Re: [PATCH v6 2/2] mm: kick writeback flusher for IOCB_DONTCACHE with targeted dirty tracking

[Andrew Morton]

On Tue, 05 May 2026 20:59:49 +0200 Jeff Layton <jlayton@xxxxxxxxxx> wrote:

> The IOCB_DONTCACHE writeback path in generic_write_sync() calls
> filemap_flush_range() on every write, submitting writeback inline in
> the writer's context.  Perf lock contention profiling shows the
> performance problem is not lock contention but the writeback submission
> work itself — walking the page tree and submitting I/O blocks the writer
> for milliseconds, inflating p99.9 latency from 23ms (buffered) to 93ms
> (dontcache).
> 
> Replace the inline filemap_flush_range() call with a flusher kick that
> drains dirty pages in the background.  This moves writeback submission
> completely off the writer's hot path.
> 
> ...
>
>                         Before    After    Change
>   seq-write/dontcache      298      897    +201%
>   rand-write/dontcache     131      236     +80%
> 
> Tail latency improvements (seq-write/dontcache):
>   p99:    135,266 us  ->  23,986 us   (-82%)
>   p99.9: 8,925,479 us ->  28,443 us   (-99.7%)
> 
> Multi-writer (4 jobs, sequential write):
>                                 Before    After    Change
>   dontcache aggregate (MB/s)     2,529    4,532     +79%
>   dontcache p99 (us)             8,553    1,002     -88%
>   dontcache p99.9 (us)         109,314    1,057     -99%
> 
> 32-file write (Axboe test):
>                                 Before    After    Change
>   dontcache aggregate (MB/s)     1,548    3,499    +126%
>   dontcache p99 (us)            10,170      602     -94%
>   Peak dirty pages (MB)          1,837      213     -88%
> 
>   Dontcache now reaches 81% of buffered throughput (was 35%).
> 
> Competing writers (dontcache vs buffered, separate files):
>                                 Before    After
>   buffered writer                  868      433 MB/s
>   dontcache writer                 415      433 MB/s
>   Aggregate                      1,284      866 MB/s
> 
> ...
>
>   The dontcache writer's p99.9 latency collapsed from 119 ms to
>   33 ms (-73%), eliminating the severe periodic stalls seen in the
>   baseline. Both writers now share identical latency profiles,
>   matching the buffered-vs-buffered pattern.
> 
> The per-bdi_writeback dirty tracking dramatically reduces peak dirty
> pages in dontcache workloads, with the 32-file test dropping from
> 1.8 GB to 213 MB. Dontcache sequential write throughput triples and
> multi-writer throughput reaches parity with buffered I/O, with tail
> latencies collapsing by 1-2 orders of magnitude.

Geeze, is that the best you can do ;)

Sashiko seems to have found more stuff:
	https://sashiko.dev/#/patchset/20260505-dontcache-v6-0-66463805dd6a@xxxxxxxxxx