On 0902T1729, Warner Losh wrote:
> Author: imp
> Date: Wed Sep 2 17:29:30 2015
> New Revision: 287405
> URL: https://svnweb.freebsd.org/changeset/base/287405
>
> Log:
> After the introduction of direct dispatch, the pacing code in g_down()
> broke in two ways. One, the pacing variable was accessed in multiple
> threads in an unsafe way. Two, since large numbers of I/O could come
> down from the buf layer at one time, large numbers of allocation
> failures could happen all at once, resulting in a huge pace value that
> would limit I/Os to 10 IOPS for minutes (or even hours) at a
> time. While a real solution to these problems requires substantial
> work (to go to a no-allocation after the first model, or to have some
> way to wait for more memory with some kind of reserve for pager and
> swapper requests), it is relatively easy to make this simplistic
> pacing less pathological.
Shouldn't we emit some warning the first time this happens, to aid
in debugging strange IO performance degradation? Something like...
> @@ -688,7 +699,7 @@ g_io_deliver(struct bio *bp, int error)
> bp->bio_driver2 = NULL;
> bp->bio_pflags = 0;
> g_io_request(bp, cp);
if (warned_about_pace == 0) {
printf("WARNING: GEOM io allocation failed; expect reduced IO
performance\n");
warned_about_pace = 1;
}
> - pace++;
> + pace = 1;
> return;
> }
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