Re: [CFT] delayed allocation and multipage I/O patches for 2.5.6.

[Daniel Phillips]

On March 12, 2002 09:29 pm, Andrew Morton wrote:
> Daniel Phillips wrote:
> > 
> > On March 12, 2002 07:00 am, Andrew Morton wrote:
> > >   Identifies readahead thrashing.
> > >
> > >     Currently, it just performs a shrink on the readahead window when thrashing
> > >     occurs.  This greatly reduces the amount of pointless I/O which we perform,
> > >     and will reduce the CPU load.  The idea is that the readahead window
> > >     dynamically adjusts to a sustainable size.  It improves things, but not
> > >     hugely, experimentally.
> > 
> > The question is, does it wipe out a nasty corner case?  If so then the improvement
> > for the averge case is just a nice fringe benefit.  A carefully constructed test
> > that triggers the corner case would be most interesting.
> 
> There are many test scenarios.  The one I use is:
> 
> - 64 megs of memory.
> 
> - Process A loops across N 10-megabyte files, reading 4k from each one
>   and terminates when all N files are fully read.
> 
> - Process B loops, repeatedly reading a one gig file off another disk.
> 
> The total wallclock time for process A exhibits *massive* step jumps
> as you vary N.  In stock 2.5.6 the runtime jumps from 40 seconds to
> ten minutes when N is increased from 40 to 60.
> 
> With my changes, the rate of increase of runtime-versus-N is lower,
> and happens at later N.  But it's still very sudden and very bad.
> 
> Yes, it's a known-and-nasty corner case.  Worth fixing if the
> fix is clean.  But IMO the problem is not common enough to
> justify significantly compromising the common case.

It's a given the common case should be optimal.  I'm sure there's an algorithm that
fixes up your test case, which by the way isn't that uncommon - it's Rik's '100 ftp
processes' case.  I'll buy the suggestion it isn't common enough to drop everything
right now and go fix it.

-- 
Daniel