Date: Thu, 3 Sep 2015 14:06:06 +0200 From: Edward Tomasz =?utf-8?Q?Napiera=C5=82a?= <trasz@FreeBSD.org> To: Warner Losh <imp@FreeBSD.org> Cc: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r287405 - head/sys/geom Message-ID: <20150903120606.GB75381@brick.home> In-Reply-To: <201509021729.t82HTULW036119@repo.freebsd.org> References: <201509021729.t82HTULW036119@repo.freebsd.org>
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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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