linux-kernel - [PATCH] cfq: enable idle for seeky processes on rotational NCQ devices

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Date:	Sun, 4 Oct 2009 18:37:34 +0200
From:	Corrado Zoccolo <czoccolo@...il.com>
To:	"Linux-Kernel" <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Jeff Moyer <jmoyer@...hat.com>
Subject: [PATCH] cfq: enable idle for seeky processes on rotational NCQ devices

Disabled idle window cause large latencies when seeky processes are competing
with async writes, for rotational NCQ devices.

This patch, based on Vivek Goyal's original idea (http://lkml.org/lkml/2009/7/12/110),
unconditionally enables idle window for seeky processes on rotational NCQ devices.
As for non-NCQ devices, a smaller idle window (2ms) is used
for seeky processes compared to normal I/O (8ms).

RAIDs should be marked as non-rotational as well (and probably a better flag
name should be devised), since they can carry multiple operations in parallel.

Signed-off-by: Corrado Zoccolo <czoccolo@...il.com>
---
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index ebab60c..576e92d 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1981,10 +1981,14 @@ cfq_update_idle_window(struct cfq_data *cfqd, struct cfq_queue *cfqq,
        enable_idle = old_idle = cfq_cfqq_idle_window(cfqq);
 
        if (!atomic_read(&cic->ioc->nr_tasks) || !cfqd->cfq_slice_idle ||
-           (!cfqd->cfq_latency && cfqd->hw_tag && CIC_SEEKY(cic)))
+           (blk_queue_nonrot(cfqd->queue) && cfqd->hw_tag && CIC_SEEKY(cic)))
                enable_idle = 0;
        else if (sample_valid(cic->ttime_samples)) {
-               if (cic->ttime_mean > cfqd->cfq_slice_idle)
+               unsigned idle_time = cfqd->cfq_slice_idle;
+               if (CIC_SEEKY(cic))
+                       idle_time = CFQ_MIN_TT;
+
+               if (cic->ttime_mean > idle_time)
                        enable_idle = 0;
                else
                        enable_idle = 1;


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