[PATCH] as: cooperating processes

Introduce the notion of cooperating processes (those that submit requests
close to one another), and use these statistics to make better choices about
whether or not to do anticipatory waiting.

Help and analysis from Seetharami Seelam <seelam@cs.utep.edu>

Performance testing from Seelam:

I set up my system and executed a couple of tests that I used for OLS.  I
tested with AS, cooperative process patch merged in -mm tree (which I called
Nick, below) and the cooperative patch with modifications to as_update_iohist
(which I called Seelam).

I used a dual-processor (2.28GHz Pentium 4 Xeon) system, with 1 GB main memory
and 1 MB L2 cache, running Linux 2.6.9.  Only a single processor is used for
the experiments.  I used 7.2K RPM Maxtor 10GB drive configured with ext2 file
system.

Experiment 1 (ex1) consists of reading  one Linux source trees using

  find . -type f -exec cat '{}' ';' > /dev/null.

Experiment 2 (ex2) consists of reading two disjoint Linux source trees
using

  find . -type f -exec cat '{}' ';' > /dev/null.

Experiment 3 (ex3) consists of streaming read of a 2GB file in the background
and 1 instance of the chunk reads in Experiment 1.

Timings for reading the Linux source are shown below:

             AS                     Nick          Seelam
ex1:      0m25.813s               0m27.859s      0m27.640s
ex2:      1m11.468s               1m13.918s      1m5.869s
ex3:      81m44.352s             10m38.572s      6m47.994s

The difference between the numbers in Experiment 3 must be due to the code in
as_update_iohist.  (akpm: that's not part of this patch.  So this patch is
"Nick").
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

drivers/block/as-iosched.c

@@ -4,7 +4,7 @@
  *  Anticipatory & deadline i/o scheduler.
  *
  *  Copyright (C) 2002 Jens Axboe <axboe@suse.de>
- *                     Nick Piggin <piggin@cyberone.com.au>
+ *                     Nick Piggin <nickpiggin@yahoo.com.au>
  *
  */
 #include <linux/kernel.h>
@@ -69,7 +69,7 @@
 
 /* Bits in as_io_context.state */
 enum as_io_states {
-	AS_TASK_RUNNING=0,	/* Process has not exitted */
+	AS_TASK_RUNNING=0,	/* Process has not exited */
 	AS_TASK_IOSTARTED,	/* Process has started some IO */
 	AS_TASK_IORUNNING,	/* Process has completed some IO */
 };
@@ -102,6 +102,9 @@ struct as_data {
 
 	unsigned long exit_prob;	/* probability a task will exit while
 					   being waited on */
+	unsigned long exit_no_coop;	/* probablility an exited task will
+					   not be part of a later cooperating
+					   request */
 	unsigned long new_ttime_total; 	/* mean thinktime on new proc */
 	unsigned long new_ttime_mean;
 	u64 new_seek_total;		/* mean seek on new proc */
@@ -636,37 +639,152 @@ static void as_antic_timeout(unsigned long data)
 		kblockd_schedule_work(&ad->antic_work);
 
