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

Christoph Hellwig
2026-04-09T06:05:08Z

On Wed, Apr 08, 2026 at 10:25:20AM -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

Please add the results into the patch. Besides having them in the patch
vs the cover letter, URLs to random hosting sites tend to get stale
very easily.

Comments on the XFS numbers / benchmark setup:

How does O_DIRECT manage to create almost the same peek dirty as
buffered? Similarly even this patched doncache show be lower, which
feels odd.

(the editorializing in these results feels odd, not sure what tool
generates them, but maybe edit it for the commit log to look a bit more
serious).

Given that this patch does not change the read path, how do the pure read
numbers for patches vs unpatched dontcache manage to differ so much?

Comments on the NFSD numbers / benchmark setup:

I though you were testing using dontcache on the server, but the
comments seem to imply it is done on the client, what is the case
here?

Again, the read changes between patches and unpatched look suspect.
Please drill down why they happen, the fact that patches is slower here
and faster in XFS makes me wonder if the results might just be very
volatile?