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    SubjectRe: 2.5.2-pre performance degradation on an old 486
    On Sat, Jan 05 2002, Davide Libenzi wrote:
    > > > (*) 100MHz 486DX4, 28MB ram, no L2 cache, two old and slow IDE disks,
    > > > small custom no-nonsense RedHat 7.2, kernels compiled with gcc 2.95.3.
    > >
    > > Is this ISA (maybe it has something to do with ISA bouncing)? Mine is:
    > >
    > > 486 DX/2 ISA, Adaptec 1542, two slow scsi disks and a self-made
    > > slackware-based system.
    > >
    > > Can you also backout the scheduler changes to verify this? I have a
    > > backout patch for 2.5.2-pre6, if you don't want to do this for yourself.
    >
    > There should be some part of the kernel that assume a certain scheduler
    > behavior. There was a guy that reported a bad hdparm performance and i
    > tried it. By running hdparm -t my system has a context switch of 20-30
    > and an irq load of about 100-110.
    > The scheduler itself, even if you code it in visual basic, cannot make
    > this with such loads.
    > Did you try to profile the kernel ?

    Davide,

    If this is caused by ISA bounce problems, then you should be able to
    reproduce by doing something ala

    [ drivers/ide/ide-dma.c ]

    ide_toggle_bounce()
    {
    ...

    + addr = BLK_BOUNCE_ISA;
    blk_queue_bounce_limit(&drive->queue, addr);
    }

    pseudo-diff, just add the addr = line. Now compare performance with and
    without your scheduler changes.

    --
    Jens Axboe

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