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

[Andrew Morton]

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.

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