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    SubjectRe: RFC on io-stalls patch


    On Mon, 14 Jul 2003, Andrea Arcangeli wrote:

    > On Mon, Jul 14, 2003 at 09:51:39PM +0200, Jens Axboe wrote:
    > > - rl = &q->rq;
    > > - if (!list_empty(&rl->free) && !blk_oversized_queue(q)) {
    > > + if ((rw == WRITE) && (blk_oversized_queue(q) || (rl->count < 4)))
    >
    > did you disable the oversized queue check completely for reads? This
    > looks unsafe, you can end with loads of ram locked up this way, the
    > request queue cannot be limited in requests anymore. this isn't the
    > "request reservation", this a "nearly unlimited amount of ram locked in
    > for reads".
    >
    > Of course, the more reads can be in the queue, the less the background
    > write loads will hurt parallel apps like a kernel compile as shown in
    > xtar_load.
    >
    > This is very different from the schedule advantage provided by the old
    > queue model. If you allow an unlimited I/O queue for reads, that means
    > the I/O queues will be filled by an huge amount of reads and a few
    > writes (no matter how fast the xtar_load is writing to disk).
    >
    > In the past (2.4.22pre4) the I/O queue would been at most 50/50, with
    > your patch it can be 90/10, hence it can generate an huge performance
    > difference, that can penealize tremendously the writers in server loads
    > using fsync plus it can hurt the VM badly if all ram is locked up by
    > parallel reads. Of course contest mostly cares about reads, not writes.
    >
    > Overall I think your patch is unsafe and shouldn't be applied.
    >
    > Still if you want to allow 50/50, go ahead, that logic in pre4 was an
    > order of magnitude more fair and generic than this patch.

    Well, I change my mind and wont apply it as-is in -pre6.

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