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[PATCHSET v5] Make background writeback great again for the first time

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  [PATCHSET v5] Make background writeback great again for the first time Jens Axboe <axboe@fb.com> - 2016-04-26 18:00 +0200
    [PATCH 6/8] block: add scalable completion tracking of requests Jens Axboe <axboe@fb.com> - 2016-04-26 18:00 +0200
    Re: [PATCHSET v5] Make background writeback great again for the  first time Jan Kara <jack@suse.cz> - 2016-04-27 20:10 +0200
      Re: [PATCHSET v5] Make background writeback great again for the first  time Jens Axboe <axboe@fb.com> - 2016-04-27 20:20 +0200
        Re: [PATCHSET v5] Make background writeback great again for the  first time Jens Axboe <axboe@kernel.dk> - 2016-04-27 22:40 +0200
          Re: [PATCHSET v5] Make background writeback great again for the  first time Jens Axboe <axboe@kernel.dk> - 2016-04-27 23:00 +0200
            Re: [PATCHSET v5] Make background writeback great again for the first  time xiakaixu <xiakaixu@huawei.com> - 2016-04-28 06:10 +0200

#1387653 — [PATCHSET v5] Make background writeback great again for the first time

Hi,

Since the dawn of time, our background buffered writeback has sucked.
When we do background buffered writeback, it should have little impact
on foreground activity. That's the definition of background activity...
But for as long as I can remember, heavy buffered writers have not
behaved like that. For instance, if I do something like this:

$ dd if=/dev/zero of=foo bs=1M count=10k

on my laptop, and then try and start chrome, it basically won't start
before the buffered writeback is done. Or, for server oriented
workloads, where installation of a big RPM (or similar) adversely
impacts database reads or sync writes. When that happens, I get people
yelling at me.

I have posted plenty of results previously, I'll keep it shorter
this time. Here's a run on my laptop, using read-to-pipe-async for
reading a 5g file, and rewriting it. You can find this test program
in the fio git repo.

4.6-rc3:

$ t/read-to-pipe-async -f ~/5g > 5g-new

Latency percentiles (usec) (READERS)
	50.0000th: 2
	75.0000th: 3
	90.0000th: 5
	95.0000th: 7
	99.0000th: 43
	99.5000th: 77
	99.9000th: 9008
	99.9900th: 91008
	99.9990th: 286208
	99.9999th: 347648
	Over=1251, min=0, max=358081
Latency percentiles (usec) (WRITERS)
	50.0000th: 4
	75.0000th: 8
	90.0000th: 13
	95.0000th: 15
	99.0000th: 32
	99.5000th: 43
	99.9000th: 81
	99.9900th: 2372
	99.9990th: 104320
	99.9999th: 349696
	Over=63, min=1, max=358321
Read rate (KB/sec) : 91859
Write rate (KB/sec): 91859

4.6-rc3 + wb-buf-throttle

Latency percentiles (usec) (READERS)
	50.0000th: 2
	75.0000th: 3
	90.0000th: 5
	95.0000th: 8
	99.0000th: 48
	99.5000th: 79
	99.9000th: 5304
	99.9900th: 22496
	99.9990th: 29408
	99.9999th: 33728
	Over=860, min=0, max=37599
Latency percentiles (usec) (WRITERS)
	50.0000th: 4
	75.0000th: 9
	90.0000th: 14
	95.0000th: 16
	99.0000th: 34
	99.5000th: 45
	99.9000th: 87
	99.9900th: 1342
	99.9990th: 13648
	99.9999th: 21280
	Over=29, min=1, max=30457
Read rate (KB/sec) : 95832
Write rate (KB/sec): 95832

Better throughput and tighter latencies, for both reads and writes.
That's hard not to like.

The above was the why. The how is basically throttling background
writeback. We still want to issue big writes from the vm side of things,
so we get nice and big extents on the file system end. But we don't need
to flood the device with THOUSANDS of requests for background writeback.
For most devices, we don't need a whole lot to get decent throughput.

This adds some simple blk-wb code that keeps limits how much buffered
writeback we keep in flight on the device end. It's all about managing
the queues on the hardware side. The big change in this version is that
it should be pretty much auto-tuning - you no longer have to set a
given percentage of writeback bandwidth. I've implemented something
similar to CoDel to manage the writeback queue. See the last patch
for a full description, but the tldr is that we monitor min latencies
over a window of time, and scale up/down the queue based on that. This
needs a minimum of tunables, and it stays out of the way, if your device
is fast enough. There's a single tunable now, wb_last_usec, that simply
sets this latency target. Most people won't have to touch this, it'll
work pretty well just being in the ballpark.

I welcome testing. If you are sick of Linux bogging down when buffered
writes are happening, then this is for you, laptop or server. The
patchset is fully stable, I have not observed problems. It passes full
xfstest runs, and a variety of benchmarks as well. It works equally well
on blk-mq/scsi-mq, and "classic" setups.

You can also find this in a branch in the block git repo:

git://git.kernel.dk/linux-block.git wb-buf-throttle

Note that I rebase this branch when I collapse patches. The
wb-buf-throttle-v5 will remain the same as this version. I've folded
the device write cache changes into my 4.7 branches, so they are not
a part of this posting. Get the full wb-buf-throttle branch, or apply
the patches here on top of my for-next. A full patch against Linus'
current tree can also be downloaded here:

http://brick.kernel.dk/snaps/wb-buf-throttle-v5.patch

Changes since v4

- Add some documentation for the two queue sysfs files
- Kill off wb_stats sysfs file. Use the trace points to get this info
  now.
- Various work around making this block layer agnostic. The main code
  now resides in lib/wbt.c and can be plugged into NFS as well, for
  instance.
- Fix an issue with double completions on the block layer side.
- Fix an issue where a long sync issue was disregarded, if the stat
  sample weren't valid.
- Speed up the division in rwb_arm_timer().
- Add logic to scale back up for 'unknown' latency events.
- Don't track sync issue timestamp of wbt is disabled.
- Drop the dirty/writeback page inc/dec patch. We don't need it, and
  it was racy.
- Move block/blk-wb.c to lib/wbt.c

Changes since v3

- Re-do the mm/ writheback parts. Add REQ_BG for background writes,
  and don't overload the wbc 'reason' for writeback decisions.
- Add tracking for when apps are sleeping waiting for a page to complete.
- Change wbc_to_write() to wbc_to_write_cmd().
- Use atomic_t for the balance_dirty_pages() sleep count.
- Add a basic scalable block stats tracking framework.
- Rewrite blk-wb core as described above, to dynamically adapt. This is
  a big change, see the last patch for a full description of it.
- Add tracing to blk-wb, instead of using debug printk's.
- Rebased to 4.6-rc3 (ish)

Changes since v2

- Switch from wb_depth to wb_percent, as that's an easier tunable.
- Add the patch to track device depth on the block layer side.
- Cleanup the limiting code.
- Don't use a fixed limit in the wb wait, since it can change
  between wakeups.
- Minor tweaks, fixups, cleanups.

