) gave me an extra
> > performance kick. 12c gave me a max of ~32MB/s, whereas your patch
> > highered this to ~41.
> >
> 
> Hmm.. suppose this is the problem anyway and that Jens patch was not enough.
> How do the disk drive sound during the test?
> 
> Does it start to sound more when performance goes down?

Yes that would be interesting to know, if the disk becomes seek bound.

> About Jens patch:
> 
> My feeling is that there should be (a lot) more  READA than READ.
> since sequential READ really only NEEDS one at a time.

Probably, my patch was really just a quick try to see if it changed
anything.

> Number of READ limits the number of concurrent streams.
> And READA limits the maximum total read ahead.

Correct, Roy you could try and change the READA balance by allocating
lots more READA requests. Simply play around with the
queue_nr_requests / 4 setting. Try something "absurd" like
queue_nr_requests << 2 or even bigger.

-- 
Jens Axboe