Re: [PATCH v4 00/28] Zone write plugging

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On 2024-04-02 14:39, Damien Le Moal wrote:
> The patch series introduces zone write plugging (ZWP) as the new
> mechanism to control the ordering of writes to zoned block devices.
> ZWP replaces zone write locking (ZWL) which is implemented only by
> mq-deadline today. ZWP also allows emulating zone append operations
> using regular writes for zoned devices that do not natively support this
> operation (e.g. SMR HDDs). This patch series removes the scsi disk
> driver and device mapper zone append emulation to use ZWP emulation.
> 
> Unlike ZWL which operates on requests, ZWP operates on BIOs. A zone
> write plug is simply a BIO list that is atomically manipulated using a
> spinlock and a kblockd submission work. A write BIO to a zone is
> "plugged" to delay its execution if a write BIO for the same zone was
> already issued, that is, if a write request for the same zone is being
> executed. The next plugged BIO is unplugged and issued once the write
> request completes.
> 
> This mechanism allows to:
>   - Untangle zone write ordering from the block IO schedulers. This
>     allows removing the restriction on using only mq-deadline for zoned
>     block devices. Any block IO scheduler, including "none" can be used.
>   - Zone write plugging operates on BIOs instead of requests. Plugged
>     BIOs waiting for execution thus do not hold scheduling tags and thus
>     do not prevent other BIOs from being submitted to the device (reads
>     or writes to other zones). Depending on the workload, this can
>     significantly improve the device use and the performance.
>   - Both blk-mq (request) based zoned devices and BIO-based devices (e.g.
>     device mapper) can use ZWP. It is mandatory for the
>     former but optional for the latter: BIO-based driver can use zone
>     write plugging to implement write ordering guarantees, or the drivers
>     can implement their own if needed.
>   - The code is less invasive in the block layer and in device drivers.
>     ZWP implementation is mostly limited to blk-zoned.c, with some small
>     changes in blk-mq.c, blk-merge.c and bio.c.
> 
> Performance evaluation results are shown below.
> 
> The series is based on block/for-next and organized as follows:
> 
>   - Patch 1 to 6 are preparatory changes for patch 7.
>   - Patch 7 and 8 introduce ZWP
>   - Patch 9 and 10 add zone append emulation to ZWP.
>   - Patch 11 to 18 modify zoned block device drivers to use ZWP and
>     prepare for the removal of ZWL.
>   - Patch 19 to 28 remove zone write locking
> 
> Overall, these changes do not significantly increase the amount of code
> (the higher number of addition shown by diff-stat is in fact due to a
> larger amount of comments in the code).
> 
> Many thanks must go to Christoph Hellwig for comments and suggestions
> he provided on earlier versions of these patches.
> 
> Performance evaluation results
> ==============================
> 
> Environments:
>   - Intel Xeon 16-cores/32-threads, 128GB of RAM
>   - Kernel:
>     - ZWL (baseline): block/for-next (based on 6.9.0-rc2)
>     - ZWP: block/for-next patched kernel to add zone write plugging
>       (both kernels were compiled with the same configuration turning
>       off most heavy debug features)
> 
> Workoads:
>   - seqw4K1: 4KB sequential write, qd=1
>   - seqw4K16: 4KB sequential write, qd=16
>   - seqw1M16: 1MB sequential write, qd=16
>   - rndw4K16: 4KB random write, qd=16
>   - rndw128K16: 128KB random write, qd=16
>   - btrfs workoad: Single fio job writing 128 MB files using 128 KB
>     direct IOs at qd=16.
> 
> Devices:
>   - nullblk (zoned): 4096 zones of 256 MB, 128 max open zones.
>   - NVMe ZNS drive: 1 TB ZNS drive with 2GB zone size, 14 max open and
>     active zones.
>   - SMR HDD: 20 TB disk with 256MB zone size, 128 max open zones.
> 
> For ZWP, the result show the performance percentage increase (or
> decrease) against ZWL (baseline) case.
> 
> 1) null_blk zoned device:
> 
>               +--------+--------+-------+--------+---------+---------+
>               |seqw4K1 |seqw4K16|seqw1M1|seqw1M16|rndw4K16|rndw128K16|
>               |(MB/s)  | (MB/s) |(MB/s) | (MB/s) | (KIOPS)| (KIOPS)  |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWL    | 940    | 840    | 18550 | 14400  | 424    | 167      |
>   |mq-deadline|        |        |       |        |        |          |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 943    | 845    | 18660 | 14770  | 461    | 165      |
>   |mq-deadline| (+0%)  | (+0%)  | (+0%) | (+1%)  | (+8%)  | (-1%)    |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 756    | 668    | 16020 | 12980  | 135    | 101      |
>   |    bfq    | (-19%) | (-20%) | (-13%)| (-9%)  | (-68%) | (-39%)   |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 2639   | 1715   | 28190 | 19760  | 344    | 150      |
>   |   none    | (+180%)| (+104%)| (+51%)| (+37%) | (-18%) | (-10%)   |
>   +-----------+--------+--------+-------+--------+--------+----------+