 		if (aic->ttime_samples == 0) {
-			/* process anticipated on has exitted or timed out*/
+			/* process anticipated on has exited or timed out*/
 			ad->exit_prob = (7*ad->exit_prob + 256)/8;
 		}
+		if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
+			/* process not "saved" by a cooperating request */
+			ad->exit_no_coop = (7*ad->exit_no_coop + 256)/8;
+		}
 	}
 	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
+static void as_update_thinktime(struct as_data *ad, struct as_io_context *aic,
+				unsigned long ttime)
+{
+	/* fixed point: 1.0 == 1<<8 */
+	if (aic->ttime_samples == 0) {
+		ad->new_ttime_total = (7*ad->new_ttime_total + 256*ttime) / 8;
+		ad->new_ttime_mean = ad->new_ttime_total / 256;
+
+		ad->exit_prob = (7*ad->exit_prob)/8;
+	}
+	aic->ttime_samples = (7*aic->ttime_samples + 256) / 8;
+	aic->ttime_total = (7*aic->ttime_total + 256*ttime) / 8;
+	aic->ttime_mean = (aic->ttime_total + 128) / aic->ttime_samples;
+}
+
+static void as_update_seekdist(struct as_data *ad, struct as_io_context *aic,
+				sector_t sdist)
+{
+	u64 total;
+
+	if (aic->seek_samples == 0) {
+		ad->new_seek_total = (7*ad->new_seek_total + 256*(u64)sdist)/8;
+		ad->new_seek_mean = ad->new_seek_total / 256;
+	}
+
+	/*
+	 * Don't allow the seek distance to get too large from the
+	 * odd fragment, pagein, etc
+	 */
+	if (aic->seek_samples <= 60) /* second&third seek */
+		sdist = min(sdist, (aic->seek_mean * 4) + 2*1024*1024);
+	else
+		sdist = min(sdist, (aic->seek_mean * 4)	+ 2*1024*64);
+
+	aic->seek_samples = (7*aic->seek_samples + 256) / 8;
+	aic->seek_total = (7*aic->seek_total + (u64)256*sdist) / 8;
+	total = aic->seek_total + (aic->seek_samples/2);
+	do_div(total, aic->seek_samples);
+	aic->seek_mean = (sector_t)total;
+}
+
+/*
+ * as_update_iohist keeps a decaying histogram of IO thinktimes, and
+ * updates @aic->ttime_mean based on that. It is called when a new
+ * request is queued.
+ */
+static void as_update_iohist(struct as_data *ad, struct as_io_context *aic,
+				struct request *rq)
+{
+	struct as_rq *arq = RQ_DATA(rq);
+	int data_dir = arq->is_sync;
+	unsigned long thinktime = 0;
+	sector_t seek_dist;
+
+	if (aic == NULL)
+		return;
+
+	if (data_dir == REQ_SYNC) {
+		unsigned long in_flight = atomic_read(&aic->nr_queued)
+					+ atomic_read(&aic->nr_dispatched);
+		spin_lock(&aic->lock);
+		if (test_bit(AS_TASK_IORUNNING, &aic->state) ||
+			test_bit(AS_TASK_IOSTARTED, &aic->state)) {
+			/* Calculate read -> read thinktime */
+			if (test_bit(AS_TASK_IORUNNING, &aic->state)
+							&& in_flight == 0) {
+				thinktime = jiffies - aic->last_end_request;
+				thinktime = min(thinktime, MAX_THINKTIME-1);
+			}
+			as_update_thinktime(ad, aic, thinktime);
+
+			/* Calculate read -> read seek distance */
+			if (aic->last_request_pos < rq->sector)
+				seek_dist = rq->sector - aic->last_request_pos;
+			else
+				seek_dist = aic->last_request_pos - rq->sector;
+			as_update_seekdist(ad, aic, seek_dist);
+		}
+		aic->last_request_pos = rq->sector + rq->nr_sectors;
+		set_bit(AS_TASK_IOSTARTED, &aic->state);
+		spin_unlock(&aic->lock);
+	}
+}
+
 /*
  * as_close_req decides if one request is considered "close" to the
  * previous one issued.
  */
-static int as_close_req(struct as_data *ad, struct as_rq *arq)
+static int as_close_req(struct as_data *ad, struct as_io_context *aic,
+				struct as_rq *arq)
 {
 	unsigned long delay;	/* milliseconds */
 	sector_t last = ad->last_sector[ad->batch_data_dir];
 	sector_t next = arq->request->sector;
 	sector_t delta; /* acceptable close offset (in sectors) */
+	sector_t s;
 
 	if (ad->antic_status == ANTIC_OFF || !ad->ioc_finished)
 		delay = 0;
 	else
 		delay = ((jiffies - ad->antic_start) * 1000) / HZ;
 
-	if (delay <= 1)
-		delta = 64;
+	if (delay == 0)
+		delta = 8192;
 	else if (delay <= 20 && delay <= ad->antic_expire)
-		delta = 64 << (delay-1);
+		delta = 8192 << delay;
 	else
 		return 1;
 
