linux-kernel - Re: trying to understand READ_META, READ_SYNC, WRITE

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Date:	Sun, 27 Jun 2010 11:44:30 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co

Vivek Goyal <vgoyal@...hat.com> writes:

> On Fri, Jun 25, 2010 at 01:03:20PM +0200, Christoph Hellwig wrote:
>> On Wed, Jun 23, 2010 at 09:44:20PM -0400, Vivek Goyal wrote:
>> I see the point of this logic for reads where various workloads have
>> dependent reads that might be close to each other, but I don't really
>> see any point for writes.
>> 
>> > So looks like fsync path will do bunch of IO and then will wait for jbd thread
>> > to finish the work. In this case idling is waste of time.
>> 
>> Given that ->writepage already does WRITE_SYNC_PLUG I/O which includes
>> REQ_NODILE I'm still confused why we still have that issue.
>
> In current form, cfq honors REQ_NOIDLE conditionally and that's why we
> still have the issue. If you look at cfq_completed_request(), we continue
> to idle in following two cases.
>
> - If we classifed the queue as SYNC_WORKLOAD.
> - If there is another random read/write happening on sync-noidle service
>   tree.
>
> SYNC_WORKLOAD means that cfq thinks this particular queue is doing sequential
> IO. For random IO queues, we don't idle on each individual queue but a
> group of queue.
>
> In jeff's testing, fsync thread/queue sometimes is viewed as sequential
> workload and goes on SYNC_WORKLOAD tree. In that case even if request is
> REQ_NOIDLE, we will continue to idle hence fsync issue.

I'm now testing OCFS2, and I'm seeing performance that is not great
(even with the blk_yield patches applied).  What happens is that we
successfully yield the queue to the journal thread, but then idle on the
journal thread (even though RQ_NOIDLE was set).

So, can we just get rid of idling when RQ_NOIDLE is set?

Vivek sent me this patch to test, and it got rid of the performance
issue for the fsync workload.  Can we discuss its merits?

Thanks,
Jeff

Index: linux-2.6/block/cfq-iosched.c
===================================================================
--- linux-2.6.orig/block/cfq-iosched.c	2010-06-25 15:57:33.832125786 -0400
+++ linux-2.6/block/cfq-iosched.c	2010-06-25 15:59:19.788876361 -0400
@@ -318,6 +318,7 @@
 	CFQ_CFQQ_FLAG_split_coop,	/* shared cfqq will be splitted */
 	CFQ_CFQQ_FLAG_deep,		/* sync cfqq experienced large depth */
 	CFQ_CFQQ_FLAG_wait_busy,	/* Waiting for next request */
+	CFQ_CFQQ_FLAG_group_idle,	/* This queue is doing group idle */
 };
 
 #define CFQ_CFQQ_FNS(name)						\
@@ -347,6 +348,7 @@
 CFQ_CFQQ_FNS(split_coop);
 CFQ_CFQQ_FNS(deep);
 CFQ_CFQQ_FNS(wait_busy);
+CFQ_CFQQ_FNS(group_idle);
 #undef CFQ_CFQQ_FNS
 
 #ifdef CONFIG_CFQ_GROUP_IOSCHED
@@ -1613,6 +1615,7 @@
 
 	cfq_clear_cfqq_wait_request(cfqq);
 	cfq_clear_cfqq_wait_busy(cfqq);
+	cfq_clear_cfqq_group_idle(cfqq);
 
 	/*
 	 * If this cfqq is shared between multiple processes, check to
@@ -3176,6 +3179,13 @@
 	if (cfq_class_rt(new_cfqq) && !cfq_class_rt(cfqq))
 		return true;
 
+	/*
+	 * If were doing group_idle and we got new request in same group,
+	 * preempt the queue
+	 */
+	if (cfq_cfqq_group_idle(cfqq))
+		return true;
+
 	if (!cfqd->active_cic || !cfq_cfqq_wait_request(cfqq))
 		return false;
 
@@ -3271,6 +3281,7 @@
 	struct cfq_queue *cfqq = RQ_CFQQ(rq);
 
 	cfq_log_cfqq(cfqd, cfqq, "insert_request");
+	cfq_clear_cfqq_group_idle(cfqq);
 	cfq_init_prio_data(cfqq, RQ_CIC(rq)->ioc);
 
 	rq_set_fifo_time(rq, jiffies + cfqd->cfq_fifo_expire[rq_is_sync(rq)]);
@@ -3416,10 +3427,12 @@
 			 * SYNC_NOIDLE_WORKLOAD idles at the end of the tree
 			 * only if we processed at least one !rq_noidle request
 			 */
-			if (cfqd->serving_type == SYNC_WORKLOAD
-			    || cfqd->noidle_tree_requires_idle
-			    || cfqq->cfqg->nr_cfqq == 1)
+			if (cfqd->noidle_tree_requires_idle)
+				cfq_arm_slice_timer(cfqd);
+			else if (cfqq->cfqg->nr_cfqq == 1) {
+				cfq_mark_cfqq_group_idle(cfqq);
 				cfq_arm_slice_timer(cfqd);
+			}
 		}
 	}
 

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