Re: trying to understand READ_META, READ_SYNC, WRITE_SYNC & co

Jeff Moyer
2010-06-27T15:44:38Z

Vivek Goyal <vgoyal@redhat.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);
+ }
}
}