Changes since v1

- Drop sync() WB_SYNC_NONE -> WB_SYNC_ALL change
- wb_start_writeback() fills in background/reclaim/sync info in
  the writeback work, based on writeback reason.
- Use WRITE_SYNC for reclaim/sync IO
- Split balance_dirty_pages() sleep change into separate patch
- Drop get_request() u64 flag change, set the bit on the request
  directly after-the-fact.
- Fix wrong sysfs return value
- Various small cleanups


 Documentation/block/queue-sysfs.txt             |   22 +
 Documentation/block/writeback_cache_control.txt |    4 
 arch/um/drivers/ubd_kern.c                      |    2 
 block/Kconfig                                   |    1 
 block/Makefile                                  |    2 
 block/blk-core.c                                |   26 +
 block/blk-flush.c                               |   11 
 block/blk-mq-sysfs.c                            |   47 ++
 block/blk-mq.c                                  |   44 +-
 block/blk-mq.h                                  |    3 
 block/blk-settings.c                            |   59 +-
 block/blk-stat.c                                |  185 ++++++++
 block/blk-stat.h                                |   17 
 block/blk-sysfs.c                               |  184 ++++++++
 drivers/block/drbd/drbd_main.c                  |    2 
 drivers/block/loop.c                            |    2 
 drivers/block/mtip32xx/mtip32xx.c               |    6 
 drivers/block/nbd.c                             |    4 
 drivers/block/osdblk.c                          |    2 
 drivers/block/ps3disk.c                         |    2 
 drivers/block/skd_main.c                        |    2 
 drivers/block/virtio_blk.c                      |    6 
 drivers/block/xen-blkback/xenbus.c              |    2 
 drivers/block/xen-blkfront.c                    |    3 
 drivers/ide/ide-disk.c                          |    6 
 drivers/md/bcache/super.c                       |    2 
 drivers/md/dm-table.c                           |   20 
 drivers/md/md.c                                 |    2 
 drivers/md/raid5-cache.c                        |    3 
 drivers/mmc/card/block.c                        |    2 
 drivers/mtd/mtd_blkdevs.c                       |    2 
 drivers/nvme/host/core.c                        |    7 
 drivers/scsi/scsi.c                             |    3 
 drivers/scsi/sd.c                               |    8 
 drivers/target/target_core_iblock.c             |    6 
 fs/block_dev.c                                  |    2 
 fs/buffer.c                                     |    2 
 fs/f2fs/data.c                                  |    2 
 fs/f2fs/node.c                                  |    2 
 fs/gfs2/meta_io.c                               |    3 
 fs/mpage.c                                      |    9 
 fs/xfs/xfs_aops.c                               |    2 
 include/linux/backing-dev-defs.h                |    2 
 include/linux/blk_types.h                       |   12 
 include/linux/blkdev.h                          |   28 +
 include/linux/fs.h                              |    4 
 include/linux/wbt.h                             |   95 ++++
 include/linux/writeback.h                       |   10 
 include/trace/events/wbt.h                      |  122 +++++
 lib/Kconfig                                     |    3 
 lib/Makefile                                    |    1 
 lib/wbt.c                                       |  524 ++++++++++++++++++++++++
 mm/backing-dev.c                                |    1 
 mm/page-writeback.c                             |    2 
 54 files changed, 1429 insertions(+), 96 deletions(-)

-- 
Jens Axboe

[toc] | [next] | [standalone]

#1387654 — [PATCH 6/8] block: add scalable completion tracking of requests

For legacy block, we simply track them in the request queue. For
blk-mq, we track them on a per-sw queue basis, which we can then
sum up through the hardware queues and finally to a per device
state.

The stats are tracked in, roughly, 0.1s interval windows.

Add sysfs files to display the stats.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 block/Makefile            |   2 +-
 block/blk-core.c          |   4 +
 block/blk-mq-sysfs.c      |  47 ++++++++++++
 block/blk-mq.c            |  14 ++++
 block/blk-mq.h            |   3 +
 block/blk-stat.c          | 184 ++++++++++++++++++++++++++++++++++++++++++++++
 block/blk-stat.h          |  17 +++++
 block/blk-sysfs.c         |  26 +++++++
 include/linux/blk_types.h |   8 ++
 include/linux/blkdev.h    |   4 +
 10 files changed, 308 insertions(+), 1 deletion(-)
 create mode 100644 block/blk-stat.c
 create mode 100644 block/blk-stat.h

diff --git a/block/Makefile b/block/Makefile
index 9eda2322b2d4..3446e0472df0 100644
--- a/block/Makefile
+++ b/block/Makefile
@@ -5,7 +5,7 @@
 obj-$(CONFIG_BLOCK) := bio.o elevator.o blk-core.o blk-tag.o blk-sysfs.o \
 			blk-flush.o blk-settings.o blk-ioc.o blk-map.o \
 			blk-exec.o blk-merge.o blk-softirq.o blk-timeout.o \
-			blk-lib.o blk-mq.o blk-mq-tag.o \
+			blk-lib.o blk-mq.o blk-mq-tag.o blk-stat.o \
 			blk-mq-sysfs.o blk-mq-cpu.o blk-mq-cpumap.o ioctl.o \
 			genhd.o scsi_ioctl.o partition-generic.o ioprio.o \
 			badblocks.o partitions/
diff --git a/block/blk-core.c b/block/blk-core.c
index 74c16fd8995d..40b57bf4852c 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2514,6 +2514,8 @@ void blk_start_request(struct request *req)
 {
 	blk_dequeue_request(req);
 
