git: 6d83b381860b - main - geom_io: Shift to pause_sbt to eliminate bogus min and update comment.

The branch main has been updated by imp:

URL: https://cgit.FreeBSD.org/src/commit/?id=6d83b381860bd8a1ea2043e26bdf2cf2ed640a38

commit 6d83b381860bd8a1ea2043e26bdf2cf2ed640a38
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: 2024-05-24 14:31:55 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: 2024-05-24 14:31:55 +0000

    geom_io: Shift to pause_sbt to eliminate bogus min and update comment.
    
    Update to eliminate bogus min to ensure 0 was never passed to
    pause. Instead, requrest 1ms with an 'infinite' precision, which
    defaults to whatever the underlying time counter can do. This should
    ensure we run fairly quickly to start processing done events, while
    still giving a small pause for the system to catch its breath. This rate
    limiter still is less than ideal, and this commit doesn't change
    that. It should really have no functional change: it just uses a better
    interface to express the desired sleep.
    
    Sponsored by:           Netflix
    Reviewed by:            kib
    Differential Revision:  https://reviews.freebsd.org/D45316
---
 sys/geom/geom_io.c | 27 ++++++++++++---------------
 1 file changed, 12 insertions(+), 15 deletions(-)

diff --git a/sys/geom/geom_io.c b/sys/geom/geom_io.c
index 25b3908e5fb5..02c8ed9f2bae 100644
--- a/sys/geom/geom_io.c
+++ b/sys/geom/geom_io.c
@@ -809,20 +809,17 @@ g_io_schedule_down(struct thread *tp __unused)
 		biotrack(bp, __func__);
 		if (pace != 0) {
 			/*
-			 * There has been at least one memory allocation
-			 * failure since the last I/O completed. Pause 1ms to
-			 * give the system a chance to free up memory. We only
-			 * do this once because a large number of allocations
-			 * can fail in the direct dispatch case and there's no
-			 * relationship between the number of these failures and
-			 * the length of the outage. If there's still an outage,
-			 * we'll pause again and again until it's
-			 * resolved. Older versions paused longer and once per
-			 * allocation failure. This was OK for a single threaded
-			 * g_down, but with direct dispatch would lead to max of
-			 * 10 IOPs for minutes at a time when transient memory
-			 * issues prevented allocation for a batch of requests
-			 * from the upper layers.
+			 * There has been at least one memory allocation failure
+			 * since the last I/O completed. Pause 1ms to give the
+			 * system a chance to free up memory. Pause time is not
+			 * scaled to the number of I/O failures since they tend
+			 * to cluster and the number is not predictive of how
+			 * long a pause is needed.
+			 *
+			 * Older versions had a longer pause, which limited the
+			 * IOPS to 10, which prolonged memory shortages that could
+			 * be alleviated by I/O completing since it eliminated
+			 * direct dispatch as well.
 			 *
 			 * XXX This pacing is really lame. It needs to be solved
 			 * by other methods. This is OK only because the worst
@@ -833,7 +830,7 @@ g_io_schedule_down(struct thread *tp __unused)
 			 */
 			CTR0(KTR_GEOM, "g_down pacing self");
 			pause_count++;		/* g_down has only one thread */
-			pause("g_down", min(hz/1000, 1));
+			pause_sbt("g_down", SBT_1MS, 0, 0);
 			pace = 0;
 		}
 		CTR2(KTR_GEOM, "g_down processing bp %p provider %s", bp,