[RFC,RFT,PATCH] cfq: autotuning support

[Corrado Zoccolo <czoccolo@gmail.com>]

Hi, this is my first attempt at autotuning cfq parameters, and should apply on top of for-2.6.33 branch.
Jeff and Vivek, if you can test this on your NCQ SSDs, it will help me to have your feedback (please include 
the output of: 'grep -r . /sys/block/sdX/queue/iosched' after you run your tests).

The patch introduces code to sample the request service time distribution, and analyze it,
in order to compute the following cfq parameters:
* slice_idle, is computed as the expected service time of random request
* cfq_slice[1] (i.e. the slice for sync queues)
* cfq_slice[0] (i.e. the slice for async queues)

The sync and async slices are scaled from default values proportionally to the new computed slice_idle.

Autotuning will be enabled by default only on kernels compiled with HZ >= 500.
With smaller HZ, I don't think jiffies is reliable to estimate those parameters.

Signed-off-by: Corrado Zoccolo <czoccolo@gmail.com>
---
 block/cfq-iosched.c |  166 ++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 files changed, 163 insertions(+), 3 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index 6925ab9..d786a0b 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -32,6 +32,12 @@ static const int cfq_target_latency = HZ * 3/10; /* 300 ms */
 static const int cfq_hist_divisor = 4;
 
 /*
+ * Number of samples to collect before updating autotune
+ * higher # makes the measurements more stable
+ */
+#define CFQ_AUTOTUNE_SAMPLES	(10)
+
+/*
  * offset from end of service tree
  */
 #define CFQ_IDLE_DELAY		(HZ / 5)
@@ -201,6 +207,21 @@ struct cfq_data {
 	unsigned int hw_tag_samples;
 
 	/*
+	 * disk performance measurements
+	 */
+	unsigned long observation_start;
+	/*
+	 * measures are split (READ vs WRITE)
+	 */
+	unsigned long processed_rq[2];
+	/*
+	 * We store an histogram of samples for the service time
+	 * in log scale [0..5]; [6] is a count, that is reset every
+	 * time autotuning is done (i.e. every CFQ_AUTOTUNE_SAMPLES)
+	 */
+	unsigned int serv_time_samples[2][7];
+
+	/*
 	 * idle window management
 	 */
 	struct timer_list idle_slice_timer;
@@ -228,6 +249,7 @@ struct cfq_data {
 	unsigned int cfq_slice_async_rq;
 	unsigned int cfq_slice_idle;
 	unsigned int cfq_latency;
+	unsigned int cfq_autotune;
 
 	struct list_head cic_list;
 
@@ -891,6 +913,12 @@ static void cfq_activate_request(struct request_queue *q, struct request *rq)
 {
 	struct cfq_data *cfqd = q->elevator->elevator_data;
 
+	if (!rq_in_driver(cfqd)) {
+		cfqd->observation_start = jiffies;
+		cfqd->processed_rq[0] = 0;
+		cfqd->processed_rq[1] = 0;
+	}
+
 	cfqd->rq_in_driver[rq_is_sync(rq)]++;
 	cfq_log_cfqq(cfqd, RQ_CFQQ(rq), "activate rq, drv=%d",
 						rq_in_driver(cfqd));
@@ -2562,14 +2590,120 @@ static void cfq_update_hw_tag(struct cfq_data *cfqd)
 		cfqd->hw_tag = 0;
 }
 
+
+/*
+ * Update the histogram to compute service time
+ * Returns true if we collected enough samples to re-run autotune
+ */
+static bool cfq_update_stime(unsigned samples[6], unsigned long stime)
+{
+	unsigned idx = (stime > 15) + (stime > 7) + (stime > 3)
+		       + (stime > 1) + (stime > 0);
+	samples[idx]++;
+	if (samples[idx] > (1U<<31))
+		for (idx = 0; idx < 6; ++idx) {
+			samples[idx]++;
+			samples[idx] >>= 1;
+		}
+
+	if (samples[6]++ < CFQ_AUTOTUNE_SAMPLES)
+		return false;
+	samples[6] = 0;
+	return true;
+}
+
+/*
+ * Currently, we measure only service time for pure READ or WRITE requests
+ * and we update autotune when we have collected enough READ requests
+ */
+static bool cfq_update_perf_measures(struct cfq_data *cfqd, unsigned long now)
+{
+	unsigned long tot_proc = cfqd->processed_rq[0] + cfqd->processed_rq[1];
+	unsigned long obstime = now - cfqd->observation_start;
+	unsigned long stime = obstime / tot_proc;
+
+	cfqd->observation_start = now;
+
+	if (!cfqd->processed_rq[READ])
+		cfq_update_stime(cfqd->serv_time_samples[WRITE], stime);
+	if (!cfqd->processed_rq[WRITE])
+		return cfq_update_stime(cfqd->serv_time_samples[READ], stime);
+	return false;
+}
+
+/*
+ * Compute service time from the sampled distribution in the histogram
+ * The real service time distribution is a super-position of two distinct
+ * distributions:
+ * the one for sequential requests (usually this has a small mean)
+ * the one for random requests (usually with a larger mean)
+ * and we want to identify the random request one
+ */
+static unsigned cfq_service_time(unsigned samples[6])
+{
+	unsigned last_max = 0, i;
+	/* Random request service time corresponds to the
+	 * largest maximum in the histogram */
+	for (i = 1; i < 6; ++i)
+		if (samples[i] > samples[i-1])
+			last_max = i;
+	/*
+	 * Unfortunately, if sequential requests overwhelm
+	 * random ones, and the two peaks are too near,
+	 * the second peak could be masked by the tail of the first.
+	 * To catch this, we check if the tail has enough weight,
+	 * and in this case we take the next bin as maximum
+	 */
+	if (last_max < 5) {
+		unsigned total = 0;
+		for (i = last_max + 1; i < 6; ++i)
+			total += samples[i];
+		if (total > samples[last_max])
+			++last_max;
+	}
+	if (!last_max)
+		return 0;
+	return 1U << last_max;
+}
+
+static void cfq_update_autotune(struct cfq_data *cfqd)
+{
+	unsigned long base, baseW;
+	if (!cfqd->cfq_autotune)
+		return;
+	base = cfq_service_time(cfqd->serv_time_samples[READ]);
+	baseW = cfq_service_time(cfqd->serv_time_samples[WRITE]);
+
+	/* Compute slice_idle */
+	if (!base)
+		base = baseW;
+	if (base > cfq_slice_idle)
+		base = min_t(unsigned long,
+			     (base + cfq_slice_idle) / 2, 2 * cfq_slice_idle);
+
+	cfqd->cfq_slice_idle = base;
+
+	/* Compute derived cfq_slice[*]
+	 * Note: those cannot be 0
+	 */
+	if (!base)
+		base = 1;
+
+	if (baseW)
+		baseW = min(base, baseW * cfq_slice_sync / cfq_slice_async);
+	else
+		baseW = base;
+
+	cfqd->cfq_slice[1] = base * cfq_slice_sync / cfq_slice_idle;
+	cfqd->cfq_slice[0] = baseW * cfq_slice_async / cfq_slice_idle;
+}
+
 static void cfq_completed_request(struct request_queue *q, struct request *rq)
 {
 	struct cfq_queue *cfqq = RQ_CFQQ(rq);
 	struct cfq_data *cfqd = cfqq->cfqd;
 	const int sync = rq_is_sync(rq);
-	unsigned long now;
-
-	now = jiffies;
+	unsigned long now = jiffies;
 	cfq_log_cfqq(cfqd, cfqq, "complete");
 
