Re: [PATCH v2 0/3] mm: improve write performance with RWF_DONTCACHE

[Jeff Layton <jlayton@kernel.org>]

On Wed, 2026-04-08 at 10:25 -0400, Jeff Layton wrote:
> This version adopts Christoph's suggest to have generic_write_sync()
> kick the flusher thread for the superblock instead of initiating
> writeback directly. This seems to perform as well or better in most
> cases than doing the writeback directly.
> 
> Here are results on XFS, both local and exported via knfsd:
> 
>     nfsd: https://markdownpastebin.com/?id=1884b9487c404ff4b7094ed41cc48f05
>     xfs: https://markdownpastebin.com/?id=3c6b262182184b25b7d58fb211374475
> 
> Ritesh had also asked about getting perf lock traces to confirm the
> source of the contention. I did that (and I can post them if you like),
> but the results from the unpatched dontcache runs didn't point out any
> specific lock contention. That leads me to believe that the bottlenecks
> were from normal queueing work, and not contention for the xa_lock after
> all.
> 
> Kicking the writeback thread seems to be a clear improvement over the
> status quo in my testing, but I do wonder if having dontcache writes
> spamming writeback for the whole bdi is the best idea.
> 
> I'm benchmarking out a patch that has the flusher do a
> writeback_single_inode() for the work. I don't expect it to perform
> measurably better in this testing, but it would better isolate the
> DONTCACHE writeback behavior to just those inodes touched by DONTCACHE
> writes.
> 
> Assuming that looks OK, I'll probably send a v3. Original cover letter
> from v1 follows:
> 

Actually, that version regressed performance in a couple of cases. I think v2 is probably the best approach, on balance. Maybe we can get this into -next so that it can make v7.2?

Here's the comparison between this version and a writeback_single_inode() flush version:

------------------8<-----------------------

● Comparing dontcache numbers against the previous whole-BDI flusher kernel (from /tmp/dontcache-local-4way-flusher.md):                                                                          

  Per-Inode vs Whole-BDI Flusher — DONTCACHE on Local XFS                                                                                                                                         

  Single-Client Writes

  ┌──────────────────┬───────────┬───────────┬─────────────┐                                                                                                                                      
  │    Benchmark     │ Whole-BDI │ Per-Inode │   Change    │                                                                                                                                      
  ├──────────────────┼───────────┼───────────┼─────────────┤      
  │ Seq write MB/s   │ 1450      │ 1438      │ -1% (noise) │
  ├──────────────────┼───────────┼───────────┼─────────────┤
  │ Seq write p99.9  │ 23.5 ms   │ 23.5 ms   │ identical   │
  ├──────────────────┼───────────┼───────────┼─────────────┤                                                                                                                                      
  │ Rand write MB/s  │ 363       │ 286       │ -21%        │
  ├──────────────────┼───────────┼───────────┼─────────────┤                                                                                                                                      
  │ Rand write p99.9 │ 1.8 ms    │ 16.7 ms   │ regression  │
  └──────────────────┴───────────┴───────────┴─────────────┘

  Seq write is identical. Rand write regressed — the whole-BDI flusher batched all dirty pages in one pass with writeback_sb_inodes() under a single blk_plug, while per-inode write_inode_now() loses that batching.

  Single-Client Reads

  ┌────────────────┬───────────┬───────────┬────────┐
  │   Benchmark    │ Whole-BDI │ Per-Inode │ Change │
  ├────────────────┼───────────┼───────────┼────────┤
  │ Seq read MB/s  │ 2950      │ 2350      │ -20%   │
  ├────────────────┼───────────┼───────────┼────────┤
  │ Rand read MB/s │ 651       │ 519       │ -20%   │
  └────────────────┴───────────┴───────────┴────────┘

  Reads shouldn't be affected by writeback path changes. Buffered reads also dropped (2888 → 2331), suggesting different system conditions between runs rather than a per-inode regression.

  Multi-Writer (Scenario A)

  ┌────────────────┬───────────┬───────────┬────────────┐
  │     Metric     │ Whole-BDI │ Per-Inode │   Change   │
  ├────────────────┼───────────┼───────────┼────────────┤
  │ Aggregate MB/s │ 1478      │ 999       │ -32%       │
  ├────────────────┼───────────┼───────────┼────────────┤
  │ p99.9          │ 46 ms     │ 77 ms     │ -67% worse │
  └────────────────┴───────────┴───────────┴────────────┘

  This is the biggest regression. With whole-BDI, the flusher did one batched pass through all dirty inodes via writeback_sb_inodes(). With per-inode, each of 4 writers queues a separate work item processed serially by write_inode_now() — losing the batch I/O merging benefit.

  Scenario C & D (Noisy Neighbor)

  ┌─────────────────────────┬───────────┬───────────┬─────────────┐
  │         Metric          │ Whole-BDI │ Per-Inode │   Change    │
  ├─────────────────────────┼───────────┼───────────┼─────────────┤
  │ Scenario C writer       │ 1468      │ 1386      │ -6%         │
  ├─────────────────────────┼───────────┼───────────┼─────────────┤
  │ Scenario C readers      │ 18.7 MB/s │ 18.7 MB/s │ identical   │
  ├─────────────────────────┼───────────┼───────────┼─────────────┤
  │ Scenario D writer       │ 1472      │ 1467      │ identical   │
  ├─────────────────────────┼───────────┼───────────┼─────────────┤
  │ Scenario D readers      │ 496 MB/s  │ 507 MB/s  │ +2%         │
  ├─────────────────────────┼───────────┼───────────┼─────────────┤
  │ Scenario D reader p99.9 │ 440 us    │ 358 us    │ +19% better │
  └─────────────────────────┴───────────┴───────────┴─────────────┘

  Mixed-mode (Scenario D) is the intended production case and it's essentially identical or slightly better — per-inode writeback creates less device contention for buffered readers.

  Summary

  The per-inode approach is neutral-to-slightly-better for the production scenario (Scenario D), but regresses on multi-writer and random write workloads. The core issue is loss of I/O batching
 — writeback_sb_inodes() processes all dirty inodes in one blk_plug'd pass, while per-inode write_inode_now() calls are processed one at a time. The read regressions likely reflect different
  system conditions since buffered/direct reads also dropped ~20%.

-- 
Jeff Layton <jlayton@kernel.org>