+	req->issue_time = ktime_to_ns(ktime_get());
+
 	/*
 	 * We are now handing the request to the hardware, initialize
 	 * resid_len to full count and add the timeout handler.
@@ -2581,6 +2583,8 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
 
 	trace_block_rq_complete(req->q, req, nr_bytes);
 
+	blk_stat_add(&req->q->rq_stats[rq_data_dir(req)], req);
+
 	if (!req->bio)
 		return false;
 
diff --git a/block/blk-mq-sysfs.c b/block/blk-mq-sysfs.c
index 4ea4dd8a1eed..2f68015f8616 100644
--- a/block/blk-mq-sysfs.c
+++ b/block/blk-mq-sysfs.c
@@ -247,6 +247,47 @@ static ssize_t blk_mq_hw_sysfs_cpus_show(struct blk_mq_hw_ctx *hctx, char *page)
 	return ret;
 }
 
+static void blk_mq_stat_clear(struct blk_mq_hw_ctx *hctx)
+{
+	struct blk_mq_ctx *ctx;
+	unsigned int i;
+
+	hctx_for_each_ctx(hctx, ctx, i) {
+		blk_stat_init(&ctx->stat[0]);
+		blk_stat_init(&ctx->stat[1]);
+	}
+}
+
+static ssize_t blk_mq_hw_sysfs_stat_store(struct blk_mq_hw_ctx *hctx,
+					  const char *page, size_t count)
+{
+	blk_mq_stat_clear(hctx);
+	return count;
+}
+
+static ssize_t print_stat(char *page, struct blk_rq_stat *stat, const char *pre)
+{
+	return sprintf(page, "%s samples=%llu, mean=%lld, min=%lld, max=%lld\n",
+			pre, (long long) stat->nr_samples,
+			(long long) stat->mean, (long long) stat->min,
+			(long long) stat->max);
+}
+
+static ssize_t blk_mq_hw_sysfs_stat_show(struct blk_mq_hw_ctx *hctx, char *page)
+{
+	struct blk_rq_stat stat[2];
+	ssize_t ret;
+
+	blk_stat_init(&stat[0]);
+	blk_stat_init(&stat[1]);
+
+	blk_hctx_stat_get(hctx, stat);
+
+	ret = print_stat(page, &stat[0], "read :");
+	ret += print_stat(page + ret, &stat[1], "write:");
+	return ret;
+}
+
 static struct blk_mq_ctx_sysfs_entry blk_mq_sysfs_dispatched = {
 	.attr = {.name = "dispatched", .mode = S_IRUGO },
 	.show = blk_mq_sysfs_dispatched_show,
@@ -304,6 +345,11 @@ static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_poll = {
 	.attr = {.name = "io_poll", .mode = S_IRUGO },
 	.show = blk_mq_hw_sysfs_poll_show,
 };
+static struct blk_mq_hw_ctx_sysfs_entry blk_mq_hw_sysfs_stat = {
+	.attr = {.name = "stats", .mode = S_IRUGO | S_IWUSR },
+	.show = blk_mq_hw_sysfs_stat_show,
+	.store = blk_mq_hw_sysfs_stat_store,
+};
 
 static struct attribute *default_hw_ctx_attrs[] = {
 	&blk_mq_hw_sysfs_queued.attr,
@@ -314,6 +360,7 @@ static struct attribute *default_hw_ctx_attrs[] = {
 	&blk_mq_hw_sysfs_cpus.attr,
 	&blk_mq_hw_sysfs_active.attr,
 	&blk_mq_hw_sysfs_poll.attr,
+	&blk_mq_hw_sysfs_stat.attr,
 	NULL,
 };
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1699baf39b78..71b4a13fbf94 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -29,6 +29,7 @@
 #include "blk.h"
 #include "blk-mq.h"
 #include "blk-mq-tag.h"
+#include "blk-stat.h"
 
 static DEFINE_MUTEX(all_q_mutex);
 static LIST_HEAD(all_q_list);
@@ -356,10 +357,19 @@ static void blk_mq_ipi_complete_request(struct request *rq)
 	put_cpu();
 }
 
+static void blk_mq_stat_add(struct request *rq)
+{
+	struct blk_rq_stat *stat = &rq->mq_ctx->stat[rq_data_dir(rq)];
+
+	blk_stat_add(stat, rq);
+}
+
 static void __blk_mq_complete_request(struct request *rq)
 {
 	struct request_queue *q = rq->q;
 
+	blk_mq_stat_add(rq);
+
 	if (!q->softirq_done_fn)
 		blk_mq_end_request(rq, rq->errors);
 	else
@@ -403,6 +413,8 @@ void blk_mq_start_request(struct request *rq)
 	if (unlikely(blk_bidi_rq(rq)))
 		rq->next_rq->resid_len = blk_rq_bytes(rq->next_rq);
 
+	rq->issue_time = ktime_to_ns(ktime_get());
+
 	blk_add_timer(rq);
 
 	/*
@@ -1761,6 +1773,8 @@ static void blk_mq_init_cpu_queues(struct request_queue *q,
 		spin_lock_init(&__ctx->lock);
 		INIT_LIST_HEAD(&__ctx->rq_list);
 		__ctx->queue = q;
+		blk_stat_init(&__ctx->stat[0]);
+		blk_stat_init(&__ctx->stat[1]);
 
 		/* If the cpu isn't online, the cpu is mapped to first hctx */
 		if (!cpu_online(i))
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 9087b11037b7..e107f700ff17 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -1,6 +1,8 @@
 #ifndef INT_BLK_MQ_H
 #define INT_BLK_MQ_H
 