-	return (last - (delta>>1) <= next) && (next <= last + delta);
+	if ((last <= next + (delta>>1)) && (next <= last + delta))
+		return 1;
+
+	if (last < next)
+		s = next - last;
+	else
+		s = last - next;
+
+	if (aic->seek_samples == 0) {
+		/*
+		 * Process has just started IO. Use past statistics to
+		 * gauge success possibility
+		 */
+		if (ad->new_seek_mean > s) {
+			/* this request is better than what we're expecting */
+			return 1;
+		}
+
+	} else {
+		if (aic->seek_mean > s) {
+			/* this request is better than what we're expecting */
+			return 1;
+		}
+	}
+
+	return 0;
 }
 
 /*
@@ -678,7 +796,7 @@ static int as_close_req(struct as_data *ad, struct as_rq *arq)
  * dispatch it ASAP, because we know that application will not be submitting
  * any new reads.
  *
- * If the task which has submitted the request has exitted, break anticipation.
+ * If the task which has submitted the request has exited, break anticipation.
  *
  * If this task has queued some other IO, do not enter enticipation.
  */
@@ -686,7 +804,6 @@ static int as_can_break_anticipation(struct as_data *ad, struct as_rq *arq)
 {
 	struct io_context *ioc;
 	struct as_io_context *aic;
-	sector_t s;
 
 	ioc = ad->io_context;
 	BUG_ON(!ioc);
@@ -708,13 +825,6 @@ static int as_can_break_anticipation(struct as_data *ad, struct as_rq *arq)
 	if (!aic)
 		return 0;
 
-	if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
-		/* process anticipated on has exitted */
-		if (aic->ttime_samples == 0)
-			ad->exit_prob = (7*ad->exit_prob + 256)/8;
-		return 1;
-	}
-
 	if (atomic_read(&aic->nr_queued) > 0) {
 		/* process has more requests queued */
 		return 1;
@@ -725,57 +835,45 @@ static int as_can_break_anticipation(struct as_data *ad, struct as_rq *arq)
 		return 1;
 	}
 
-	if (arq && arq->is_sync == REQ_SYNC && as_close_req(ad, arq)) {
+	if (arq && arq->is_sync == REQ_SYNC && as_close_req(ad, aic, arq)) {
 		/*
 		 * Found a close request that is not one of ours.
 		 *
-		 * This makes close requests from another process reset
-		 * our thinktime delay. Is generally useful when there are
+		 * This makes close requests from another process update
+		 * our IO history. Is generally useful when there are
 		 * two or more cooperating processes working in the same
 		 * area.
 		 */
-		spin_lock(&aic->lock);
-		aic->last_end_request = jiffies;
-		spin_unlock(&aic->lock);
+		if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
+			if (aic->ttime_samples == 0)
+				ad->exit_prob = (7*ad->exit_prob + 256)/8;
+
+			ad->exit_no_coop = (7*ad->exit_no_coop)/8;
+		}
+
+		as_update_iohist(ad, aic, arq->request);
 		return 1;
 	}
 
+	if (!test_bit(AS_TASK_RUNNING, &aic->state)) {
+		/* process anticipated on has exited */
+		if (aic->ttime_samples == 0)
+			ad->exit_prob = (7*ad->exit_prob + 256)/8;
+
+		if (ad->exit_no_coop > 128)
+			return 1;
+	}
 
 	if (aic->ttime_samples == 0) {
 		if (ad->new_ttime_mean > ad->antic_expire)
 			return 1;
-		if (ad->exit_prob > 128)
+		if (ad->exit_prob * ad->exit_no_coop > 128*256)
 			return 1;
 	} else if (aic->ttime_mean > ad->antic_expire) {
 		/* the process thinks too much between requests */
 		return 1;
 	}
 
-	if (!arq)
-		return 0;
-
-	if (ad->last_sector[REQ_SYNC] < arq->request->sector)
-		s = arq->request->sector - ad->last_sector[REQ_SYNC];
-	else
-		s = ad->last_sector[REQ_SYNC] - arq->request->sector;
-
-	if (aic->seek_samples == 0) {
-		/*
-		 * Process has just started IO. Use past statistics to
-		 * guage success possibility
-		 */
-		if (ad->new_seek_mean > s) {
-			/* this request is better than what we're expecting */
-			return 1;
-		}
-
-	} else {
-		if (aic->seek_mean > s) {
-			/* this request is better than what we're expecting */
-			return 1;
-		}
-	}
-
 	return 0;
 }
 
