svn commit: r220524 - stable/8/sys/geom/gate

Mikolaj Golub trociny at
Sun Apr 10 15:40:57 UTC 2011

Author: trociny
Date: Sun Apr 10 15:40:57 2011
New Revision: 220524

  MFC r220264:
  r220264 (pjd):
  GEOM has an internal mechanism to deal with ENOMEM errors returned via
  g_io_deliver(). In such case it increases 'pace' counter on each ENOMEM and
  reschedules the request. The 'pace' counter is decreased for each request going
  down, but until 'pace' is greater than zero, GEOM will handle at most 10
  requests per second. For GEOM GATE users that are proxy to local GEOM providers
  (like ggatel(8) and HAST) we can end up with almost permanent slow down of GEOM
  down queue. This is because once we reach GEOM GATE queue limit, we return
  ENOMEM to the GEOM. This means that we have, eg. 1024 I/O requests in the GEOM
  GATE queue. To make room in the queue and stop returning ENOMEM we need to
  proceed the requests of course, but those requests are handled by userland
  daemons that handle them by reading/writing also from/to local GEOM providers.
  For example with HAST, a new requests comes to /dev/hast/data, which is GEOM
  GATE provider. GEOM GATE passes the request to hastd(8) and hastd(8)
  reads/writes from/to /dev/da0. Once we reach GEOM GATE queue limit, to free up
  a slot in GEOM GATE queue, hastd(8) has to read/write from/to /dev/da0, but
  this request will also be very slow, because GEOM now slows down all the
  requests. We end up with full queue that we can unload at the speed of 10
  requests per second. This simply looks like a deadlock.
  Fix it by allowing userland daemons that work with both GEOM GATE and local
  GEOM providers to specify unlimited queue size, so GEOM GATE will never return
  ENOMEM to the GEOM.
  Approved by:	pjd (mentor)

Directory Properties:
  stable/8/sys/   (props changed)
  stable/8/sys/amd64/include/xen/   (props changed)
  stable/8/sys/cddl/contrib/opensolaris/   (props changed)
  stable/8/sys/contrib/dev/acpica/   (props changed)
  stable/8/sys/contrib/pf/   (props changed)

Modified: stable/8/sys/geom/gate/g_gate.c
--- stable/8/sys/geom/gate/g_gate.c	Sun Apr 10 15:28:37 2011	(r220523)
+++ stable/8/sys/geom/gate/g_gate.c	Sun Apr 10 15:40:57 2011	(r220524)
@@ -192,7 +192,7 @@ g_gate_start(struct bio *bp)
-	if (sc->sc_queue_count > sc->sc_queue_size) {
+	if (sc->sc_queue_size > 0 && sc->sc_queue_count > sc->sc_queue_size) {
 		G_GATE_LOGREQ(1, bp, "Queue full, request canceled.");
 		g_io_deliver(bp, ENOMEM);

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