+#include "blk-stat.h"
+
 struct blk_mq_tag_set;
 
 struct blk_mq_ctx {
@@ -20,6 +22,7 @@ struct blk_mq_ctx {
 
 	/* incremented at completion time */
 	unsigned long		____cacheline_aligned_in_smp rq_completed[2];
+	struct blk_rq_stat	stat[2];
 
 	struct request_queue	*queue;
 	struct kobject		kobj;
diff --git a/block/blk-stat.c b/block/blk-stat.c
new file mode 100644
index 000000000000..b38776a83173
--- /dev/null
+++ b/block/blk-stat.c
@@ -0,0 +1,184 @@
+/*
+ * Block stat tracking code
+ *
+ * Copyright (C) 2016 Jens Axboe
+ */
+#include <linux/kernel.h>
+#include <linux/blk-mq.h>
+
+#include "blk-stat.h"
+#include "blk-mq.h"
+
+void blk_stat_sum(struct blk_rq_stat *dst, struct blk_rq_stat *src)
+{
+	if (!src->nr_samples)
+		return;
+
+	dst->min = min(dst->min, src->min);
+	dst->max = max(dst->max, src->max);
+
+	if (!dst->nr_samples)
+		dst->mean = src->mean;
+	else {
+		dst->mean = div64_s64((src->mean * src->nr_samples) +
+					(dst->mean * dst->nr_samples),
+					dst->nr_samples + src->nr_samples);
+	}
+	dst->nr_samples += src->nr_samples;
+}
+
+static void blk_mq_stat_get(struct request_queue *q, struct blk_rq_stat *dst)
+{
+	struct blk_mq_hw_ctx *hctx;
+	struct blk_mq_ctx *ctx;
+	int i, j, nr;
+
+	blk_stat_init(&dst[0]);
+	blk_stat_init(&dst[1]);
+
+	nr = 0;
+	do {
+		uint64_t newest = 0;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				if (!ctx->stat[0].nr_samples &&
+				    !ctx->stat[1].nr_samples)
+					continue;
+				if (ctx->stat[0].time > newest)
+					newest = ctx->stat[0].time;
+				if (ctx->stat[1].time > newest)
+					newest = ctx->stat[1].time;
+			}
+		}
+
+		/*
+		 * No samples
+		 */
+		if (!newest)
+			break;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				if (ctx->stat[0].time == newest) {
+					blk_stat_sum(&dst[0], &ctx->stat[0]);
+					nr++;
+				}
+				if (ctx->stat[1].time == newest) {
+					blk_stat_sum(&dst[1], &ctx->stat[1]);
+					nr++;
+				}
+			}
+		}
+		/*
+		 * If we race on finding an entry, just loop back again.
+		 * Should be very rare.
+		 */
+	} while (!nr);
+}
+
+void blk_queue_stat_get(struct request_queue *q, struct blk_rq_stat *dst)
+{
+	if (q->mq_ops)
+		blk_mq_stat_get(q, dst);
+	else {
+		memcpy(&dst[0], &q->rq_stats[0], sizeof(struct blk_rq_stat));
+		memcpy(&dst[1], &q->rq_stats[1], sizeof(struct blk_rq_stat));
+	}
+}
+
+void blk_hctx_stat_get(struct blk_mq_hw_ctx *hctx, struct blk_rq_stat *dst)
+{
+	struct blk_mq_ctx *ctx;
+	unsigned int i, nr;
+
+	nr = 0;
+	do {
+		uint64_t newest = 0;
+
+		hctx_for_each_ctx(hctx, ctx, i) {
+			if (!ctx->stat[0].nr_samples &&
+			    !ctx->stat[1].nr_samples)
+				continue;
+
+			if (ctx->stat[0].time > newest)
+				newest = ctx->stat[0].time;
+			if (ctx->stat[1].time > newest)
+				newest = ctx->stat[1].time;
+		}
+
+		if (!newest)
+			break;
+
+		hctx_for_each_ctx(hctx, ctx, i) {
+			if (ctx->stat[0].time == newest) {
+				blk_stat_sum(&dst[0], &ctx->stat[0]);
+				nr++;
+			}
+			if (ctx->stat[1].time == newest) {
+				blk_stat_sum(&dst[1], &ctx->stat[1]);
+				nr++;
+			}
+		}
+		/*
+		 * If we race on finding an entry, just loop back again.
+		 * Should be very rare, as the window is only updated
+		 * occasionally
+		 */
+	} while (!nr);
+}
+
+static void __blk_stat_init(struct blk_rq_stat *stat, s64 time_now)
+{
+	stat->min = -1ULL;
+	stat->max = stat->nr_samples = stat->mean = 0;
+	stat->time = time_now & BLK_STAT_MASK;
+}
+
+void blk_stat_init(struct blk_rq_stat *stat)
+{
+	__blk_stat_init(stat, ktime_to_ns(ktime_get()));
+}
+
+void blk_stat_add(struct blk_rq_stat *stat, struct request *rq)
+{
+	s64 delta, now, value;
+
+	now = ktime_to_ns(ktime_get());
+	if (now < rq->issue_time)
+		return;
+
+	if ((now & BLK_STAT_MASK) != (stat->time & BLK_STAT_MASK))
+		__blk_stat_init(stat, now);
+
+	value = now - rq->issue_time;
+	if (value > stat->max)
+		stat->max = value;
+	if (value < stat->min)
+		stat->min = value;
+
+	delta = value - stat->mean;
+	if (delta)
+		stat->mean += div64_s64(delta, stat->nr_samples + 1);
+
+	stat->nr_samples++;
+}
+
+void blk_stat_clear(struct request_queue *q)
+{
+	if (q->mq_ops) {
+		struct blk_mq_hw_ctx *hctx;
+		struct blk_mq_ctx *ctx;
+		int i, j;
+
+		queue_for_each_hw_ctx(q, hctx, i) {
+			hctx_for_each_ctx(hctx, ctx, j) {
+				blk_stat_init(&ctx->stat[0]);
+				blk_stat_init(&ctx->stat[1]);
+			}
+		}
+	} else {
+		blk_stat_init(&q->rq_stats[0]);
+		blk_stat_init(&q->rq_stats[1]);
+	}
+}
diff --git a/block/blk-stat.h b/block/blk-stat.h
new file mode 100644
index 000000000000..d77548dbf196
--- /dev/null
+++ b/block/blk-stat.h
@@ -0,0 +1,17 @@
+#ifndef BLK_STAT_H
+#define BLK_STAT_H
+
+/*
+ * ~0.13s window as a power-of-2 (2^27 nsecs)
+ */
+#define BLK_STAT_NSEC	134217728ULL
+#define BLK_STAT_MASK	~(BLK_STAT_NSEC - 1)
+
+void blk_stat_add(struct blk_rq_stat *, struct request *);
+void blk_hctx_stat_get(struct blk_mq_hw_ctx *, struct blk_rq_stat *);
+void blk_queue_stat_get(struct request_queue *, struct blk_rq_stat *);
+void blk_stat_clear(struct request_queue *q);
+void blk_stat_init(struct blk_rq_stat *);
+void blk_stat_sum(struct blk_rq_stat *, struct blk_rq_stat *);
+
+#endif
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 99205965f559..6e516cc0d3d0 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -379,6 +379,26 @@ static ssize_t queue_wc_store(struct request_queue *q, const char *page,
 	return count;
 }
 