@@ -809,94 +907,11 @@ static int as_can_anticipate(struct as_data *ad, struct as_rq *arq)
 	 * Status is either ANTIC_OFF so start waiting,
 	 * ANTIC_WAIT_REQ so continue waiting for request to finish
 	 * or ANTIC_WAIT_NEXT so continue waiting for an acceptable request.
-	 *
 	 */
 
 	return 1;
 }
 
-static void as_update_thinktime(struct as_data *ad, struct as_io_context *aic, unsigned long ttime)
-{
-	/* fixed point: 1.0 == 1<<8 */
-	if (aic->ttime_samples == 0) {
-		ad->new_ttime_total = (7*ad->new_ttime_total + 256*ttime) / 8;
-		ad->new_ttime_mean = ad->new_ttime_total / 256;
-
-		ad->exit_prob = (7*ad->exit_prob)/8;
-	}
-	aic->ttime_samples = (7*aic->ttime_samples + 256) / 8;
-	aic->ttime_total = (7*aic->ttime_total + 256*ttime) / 8;
-	aic->ttime_mean = (aic->ttime_total + 128) / aic->ttime_samples;
-}
-
-static void as_update_seekdist(struct as_data *ad, struct as_io_context *aic, sector_t sdist)
-{
-	u64 total;
-
-	if (aic->seek_samples == 0) {
-		ad->new_seek_total = (7*ad->new_seek_total + 256*(u64)sdist)/8;
-		ad->new_seek_mean = ad->new_seek_total / 256;
-	}
-
-	/*
-	 * Don't allow the seek distance to get too large from the
-	 * odd fragment, pagein, etc
-	 */
-	if (aic->seek_samples <= 60) /* second&third seek */
-		sdist = min(sdist, (aic->seek_mean * 4) + 2*1024*1024);
-	else
-		sdist = min(sdist, (aic->seek_mean * 4)	+ 2*1024*64);
-
-	aic->seek_samples = (7*aic->seek_samples + 256) / 8;
-	aic->seek_total = (7*aic->seek_total + (u64)256*sdist) / 8;
-	total = aic->seek_total + (aic->seek_samples/2);
-	do_div(total, aic->seek_samples);
-	aic->seek_mean = (sector_t)total;
-}
-
-/*
- * as_update_iohist keeps a decaying histogram of IO thinktimes, and
- * updates @aic->ttime_mean based on that. It is called when a new
- * request is queued.
- */
-static void as_update_iohist(struct as_data *ad, struct as_io_context *aic, struct request *rq)
-{
-	struct as_rq *arq = RQ_DATA(rq);
-	int data_dir = arq->is_sync;
-	unsigned long thinktime;
-	sector_t seek_dist;
-
-	if (aic == NULL)
-		return;
-
-	if (data_dir == REQ_SYNC) {
-		unsigned long in_flight = atomic_read(&aic->nr_queued)
-					+ atomic_read(&aic->nr_dispatched);
-		spin_lock(&aic->lock);
-		if (test_bit(AS_TASK_IORUNNING, &aic->state) ||
-			test_bit(AS_TASK_IOSTARTED, &aic->state)) {
-			/* Calculate read -> read thinktime */
-			if (test_bit(AS_TASK_IORUNNING, &aic->state)
-							&& in_flight == 0) {
-				thinktime = jiffies - aic->last_end_request;
-				thinktime = min(thinktime, MAX_THINKTIME-1);
-			} else
-				thinktime = 0;
-			as_update_thinktime(ad, aic, thinktime);
-
-			/* Calculate read -> read seek distance */
-			if (aic->last_request_pos < rq->sector)
-				seek_dist = rq->sector - aic->last_request_pos;
-			else
-				seek_dist = aic->last_request_pos - rq->sector;
-			as_update_seekdist(ad, aic, seek_dist);
-		}
-		aic->last_request_pos = rq->sector + rq->nr_sectors;
-		set_bit(AS_TASK_IOSTARTED, &aic->state);
-		spin_unlock(&aic->lock);
-	}
-}
-
 /*
  * as_update_arq must be called whenever a request (arq) is added to
  * the sort_list. This function keeps caches up to date, and checks if the
@@ -1201,7 +1216,7 @@ static int as_dispatch_request(request_queue_t *q, int force)
 		|| ad->changed_batch)
 		return 0;
 