> 
> ZWP with mq-deadline gives performance very similar to zone write
> locking, showing that zone write plugging overhead is acceptable.
> But ZWP ability to run fast block devices with the none scheduler
> shows brings all the benefits of zone write plugging and results in
> significant performance increase for all workloads. The exception to
> this are random write workloads with multiple jobs: for these, the
> faster request submission rate achieved by zone write plugging results
> in higher contention on null-blk zone spinlock, which degrades
> performance.
> 
> 2) NVMe ZNS drive:
> 
>               +--------+--------+-------+--------+--------+----------+
>               |seqw4K1 |seqw4K16|seqw1M1|seqw1M16|rndw4K16|rndw128K16|
>               |(MB/s)  | (MB/s) |(MB/s) | (MB/s) | (KIOPS)|  (KIOPS) |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWL    | 183    | 702    | 1066  | 1103   | 53.5   | 14.5     |
>   |mq-deadline|        |        |       |        |        |          |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 183    | 719    | 1086  | 1108   | 55.6   | 14.7     |
>   |mq-deadline| (-0%)  | (+1%)  | (+0%) | (+0%)  | (+3%)  | (+1%)    |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 178    | 691    | 1082  | 1106   | 30.8   | 11.5     |
>   |    bfq    | (-3%)  | (-2%)  | (-0%) | (+0%)  | (-42%) | (-20%)   |
>   +-----------+--------+--------+-------+--------+--------+----------+
>   |    ZWP    | 190    | 666    | 1083  | 1108   | 51.4   | 14.7     |
>   |   none    | (+4%)  | (-5%)  | (+0%) | (+0%)  | (-4%)  | (+0%)    |
>   +-----------+--------+--------+-------+--------+--------+----------+
> 
> Zone write plugging overhead does not significantly impact performance.
> Similar to nullblk, using the none scheduler leads to performance
> increase for most workloads.
> 
> 3) SMR SATA HDD:
> 
>               +-------+--------+-------+--------+--------+----------+
>               |seqw4K1|seqw4K16|seqw1M1|seqw1M16|rndw4K16|rndw128K16|
>               |(MB/s) | (MB/s) |(MB/s) | (MB/s) | (KIOPS)|  (KIOPS) |
>   +-----------+-------+--------+-------+--------+--------+----------+
>   |    ZWL    | 107   | 243    | 245   | 246    | 2.2    | 0.763    |
>   |mq-deadline|       |        |       |        |        |          |
>   +-----------+-------+--------+-------+--------+--------+----------+
>   |    ZWP    | 107   | 242    | 245   | 245    | 2.2    | 0.772    |
>   |mq-deadline| (+0%) | (-0%)  | (+0%) | (-0%)  | (+0%)  | (+0%)    |
>   +-----------+-------+--------+-------+--------+--------+----------+
>   |    ZWP    | 104   | 241    | 246   | 242    | 2.2    | 0.765    |
>   |    bfq    | (-2%) | (-0%)  | (+0%) | (-0%)  | (+0%)  | (+0%)    |
>   +-----------+-------+--------+-------+--------+--------+----------+
>   |    ZWP    | 115   | 235    | 249   | 242    | 2.2    | 0.763    |
>   |   none    | (+7%) | (-3%)  | (+1%) | (-1%)  | (+0%)  | (+0%)    |
>   +-----------+-------+--------+-------+--------+--------+----------+
> 
> Performance with purely sequential write workloads at high queue depth
> somewhat decrease a little when using zone write plugging. This is due
> to the different IO pattern that ZWP generates where the first writes to
> a zone start being issued when the end of the previous zone are still
> being written. Depending on how the disk handles queued commands, seek
> may be generated, slightly impacting the throughput achieved. Such pure
> sequential write workloads are however rare with SMR drives.
> 
> 4) Zone append tests using btrfs:
> 
>               +-------------+-------------+-----------+-------------+
>               |  null-blk   |  null_blk   |    ZNS    |     SMR     |
>               |  native ZA  | emulated ZA | native ZA | emulated ZA |
>               |    (MB/s)   |   (MB/s)    |   (MB/s)  |    (MB/s)   |
>   +-----------+-------------+-------------+-----------+-------------+
>   |    ZWL    | 2441        | N/A         | 1081      | 243         |
>   |mq-deadline|             |             |           |             |
>   +-----------+-------------+-------------+-----------+-------------+
>   |    ZWP    | 2361        | 2999        | 1085      | 239         |
>   |mq-deadline| (-1%)       |             | (+0%)     | (-2%)       |
>   +-----------+-------------+-------------+-----------+-------------+
>   |    ZWP    | 2299        | 2730        | 1080      | 240         |
>   |    bfq    | (-4%)       |             | (+0%)     | (-2%)       |
>   +-----------+-------------+-------------+-----------+-------------+
>   |    ZWP    | 2443        | 3152        | 1083      | 240         |
>   |    none   | (+0%)       |             | (+0%)     | (-1%)       |
>   +-----------+-------------+-------------+-----------+-------------+
> 
> With a more realistic use of the device though a file system, ZWP does
> not introduce significant performance differences, except for SMR for
> the same reason as with the fio sequential workloads at high queue
> depth.