+static ssize_t print_stat(char *page, struct blk_rq_stat *stat, const char *pre)
+{
+	return sprintf(page, "%s samples=%llu, mean=%lld, min=%lld, max=%lld\n",
+			pre, (long long) stat->nr_samples,
+			(long long) stat->mean, (long long) stat->min,
+			(long long) stat->max);
+}
+
+static ssize_t queue_stats_show(struct request_queue *q, char *page)
+{
+	struct blk_rq_stat stat[2];
+	ssize_t ret;
+
+	blk_queue_stat_get(q, stat);
+
+	ret = print_stat(page, &stat[0], "read :");
+	ret += print_stat(page + ret, &stat[1], "write:");
+	return ret;
+}
+
 static struct queue_sysfs_entry queue_requests_entry = {
 	.attr = {.name = "nr_requests", .mode = S_IRUGO | S_IWUSR },
 	.show = queue_requests_show,
@@ -516,6 +536,11 @@ static struct queue_sysfs_entry queue_wc_entry = {
 	.store = queue_wc_store,
 };
 
+static struct queue_sysfs_entry queue_stats_entry = {
+	.attr = {.name = "stats", .mode = S_IRUGO },
+	.show = queue_stats_show,
+};
+
 static struct attribute *default_attrs[] = {
 	&queue_requests_entry.attr,
 	&queue_ra_entry.attr,
@@ -542,6 +567,7 @@ static struct attribute *default_attrs[] = {
 	&queue_random_entry.attr,
 	&queue_poll_entry.attr,
 	&queue_wc_entry.attr,
+	&queue_stats_entry.attr,
 	NULL,
 };
 
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 223012451c7a..2b4414fb4d8e 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -268,4 +268,12 @@ static inline unsigned int blk_qc_t_to_tag(blk_qc_t cookie)
 	return cookie & ((1u << BLK_QC_T_SHIFT) - 1);
 }
 
+struct blk_rq_stat {
+	s64 mean;
+	u64 min;
+	u64 max;
+	s64 nr_samples;
+	s64 time;
+};
+
 #endif /* __LINUX_BLK_TYPES_H */
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index eee94bd6de52..87f6703ced71 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -153,6 +153,7 @@ struct request {
 	struct gendisk *rq_disk;
 	struct hd_struct *part;
 	unsigned long start_time;
+	s64 issue_time;
 #ifdef CONFIG_BLK_CGROUP
 	struct request_list *rl;		/* rl this rq is alloced from */
 	unsigned long long start_time_ns;
@@ -402,6 +403,9 @@ struct request_queue {
 
 	unsigned int		nr_sorted;
 	unsigned int		in_flight[2];
+
+	struct blk_rq_stat	rq_stats[2];
+
 	/*
 	 * Number of active block driver functions for which blk_drain_queue()
 	 * must wait. Must be incremented around functions that unlock the
-- 
2.8.0.rc4.6.g7e4ba36

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#1389427 — Re: [PATCHSET v5] Make background writeback great again for the first time

Hi,

On Tue 26-04-16 09:55:23, Jens Axboe wrote:
> Since the dawn of time, our background buffered writeback has sucked.
> When we do background buffered writeback, it should have little impact
> on foreground activity. That's the definition of background activity...
> But for as long as I can remember, heavy buffered writers have not
> behaved like that. For instance, if I do something like this:
> 
> $ dd if=/dev/zero of=foo bs=1M count=10k
> 
> on my laptop, and then try and start chrome, it basically won't start
> before the buffered writeback is done. Or, for server oriented
> workloads, where installation of a big RPM (or similar) adversely
> impacts database reads or sync writes. When that happens, I get people
> yelling at me.
> 
> I have posted plenty of results previously, I'll keep it shorter
> this time. Here's a run on my laptop, using read-to-pipe-async for
> reading a 5g file, and rewriting it. You can find this test program
> in the fio git repo.

I have tested your patchset on my test system. Generally I have observed
noticeable drop in average throughput for heavy background writes without
any other disk activity and also somewhat increased variance in the
runtimes. It is most visible on this simple testcases:

dd if=/dev/zero of=/mnt/file bs=1M count=10000

and

dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync

The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
created before each dd run on a dedicated disk.

Without your patches I get pretty stable dd runtimes for both cases:

dd if=/dev/zero of=/mnt/file bs=1M count=10000
Runtimes: 87.9611 87.3279 87.2554

dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
Runtimes: 93.3502 93.2086 93.541

With your patches the numbers look like:

dd if=/dev/zero of=/mnt/file bs=1M count=10000
Runtimes: 108.183, 97.184, 99.9587

dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
Runtimes: 104.9, 102.775, 102.892

I have checked whether the variance is due to some interaction with CFQ
which is used for the disk. When I switched the disk to deadline, I still
get some variance although, the throughput is still ~10% lower:

dd if=/dev/zero of=/mnt/file bs=1M count=10000
Runtimes: 100.417 100.643 100.866

dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
Runtimes: 104.208 106.341 105.483

The disk is rotational SATA drive with writeback cache, queue depth of the
disk reported in /sys/block/sdb/device/queue_depth is 1.

