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Index: sys/geom/geom_io.c
===================================================================
--- sys/geom/geom_io.c
+++ sys/geom/geom_io.c
@@ -71,7 +71,17 @@
static struct g_bioq g_bio_run_up;
static struct g_bioq g_bio_run_task;
-static u_int pace;
+/*
+ * Pace is a hint that we've had some trouble recently allocating
+ * bios, so we should back off trying to send I/O down the stack
+ * a bit to let the problem resolve. When pacing, we also turn
+ * off direct dispatch to also reduce memory pressure from I/Os
+ * there, at the expxense of some added latency while the memory
+ * pressures exist. See g_io_schedule_down() for more details
+ * and limitations.
+ */
+static volatile u_int pace;
+
static uma_zone_t biozone;
/*
@@ -521,7 +531,8 @@
(pp->flags & G_PF_DIRECT_RECEIVE) != 0 &&
!g_is_geom_thread(curthread) &&
((pp->flags & G_PF_ACCEPT_UNMAPPED) != 0 ||
- (bp->bio_flags & BIO_UNMAPPED) == 0 || THREAD_CAN_SLEEP());
+ (bp->bio_flags & BIO_UNMAPPED) == 0 || THREAD_CAN_SLEEP()) &&
+ pace == 0;
if (direct) {
/* Block direct execution if less then half of stack left. */
size_t st, su;
@@ -688,7 +699,7 @@
bp->bio_driver2 = NULL;
bp->bio_pflags = 0;
g_io_request(bp, cp);
- pace++;
+ pace = 1;
return;
}
@@ -777,10 +788,32 @@
}
CTR0(KTR_GEOM, "g_down has work to do");
g_bioq_unlock(&g_bio_run_down);
- if (pace > 0) {
- CTR1(KTR_GEOM, "g_down pacing self (pace %d)", pace);
- pause("g_down", hz/10);
- pace--;
+ 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.
+ *
+ * XXX This pacing is really lame. It needs to be solved
+ * by other methods. This is OK, but in the worst case
+ * scenario all memory is tied up waiting for I/O to
+ * complete which can never happen since we can't
+ * allocate bios for that I/O.
+ */
+ CTR0(KTR_GEOM, "g_down pacing self");
+ pause("g_down", min(hz/1000, 1));
+ pace = 0;
}
CTR2(KTR_GEOM, "g_down processing bp %p provider %s", bp,
bp->bio_to->name);

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