I ran this patch set through some quick RocksDB testing
which has been very good at smoking out issues in this area before.

Setup
-----

Workloads: filluniquerandom followed by three iterations of overwrites, 
readrandom, fwdrange, revrange and varieties of readwhilewriting,
Config: Direct IO, 400M key/values

Userspace: db_bench / RocksDB 7.7.3, using ZenFS with libzbd as backend

IO Scheduler: [none]

NVMe ZNS drive: 1 TB ZNS drive with 2GB zone size, 14 max open and 
active zones.

System: 16-cores (AMD Epyc 7302P) with 128 GB of DRAM

Results:
--------

iostat: 33.6T read, 6.5T written. No errors/failures.

Comparing performance with the baseline requiring [mq-deadline] to avoid 
write reordering, we can see a ~8-10% throughput improvement for read 
heavy workloads.

No regressions seen for these workloads, db_bench write throughput and 
read and write tail latencies look unaffected.

Looks great, cheers!

Tested-by: Hans Holmberg <hans.holmberg@xxxxxxx>

> 
> Changes from v3:
>   - Rebase on block/for-next
>   - Removed old patch 1 as it is already applied
>   - Addressed Bart and Christoph comment in patch 4
>   - Merged former patch 8 and 9 together and changed the zone write plug
>     allocation to use a mempool
>   - Removed the zone_wplugs_mempool_size filed from patch 8 and instead
>     directly reference mempool->min_nr
>   - Added review tags
> 
> Changes from v2:
>   - Added Patch 1 (Christoph's comment)
>   - Fixed error code setup in Patch 3 (Bart's comment)
>   - Split former patch 26 into patches 27 and 28
>   - Modified patch 8 (zone write plugging) introduction to remove the
>     kmem_cache use and address Bart's and Christoph comments.
>   - Changed from using a mempool of zone write plugs to using a simple
>     free-list (patch 9)
>   - Simplified patch 10 as suggested by Christoph
>   - Moved common code to a helper in patch 13 as suggested by Christoph
> 
> Changes from v1:
>   - Added patch 6
>   - Rewrite of patch 7 to use a hash table of dynamically allocated zone
>     write plugs. This results in changes in patch 11 and the addition of
>     patch 8 and 9.
>   - Rebased everything on 6.9.0-rc1
>   - Added review tags for patches that did not change
> 
> Damien Le Moal (28):
>    block: Restore sector of flush requests
>    block: Remove req_bio_endio()
>    block: Introduce blk_zone_update_request_bio()
>    block: Introduce bio_straddles_zones() and bio_offset_from_zone_start()
>    block: Allow using bio_attempt_back_merge() internally
>    block: Remember zone capacity when revalidating zones
>    block: Introduce zone write plugging
>    block: Fake max open zones limit when there is no limit
>    block: Allow zero value of max_zone_append_sectors queue limit
>    block: Implement zone append emulation
>    block: Allow BIO-based drivers to use blk_revalidate_disk_zones()
>    dm: Use the block layer zone append emulation
