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