[PATCHSET v4 0/8] Make background writeback not suck

From: Jens Axboe
Date: Mon Apr 18 2016 - 00:27:05 EST


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.


$ 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-v4 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:


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 | 9
Documentation/block/writeback_cache_control.txt | 4
arch/um/drivers/ubd_kern.c | 2
block/Makefile | 2
block/blk-core.c | 22 +
block/blk-flush.c | 11
block/blk-mq-sysfs.c | 47 ++
block/blk-mq.c | 45 ++
block/blk-mq.h | 3
block/blk-settings.c | 58 +-
block/blk-stat.c | 184 ++++++++
block/blk-stat.h | 17
block/blk-sysfs.c | 122 +++++
block/blk-wb.c | 495 ++++++++++++++++++++++++
block/blk-wb.h | 42 ++
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 | 14
include/linux/blkdev.h | 27 +
include/linux/fs.h | 4
include/linux/writeback.h | 10
include/trace/events/block.h | 98 ++++
mm/backing-dev.c | 1
mm/filemap.c | 42 +-
mm/page-writeback.c | 2
52 files changed, 1281 insertions(+), 96 deletions(-)

Jens Axboe