 	cfq_update_hw_tag(cfqd);
@@ -2578,6 +2712,10 @@ static void cfq_completed_request(struct request_queue *q, struct request *rq)
 	WARN_ON(!cfqq->dispatched);
 	cfqd->rq_in_driver[sync]--;
 	cfqq->dispatched--;
+	cfqd->processed_rq[rq_data_dir(rq)]++;
+
+	if (!rq_in_driver(cfqd) && cfq_update_perf_measures(cfqd, now))
+		cfq_update_autotune(cfqd);
 
 	if (cfq_cfqq_sync(cfqq))
 		cfqd->sync_flight--;
@@ -2966,6 +3104,7 @@ static void *cfq_init_queue(struct request_queue *q)
 	cfqd->cfq_slice_async_rq = cfq_slice_async_rq;
 	cfqd->cfq_slice_idle = cfq_slice_idle;
 	cfqd->cfq_latency = 1;
+	cfqd->cfq_autotune = (HZ >= 500);
 	cfqd->hw_tag = -1;
 	cfqd->last_end_sync_rq = jiffies;
 	return cfqd;
@@ -3002,6 +3141,23 @@ fail:
 /*
  * sysfs parts below -->
  */
+static ssize_t autotune_stats_show(struct elevator_queue *e, char *page)
+{
+	struct cfq_data *cfqd = e->elevator_data;
+	int pos = 0, i, j;
+#if HZ < 500
+	pos += sprintf(page, "Autotune may work incorrectly with HZ < 500\n");
+#endif
+	for (j = 0; j < 2; ++j) {
+		for (i = 0; i < 6; ++i)
+			pos += sprintf(page+pos, "[%2u]",
+				cfqd->serv_time_samples[j][i]);
+		page[pos++] = '\n';
+	}
+	page[pos] = '\0';
+	return pos;
+}
+
 static ssize_t
 cfq_var_show(unsigned int var, char *page)
 {
@@ -3036,6 +3192,7 @@ SHOW_FUNCTION(cfq_slice_sync_show, cfqd->cfq_slice[1], 1);
 SHOW_FUNCTION(cfq_slice_async_show, cfqd->cfq_slice[0], 1);
 SHOW_FUNCTION(cfq_slice_async_rq_show, cfqd->cfq_slice_async_rq, 0);
 SHOW_FUNCTION(cfq_low_latency_show, cfqd->cfq_latency, 0);
+SHOW_FUNCTION(cfq_autotune_show, cfqd->cfq_autotune, 0);
 #undef SHOW_FUNCTION
 
 #define STORE_FUNCTION(__FUNC, __PTR, MIN, MAX, __CONV)			\
@@ -3068,6 +3225,7 @@ STORE_FUNCTION(cfq_slice_async_store, &cfqd->cfq_slice[0], 1, UINT_MAX, 1);
 STORE_FUNCTION(cfq_slice_async_rq_store, &cfqd->cfq_slice_async_rq, 1,
 		UINT_MAX, 0);
 STORE_FUNCTION(cfq_low_latency_store, &cfqd->cfq_latency, 0, 1, 0);
+STORE_FUNCTION(cfq_autotune_store, &cfqd->cfq_autotune, 0, 1, 0);
 #undef STORE_FUNCTION
 
 #define CFQ_ATTR(name) \
@@ -3084,6 +3242,8 @@ static struct elv_fs_entry cfq_attrs[] = {
 	CFQ_ATTR(slice_async_rq),
 	CFQ_ATTR(slice_idle),
 	CFQ_ATTR(low_latency),
+	CFQ_ATTR(autotune),
+	__ATTR_RO(autotune_stats),
 	__ATTR_NULL
 };
 
-- 
1.6.2.5