-	if (!(reads && writes && as_batch_expired(ad)) ) {
+	if (!(reads && writes && as_batch_expired(ad))) {
 		/*
 		 * batch is still running or no reads or no writes
 		 */
@@ -1316,7 +1331,8 @@ fifo_expired:
  * Add arq to a list behind alias
  */
 static inline void
-as_add_aliased_request(struct as_data *ad, struct as_rq *arq, struct as_rq *alias)
+as_add_aliased_request(struct as_data *ad, struct as_rq *arq,
+				struct as_rq *alias)
 {
 	struct request  *req = arq->request;
 	struct list_head *insert = alias->request->queuelist.prev;
@@ -1441,8 +1457,8 @@ static int as_queue_empty(request_queue_t *q)
 		&& list_empty(&ad->fifo_list[REQ_SYNC]);
 }
 
-static struct request *
-as_former_request(request_queue_t *q, struct request *rq)
+static struct request *as_former_request(request_queue_t *q,
+					struct request *rq)
 {
 	struct as_rq *arq = RQ_DATA(rq);
 	struct rb_node *rbprev = rb_prev(&arq->rb_node);
@@ -1454,8 +1470,8 @@ as_former_request(request_queue_t *q, struct request *rq)
 	return ret;
 }
 
-static struct request *
-as_latter_request(request_queue_t *q, struct request *rq)
+static struct request *as_latter_request(request_queue_t *q,
+					struct request *rq)
 {
 	struct as_rq *arq = RQ_DATA(rq);
 	struct rb_node *rbnext = rb_next(&arq->rb_node);
@@ -1537,7 +1553,7 @@ static void as_merged_request(request_queue_t *q, struct request *req)
 		 * currently don't bother. Ditto the next function.
 		 */
 		as_del_arq_rb(ad, arq);
-		if ((alias = as_add_arq_rb(ad, arq)) ) {
+		if ((alias = as_add_arq_rb(ad, arq))) {
 			list_del_init(&arq->fifo);
 			as_add_aliased_request(ad, arq, alias);
 			if (next_arq)
@@ -1551,8 +1567,7 @@ static void as_merged_request(request_queue_t *q, struct request *req)
 	}
 }
 
-static void
-as_merged_requests(request_queue_t *q, struct request *req,
+static void as_merged_requests(request_queue_t *q, struct request *req,
 			 	struct request *next)
 {
 	struct as_data *ad = q->elevator->elevator_data;
@@ -1576,7 +1591,7 @@ as_merged_requests(request_queue_t *q, struct request *req,
 			next_arq = as_find_next_arq(ad, arq);
 
 		as_del_arq_rb(ad, arq);
-		if ((alias = as_add_arq_rb(ad, arq)) ) {
+		if ((alias = as_add_arq_rb(ad, arq))) {
 			list_del_init(&arq->fifo);
 			as_add_aliased_request(ad, arq, alias);
 			if (next_arq)
@@ -1806,9 +1821,14 @@ static ssize_t as_est_show(struct as_data *ad, char *page)
 {
 	int pos = 0;
 
-	pos += sprintf(page+pos, "%lu %% exit probability\n", 100*ad->exit_prob/256);
+	pos += sprintf(page+pos, "%lu %% exit probability\n",
+				100*ad->exit_prob/256);
+	pos += sprintf(page+pos, "%lu %% probability of exiting without a "
+				"cooperating process submitting IO\n",
+				100*ad->exit_no_coop/256);
 	pos += sprintf(page+pos, "%lu ms new thinktime\n", ad->new_ttime_mean);
-	pos += sprintf(page+pos, "%llu sectors new seek distance\n", (unsigned long long)ad->new_seek_mean);
+	pos += sprintf(page+pos, "%llu sectors new seek distance\n",
+				(unsigned long long)ad->new_seek_mean);
 
 	return pos;
 }