zram: introduce writeback bio batching

Patch series "zram: introduce writeback bio batching", v6.

As writeback is becoming more and more common the longstanding limitations
of zram writeback throughput are becoming more visible.  Introduce
writeback bio batching so that multiple writeback bios can be processed
simultaneously.


This patch (of 6):

As was stated in a comment [1] a single page writeback IO is not
efficient, but it works.  It's time to address this throughput limitation
as writeback becomes used more often.  Introduce batched (multiple) bio
writeback support to take advantage of parallel requests processing and
better requests scheduling.

Approach used in this patch doesn't use a dedicated kthread like in [2],
or blk-plug like in [3].  Dedicated kthread adds complexity, which can be
avoided.  Apart from that not all zram setups use writeback, so having
numerous per-device kthreads (on systems that create multiple zram
devices) hanging around is not the most optimal thing to do.  blk-plug, on
the other hand, works best when request are sequential, which doesn't
particularly fit zram writebck IO patterns: zram writeback IO patterns are
expected to be random, due to how bdev block reservation/release are
handled.  blk-plug approach also works in cycles: idle IO, when zram sets
up requests in a batch, is followed by bursts of IO, when zram submits the
entire batch.

Instead we use a batch of requests and submit new bio as soon as one of
the in-flight requests completes.

For the time being the writeback batch size (maximum number of in-flight
bio requests) is set to 32 for all devices.  A follow up patch adds a
writeback_batch_size device attribute, so the batch size becomes run-time
configurable.

Link: https://lkml.kernel.org/r/20251122074029.3948921-1-senozhatsky@chromium.org
Link: https://lkml.kernel.org/r/20251122074029.3948921-2-senozhatsky@chromium.org
Link: https://lore.kernel.org/all/20181203024045.153534-6-minchan@kernel.org/ [1]
Link: https://lore.kernel.org/all/20250731064949.1690732-1-richardycc@google.com/ [2]
Link: https://lore.kernel.org/all/tencent_78FC2C4FE16BA1EBAF0897DB60FCD675ED05@qq.com/ [3]
Signed-off-by: Sergey Senozhatsky <senozhatsky@chromium.org>
Co-developed-by: Yuwen Chen <ywen.chen@foxmail.com>
Co-developed-by: Richard Chang <richardycc@google.com>
Suggested-by: Minchan Kim <minchan@google.com>
Cc: Brian Geffon <bgeffon@google.com>
Cc: Richard Chang <richardycc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

0 files changed, 0 insertions, 0 deletions