So I think we still need some tweaking on the low end of the storage
spectrum so that we don't lose 10% of throughput for simple cases like
this.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR

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#1389445 — Re: [PATCHSET v5] Make background writeback great again for the first time

On 04/27/2016 12:01 PM, Jan Kara wrote:
> Hi,
>
> On Tue 26-04-16 09:55:23, Jens Axboe wrote:
>> Since the dawn of time, our background buffered writeback has sucked.
>> When we do background buffered writeback, it should have little impact
>> on foreground activity. That's the definition of background activity...
>> But for as long as I can remember, heavy buffered writers have not
>> behaved like that. For instance, if I do something like this:
>>
>> $ dd if=/dev/zero of=foo bs=1M count=10k
>>
>> on my laptop, and then try and start chrome, it basically won't start
>> before the buffered writeback is done. Or, for server oriented
>> workloads, where installation of a big RPM (or similar) adversely
>> impacts database reads or sync writes. When that happens, I get people
>> yelling at me.
>>
>> I have posted plenty of results previously, I'll keep it shorter
>> this time. Here's a run on my laptop, using read-to-pipe-async for
>> reading a 5g file, and rewriting it. You can find this test program
>> in the fio git repo.
>
> I have tested your patchset on my test system. Generally I have observed
> noticeable drop in average throughput for heavy background writes without
> any other disk activity and also somewhat increased variance in the
> runtimes. It is most visible on this simple testcases:
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>
> and
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>
> The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
> created before each dd run on a dedicated disk.
>
> Without your patches I get pretty stable dd runtimes for both cases:
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000
> Runtimes: 87.9611 87.3279 87.2554
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> Runtimes: 93.3502 93.2086 93.541
>
> With your patches the numbers look like:
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000
> Runtimes: 108.183, 97.184, 99.9587
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> Runtimes: 104.9, 102.775, 102.892
>
> I have checked whether the variance is due to some interaction with CFQ
> which is used for the disk. When I switched the disk to deadline, I still
> get some variance although, the throughput is still ~10% lower:
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000
> Runtimes: 100.417 100.643 100.866
>
> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> Runtimes: 104.208 106.341 105.483
>
> The disk is rotational SATA drive with writeback cache, queue depth of the
> disk reported in /sys/block/sdb/device/queue_depth is 1.
>
> So I think we still need some tweaking on the low end of the storage
> spectrum so that we don't lose 10% of throughput for simple cases like
> this.

Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if you 
are seeing smaller requests, and that is why it both varies and you get 
lower throughput? I'll try and setup a test here similar to yours.

-- 
Jens Axboe

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#1389573 — Re: [PATCHSET v5] Make background writeback great again for the first time

On Wed, Apr 27 2016, Jens Axboe wrote:
> On 04/27/2016 12:01 PM, Jan Kara wrote:
> >Hi,
> >
> >On Tue 26-04-16 09:55:23, Jens Axboe wrote:
> >>Since the dawn of time, our background buffered writeback has sucked.
> >>When we do background buffered writeback, it should have little impact
> >>on foreground activity. That's the definition of background activity...
> >>But for as long as I can remember, heavy buffered writers have not
> >>behaved like that. For instance, if I do something like this:
> >>
> >>$ dd if=/dev/zero of=foo bs=1M count=10k
> >>
> >>on my laptop, and then try and start chrome, it basically won't start
> >>before the buffered writeback is done. Or, for server oriented
> >>workloads, where installation of a big RPM (or similar) adversely
> >>impacts database reads or sync writes. When that happens, I get people
> >>yelling at me.
> >>
> >>I have posted plenty of results previously, I'll keep it shorter
> >>this time. Here's a run on my laptop, using read-to-pipe-async for
> >>reading a 5g file, and rewriting it. You can find this test program
> >>in the fio git repo.
> >
> >I have tested your patchset on my test system. Generally I have observed
> >noticeable drop in average throughput for heavy background writes without
> >any other disk activity and also somewhat increased variance in the
> >runtimes. It is most visible on this simple testcases:
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> >
> >and
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> >
> >The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
> >created before each dd run on a dedicated disk.
> >
> >Without your patches I get pretty stable dd runtimes for both cases:
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> >Runtimes: 87.9611 87.3279 87.2554
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> >Runtimes: 93.3502 93.2086 93.541
> >
> >With your patches the numbers look like:
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> >Runtimes: 108.183, 97.184, 99.9587
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> >Runtimes: 104.9, 102.775, 102.892
> >
> >I have checked whether the variance is due to some interaction with CFQ
> >which is used for the disk. When I switched the disk to deadline, I still
> >get some variance although, the throughput is still ~10% lower:
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> >Runtimes: 100.417 100.643 100.866
> >
> >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> >Runtimes: 104.208 106.341 105.483
> >
> >The disk is rotational SATA drive with writeback cache, queue depth of the
> >disk reported in /sys/block/sdb/device/queue_depth is 1.
> >
> >So I think we still need some tweaking on the low end of the storage
> >spectrum so that we don't lose 10% of throughput for simple cases like
> >this.
> 
> Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
> you are seeing smaller requests, and that is why it both varies and
> you get lower throughput? I'll try and setup a test here similar to
> yours.

Jan, care to try the below patch? I can't fully reproduce your issue on
a SCSI disk limited to QD=1, but I have a feeling this might help. It's
a bit of a hack, but the general idea is to allow one more request to
build up for QD=1 devices. That eliminates wait time between one request
finishing, and the next being submitted.


diff --git a/lib/wbt.c b/lib/wbt.c
index 650da911f24f..6b24c8525ace 100644
--- a/lib/wbt.c
+++ b/lib/wbt.c
@@ -93,23 +93,30 @@ void __wbt_done(struct rq_wb *rwb)
 	 * If the device does write back caching, drop further down
 	 * before we wake people up.
 	 */
-	if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
+	if (rwb->queue_depth == 1)
+		limit = 2;
+	else if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
 		limit = 0;
 	else
 		limit = rwb->wb_normal;
 
+	inflight = atomic_dec_return(&rwb->inflight);
+
 	/*
-	 * Don't wake anyone up if we are above the normal limit. If
-	 * throttling got disabled (limit == 0) with waiters, ensure
-	 * that we wake them up.
+	 * wbt got disabled with IO in flight. Wake up any potential
+	 * waiters, we don't have to do more than that.
 	 */
-	inflight = atomic_dec_return(&rwb->inflight);
-	if (limit && inflight >= limit) {
-		if (!rwb->wb_max)
-			wake_up_all(&rwb->wait);
+	if (!rwb_enabled(rwb)) {
+		wake_up_all(&rwb->wait);
 		return;
 	}
 
+	/*
+	 * Don't wake anyone up if we are above the normal limit.
+	 */
+	if (inflight >= limit)
+		return;
+
 	if (waitqueue_active(&rwb->wait)) {
 		int diff = limit - inflight;
 
@@ -366,6 +373,9 @@ static inline unsigned int get_limit(struct rq_wb *rwb, unsigned long rw)
 	} else
 		limit = rwb->wb_normal;
 