>    scsi: sd: Use the block layer zone append emulation
>    ublk_drv: Do not request ELEVATOR_F_ZBD_SEQ_WRITE elevator feature
>    null_blk: Do not request ELEVATOR_F_ZBD_SEQ_WRITE elevator feature
>    null_blk: Introduce zone_append_max_sectors attribute
>    null_blk: Introduce fua attribute
>    nvmet: zns: Do not reference the gendisk conv_zones_bitmap
>    block: Remove BLK_STS_ZONE_RESOURCE
>    block: Simplify blk_revalidate_disk_zones() interface
>    block: mq-deadline: Remove support for zone write locking
>    block: Remove elevator required features
>    block: Do not check zone type in blk_check_zone_append()
>    block: Move zone related debugfs attribute to blk-zoned.c
>    block: Replace zone_wlock debugfs entry with zone_wplugs entry
>    block: Remove zone write locking
>    block: Do not force select mq-deadline with CONFIG_BLK_DEV_ZONED
>    block: Do not special-case plugging of zone write operations
> 
>   block/Kconfig                     |    5 -
>   block/Makefile                    |    1 -
>   block/bio.c                       |    7 +
>   block/blk-core.c                  |   11 +-
>   block/blk-flush.c                 |    1 +
>   block/blk-merge.c                 |   22 +-
>   block/blk-mq-debugfs-zoned.c      |   22 -
>   block/blk-mq-debugfs.c            |    3 +-
>   block/blk-mq-debugfs.h            |    6 +-
>   block/blk-mq.c                    |  131 ++-
>   block/blk-mq.h                    |   31 -
>   block/blk-settings.c              |   46 +-
>   block/blk-sysfs.c                 |    2 +-
>   block/blk-zoned.c                 | 1353 +++++++++++++++++++++++++++--
>   block/blk.h                       |   69 +-
>   block/elevator.c                  |   46 +-
>   block/elevator.h                  |    1 -
>   block/genhd.c                     |    3 +-
>   block/mq-deadline.c               |  176 +---
>   drivers/block/null_blk/main.c     |   22 +-
>   drivers/block/null_blk/null_blk.h |    2 +
>   drivers/block/null_blk/zoned.c    |   23 +-
>   drivers/block/ublk_drv.c          |    5 +-
>   drivers/block/virtio_blk.c        |    2 +-
>   drivers/md/dm-core.h              |    2 +-
>   drivers/md/dm-zone.c              |  476 +---------
>   drivers/md/dm.c                   |   75 +-
>   drivers/md/dm.h                   |    4 +-
>   drivers/nvme/host/core.c          |    2 +-
>   drivers/nvme/target/zns.c         |   10 +-
>   drivers/scsi/scsi_lib.c           |    1 -
>   drivers/scsi/sd.c                 |    8 -
>   drivers/scsi/sd.h                 |   19 -
>   drivers/scsi/sd_zbc.c             |  335 +------
>   include/linux/blk-mq.h            |   85 +-
>   include/linux/blk_types.h         |   30 +-
>   include/linux/blkdev.h            |  104 ++-
>   37 files changed, 1677 insertions(+), 1464 deletions(-)
>   delete mode 100644 block/blk-mq-debugfs-zoned.c
> 





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