[Devel] Re: [PATCH 08/18] io-controller: idle for sometime on sync queue before expiring it

[Vivek Goyal]

On Tue, May 05, 2009 at 03:58:35PM -0400, Vivek Goyal wrote:
> o When a sync queue expires, in many cases it might be empty and then
>   it will be deleted from the active tree. This will lead to a scenario
>   where out of two competing queues, only one is on the tree and when a
>   new queue is selected, vtime jump takes place and we don't see services
>   provided in proportion to weight.
> 
> o In general this is a fundamental problem with fairness of sync queues
>   where queues are not continuously backlogged. Looks like idling is
>   only solution to make sure such kind of queues can get some decent amount
>   of disk bandwidth in the face of competion from continusouly backlogged
>   queues. But excessive idling has potential to reduce performance on SSD
>   and disks with commnad queuing.
> 
> o This patch experiments with waiting for next request to come before a
>   queue is expired after it has consumed its time slice. This can ensure
>   more accurate fairness numbers in some cases.
> 
> o Introduced a tunable "fairness". If set, io-controller will put more
>   focus on getting fairness right than getting throughput right.
> 
> Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
> ---

Following is a fix which should go here. This patch helps me get much 
better fairness numbers for sync queues.


o Fix a window where a queue can be expired without doing busy wait for
  next request. This fix allows better fairness number for sync queues.

Signed-off-by: Vivek Goyal <vgoyal at redhat.com>
---
 block/elevator-fq.c |   18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

Index: linux14/block/elevator-fq.c
===================================================================
--- linux14.orig/block/elevator-fq.c	2009-05-13 10:55:44.000000000 -0400
+++ linux14/block/elevator-fq.c	2009-05-13 10:55:50.000000000 -0400
@@ -3368,8 +3368,22 @@ void *elv_fq_select_ioq(struct request_q
 	/*
 	 * The active queue has run out of time, expire it and select new.
 	 */
-	if (elv_ioq_slice_used(ioq) && !elv_ioq_must_dispatch(ioq))
-		goto expire;
+	if (elv_ioq_slice_used(ioq) && !elv_ioq_must_dispatch(ioq)) {
+		/*
+		 * Queue has used up its slice. Wait busy is not on otherwise
+		 * we wouldn't have been here. There is a chance that after
+		 * slice expiry no request from the queue completed hence
+		 * wait busy timer could not be turned on. If that's the case
+		 * don't expire the queue yet. Next request completion from
+		 * the queue will arm the wait busy timer.
+		 */
+		if (efqd->fairness && !ioq->nr_queued
+		    && elv_ioq_nr_dispatched(ioq)) {
+			ioq = NULL;
+			goto keep_queue;
+		} else
+			goto expire;
+	}
 
 	/*
 	 * If we have a RT cfqq waiting, then we pre-empt the current non-rt
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