+	if (rwb->queue_depth == 1)
+		limit = 2;
+
 	return limit;
 }
 

-- 
Jens Axboe

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#1389580 — Re: [PATCHSET v5] Make background writeback great again for the first time

On Wed, Apr 27 2016, Jens Axboe wrote:
> On Wed, Apr 27 2016, Jens Axboe wrote:
> > On 04/27/2016 12:01 PM, Jan Kara wrote:
> > >Hi,
> > >
> > >On Tue 26-04-16 09:55:23, Jens Axboe wrote:
> > >>Since the dawn of time, our background buffered writeback has sucked.
> > >>When we do background buffered writeback, it should have little impact
> > >>on foreground activity. That's the definition of background activity...
> > >>But for as long as I can remember, heavy buffered writers have not
> > >>behaved like that. For instance, if I do something like this:
> > >>
> > >>$ dd if=/dev/zero of=foo bs=1M count=10k
> > >>
> > >>on my laptop, and then try and start chrome, it basically won't start
> > >>before the buffered writeback is done. Or, for server oriented
> > >>workloads, where installation of a big RPM (or similar) adversely
> > >>impacts database reads or sync writes. When that happens, I get people
> > >>yelling at me.
> > >>
> > >>I have posted plenty of results previously, I'll keep it shorter
> > >>this time. Here's a run on my laptop, using read-to-pipe-async for
> > >>reading a 5g file, and rewriting it. You can find this test program
> > >>in the fio git repo.
> > >
> > >I have tested your patchset on my test system. Generally I have observed
> > >noticeable drop in average throughput for heavy background writes without
> > >any other disk activity and also somewhat increased variance in the
> > >runtimes. It is most visible on this simple testcases:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >
> > >and
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >
> > >The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
> > >created before each dd run on a dedicated disk.
> > >
> > >Without your patches I get pretty stable dd runtimes for both cases:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 87.9611 87.3279 87.2554
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 93.3502 93.2086 93.541
> > >
> > >With your patches the numbers look like:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 108.183, 97.184, 99.9587
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 104.9, 102.775, 102.892
> > >
> > >I have checked whether the variance is due to some interaction with CFQ
> > >which is used for the disk. When I switched the disk to deadline, I still
> > >get some variance although, the throughput is still ~10% lower:
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000
> > >Runtimes: 100.417 100.643 100.866
> > >
> > >dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
> > >Runtimes: 104.208 106.341 105.483
> > >
> > >The disk is rotational SATA drive with writeback cache, queue depth of the
> > >disk reported in /sys/block/sdb/device/queue_depth is 1.
> > >
> > >So I think we still need some tweaking on the low end of the storage
> > >spectrum so that we don't lose 10% of throughput for simple cases like
> > >this.
> > 
> > Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
> > you are seeing smaller requests, and that is why it both varies and
> > you get lower throughput? I'll try and setup a test here similar to
> > yours.
> 
> Jan, care to try the below patch? I can't fully reproduce your issue on
> a SCSI disk limited to QD=1, but I have a feeling this might help. It's
> a bit of a hack, but the general idea is to allow one more request to
> build up for QD=1 devices. That eliminates wait time between one request
> finishing, and the next being submitted.

That accidentally added a potentially stall, this one is both cleaner
and should have that fixed.

diff --git a/lib/wbt.c b/lib/wbt.c
index 650da911f24f..322f5e04e994 100644
--- a/lib/wbt.c
+++ b/lib/wbt.c
@@ -98,18 +98,23 @@ void __wbt_done(struct rq_wb *rwb)
 	else
 		limit = rwb->wb_normal;
 
+	inflight = atomic_dec_return(&rwb->inflight);
+
 	/*
-	 * Don't wake anyone up if we are above the normal limit. If
-	 * throttling got disabled (limit == 0) with waiters, ensure
-	 * that we wake them up.
+	 * wbt got disabled with IO in flight. Wake up any potential
+	 * waiters, we don't have to do more than that.
 	 */
-	inflight = atomic_dec_return(&rwb->inflight);
-	if (limit && inflight >= limit) {
-		if (!rwb->wb_max)
-			wake_up_all(&rwb->wait);
+	if (!rwb_enabled(rwb)) {
+		wake_up_all(&rwb->wait);
 		return;
 	}
 
+	/*
+	 * Don't wake anyone up if we are above the normal limit.
+	 */
+	if (inflight && inflight >= limit)
+		return;
+
 	if (waitqueue_active(&rwb->wait)) {
 		int diff = limit - inflight;
 
@@ -150,14 +155,26 @@ static void calc_wb_limits(struct rq_wb *rwb)
 		return;
 	}
 
-	depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
-
 	/*
-	 * Reduce max depth by 50%, and re-calculate normal/bg based on that
+	 * For QD=1 devices, this is a special case. It's important for those
+	 * to have one request ready when one completes, so force a depth of
+	 * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
+	 * since the device can't have more than that in flight.
 	 */
-	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
-	rwb->wb_normal = (rwb->wb_max + 1) / 2;
-	rwb->wb_background = (rwb->wb_max + 3) / 4;
+	if (rwb->queue_depth == 1) {
+		rwb->wb_max = rwb->wb_normal = 2;
+		rwb->wb_background = 1;
+	} else {
+		depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
+
+		/*
+		 * Reduce max depth by 50%, and re-calculate normal/bg based on
+		 * that.
+		 */
+		rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
+		rwb->wb_normal = (rwb->wb_max + 1) / 2;
+		rwb->wb_background = (rwb->wb_max + 3) / 4;
+	}
 }
 
 static bool inline stat_sample_valid(struct blk_rq_stat *stat)

-- 
Jens Axboe

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#1389781 — Re: [PATCHSET v5] Make background writeback great again for the first time

