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 -- To unsubscribe from this list: send the line "unsubscribe linux-block" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
