From: Jeff Moyer <jmoyer@redhat.com>
Date: Fri, 5 Mar 2010 14:59:28 -0500
Subject: [block] cfq: kick busy queues w/o waiting for merged req
Message-id: <x49635a24rz.fsf@segfault.boston.devel.redhat.com>
Patchwork-id: 23502
O-Subject: [RHEL 5.5 Patch] cfq: Don't wait for request merges when there are
	other busy queues
Bugzilla: 570814
RH-Acked-by: Rik van Riel <riel@redhat.com>
RH-Acked-by: Doug Ledford <dledford@redhat.com>
RH-Acked-by: Dean Nelson <dnelson@redhat.com>

Hi,

During 5.5 beta testing, Fujitsu identified a pretty significant
performance regression when testing multiple O_DIRECT readers:

  When two or more O_DIRECT reads simultaneously run on different
  partitions of a disk, each performance critically slows down.  An
  example of using multiple dd commands:

  # dd if=/dev/sdb1 of=/dev/null iflag=direct &
  # dd if=/dev/sdb2 of=/dev/null iflag=direct &

  The result of read performance is as follows:

               |      number of dds    |
               |   1   |   2   |   4   |
  -------------+-------+-------+-------|
  RHEL5.5 beta | 107.6 |   3.7 |   3.7 |
  RHEL5.4      | 109.1 |  78.1 |  75.0 |
                       read speed (MB/s)

They also identified the upstream patch that fixes the problem, and
verified that it fixed the problem in their test environment.  In
addition, they have committed QA resources to ensure that this patch
does not regress other workloads.

The attached patch addresses bug 570814.  Comments, as always, are
appreciated.

Cheers,
Jeff

commit 2d870722965211de072bb36b446a4df99dae07e1
Author: Jens Axboe <jens.axboe@oracle.com>
Date:   Wed Apr 15 12:12:46 2009 +0200

    cfq-iosched: tweak kick logic a bit more

    We only kick the dispatch for an idling queue, if we think it's a
    (somewhat) fully merged request. Also allow a kick if we have other
    busy queues in the system, since we don't want to risk waiting for
    a potential merge in that case. It's better to get some work done and
    proceed.

    Signed-off-by: Jens Axboe <jens.axboe@oracle.com>

Signed-off-by: Jarod Wilson <jarod@redhat.com>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 8ead467..ab0badf 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -2085,10 +2085,13 @@ cfq_crq_enqueued(struct cfq_data *cfqd, struct cfq_queue *cfqq,
 		 * of tiny requests, because we disrupt the normal plugging
 		 * and merging. If the request is already larger than a single
 		 * page, let it rip immediately. For that case we assume that
-		 * merging is already done.
+		 * merging is already done. Ditto for a busy system that
+		 * has other work pending, don't risk delaying until the
+		 * idle timer unplug to continue working.
 		 */
 		if (cfq_cfqq_wait_request(cfqq)) {
-			if (crq->request->data_len > PAGE_CACHE_SIZE) {
+			if (crq->request->data_len > PAGE_CACHE_SIZE ||
+			    cfqd->busy_queues > 1) {
 				del_timer(&cfqd->idle_slice_timer);
 				cfq_start_queueing(cfqd, cfqq);
 			}