于 2016/4/28 4:59, Jens Axboe 写道:
> On Wed, Apr 27 2016, Jens Axboe wrote:
>> On Wed, Apr 27 2016, Jens Axboe wrote:
>>> On 04/27/2016 12:01 PM, Jan Kara wrote:
>>>> Hi,
>>>>
>>>> On Tue 26-04-16 09:55:23, Jens Axboe wrote:
>>>>> Since the dawn of time, our background buffered writeback has sucked.
>>>>> When we do background buffered writeback, it should have little impact
>>>>> on foreground activity. That's the definition of background activity...
>>>>> But for as long as I can remember, heavy buffered writers have not
>>>>> behaved like that. For instance, if I do something like this:
>>>>>
>>>>> $ dd if=/dev/zero of=foo bs=1M count=10k
>>>>>
>>>>> on my laptop, and then try and start chrome, it basically won't start
>>>>> before the buffered writeback is done. Or, for server oriented
>>>>> workloads, where installation of a big RPM (or similar) adversely
>>>>> impacts database reads or sync writes. When that happens, I get people
>>>>> yelling at me.
>>>>>
>>>>> I have posted plenty of results previously, I'll keep it shorter
>>>>> this time. Here's a run on my laptop, using read-to-pipe-async for
>>>>> reading a 5g file, and rewriting it. You can find this test program
>>>>> in the fio git repo.
>>>>
>>>> I have tested your patchset on my test system. Generally I have observed
>>>> noticeable drop in average throughput for heavy background writes without
>>>> any other disk activity and also somewhat increased variance in the
>>>> runtimes. It is most visible on this simple testcases:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>>
>>>> and
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>>
>>>> The machine has 4GB of ram, /mnt is an ext3 filesystem that is freshly
>>>> created before each dd run on a dedicated disk.
>>>>
>>>> Without your patches I get pretty stable dd runtimes for both cases:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 87.9611 87.3279 87.2554
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 93.3502 93.2086 93.541
>>>>
>>>> With your patches the numbers look like:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 108.183, 97.184, 99.9587
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 104.9, 102.775, 102.892
>>>>
>>>> I have checked whether the variance is due to some interaction with CFQ
>>>> which is used for the disk. When I switched the disk to deadline, I still
>>>> get some variance although, the throughput is still ~10% lower:
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000
>>>> Runtimes: 100.417 100.643 100.866
>>>>
>>>> dd if=/dev/zero of=/mnt/file bs=1M count=10000 conv=fsync
>>>> Runtimes: 104.208 106.341 105.483
>>>>
>>>> The disk is rotational SATA drive with writeback cache, queue depth of the
>>>> disk reported in /sys/block/sdb/device/queue_depth is 1.
>>>>
>>>> So I think we still need some tweaking on the low end of the storage
>>>> spectrum so that we don't lose 10% of throughput for simple cases like
>>>> this.
>>>
>>> Thanks for testing, Jan! I haven't tried old QD=1 SATA. I wonder if
>>> you are seeing smaller requests, and that is why it both varies and
>>> you get lower throughput? I'll try and setup a test here similar to
>>> yours.
>>
>> Jan, care to try the below patch? I can't fully reproduce your issue on
>> a SCSI disk limited to QD=1, but I have a feeling this might help. It's
>> a bit of a hack, but the general idea is to allow one more request to
>> build up for QD=1 devices. That eliminates wait time between one request
>> finishing, and the next being submitted.
> 
> That accidentally added a potentially stall, this one is both cleaner
> and should have that fixed.
> 
> diff --git a/lib/wbt.c b/lib/wbt.c
> index 650da911f24f..322f5e04e994 100644
> --- a/lib/wbt.c
> +++ b/lib/wbt.c
> @@ -98,18 +98,23 @@ void __wbt_done(struct rq_wb *rwb)
>  	else
>  		limit = rwb->wb_normal;
Hi Jens,

This statement 'limit = rwb->wb_normal' is executed twice, maybe once is
enough. It is not a big deal anyway :)


Another question about this if branch:

   if (rwb->wc && !atomic_read(&rwb->bdi->wb.dirty_sleeping))
	limit = 0;

I can't follow the logic of this if branch. why set limit equal to 0
when the device supports write back caches and there are tasks being
limited in balance_dirty_pages(). Could you pelase give more info
about this ?  Thanks!
>  
> +	inflight = atomic_dec_return(&rwb->inflight);
> +
>  	/*
> -	 * Don't wake anyone up if we are above the normal limit. If
> -	 * throttling got disabled (limit == 0) with waiters, ensure
> -	 * that we wake them up.
> +	 * wbt got disabled with IO in flight. Wake up any potential
> +	 * waiters, we don't have to do more than that.
>  	 */
> -	inflight = atomic_dec_return(&rwb->inflight);
> -	if (limit && inflight >= limit) {
> -		if (!rwb->wb_max)
> -			wake_up_all(&rwb->wait);
> +	if (!rwb_enabled(rwb)) {
> +		wake_up_all(&rwb->wait);
>  		return;
>  	}

Maybe it is better that executing this if branch earlier. So we can wake up
potential waiters in time when wbt got disabled.
>  
> +	/*
> +	 * Don't wake anyone up if we are above the normal limit.
> +	 */
> +	if (inflight && inflight >= limit)
> +		return;
> +
>  	if (waitqueue_active(&rwb->wait)) {
>  		int diff = limit - inflight;
>  
> @@ -150,14 +155,26 @@ static void calc_wb_limits(struct rq_wb *rwb)
>  		return;
>  	}
>  
> -	depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
> -
>  	/*
> -	 * Reduce max depth by 50%, and re-calculate normal/bg based on that
> +	 * For QD=1 devices, this is a special case. It's important for those
> +	 * to have one request ready when one completes, so force a depth of
> +	 * 2 for those devices. On the backend, it'll be a depth of 1 anyway,
> +	 * since the device can't have more than that in flight.
>  	 */
> -	rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> -	rwb->wb_normal = (rwb->wb_max + 1) / 2;
> -	rwb->wb_background = (rwb->wb_max + 3) / 4;
> +	if (rwb->queue_depth == 1) {
> +		rwb->wb_max = rwb->wb_normal = 2;
> +		rwb->wb_background = 1;
> +	} else {
> +		depth = min_t(unsigned int, RWB_MAX_DEPTH, rwb->queue_depth);
> +
> +		/*
> +		 * Reduce max depth by 50%, and re-calculate normal/bg based on
> +		 * that.
> +		 */
> +		rwb->wb_max = 1 + ((depth - 1) >> min(31U, rwb->scale_step));
> +		rwb->wb_normal = (rwb->wb_max + 1) / 2;
> +		rwb->wb_background = (rwb->wb_max + 3) / 4;
> +	}
>  }
>  
>  static bool inline stat_sample_valid(struct blk_rq_stat *stat)
> 


-- 
Regards
Kaixu Xia

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