Re: [PATCHSET v2 0/3] Improve IOCB_NOWAIT O_DIRECT reads

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On Tue, Feb 9, 2021 at 10:25 PM Jens Axboe <axboe@xxxxxxxxx> wrote:
>
> On 2/9/21 12:55 PM, Andrew Morton wrote:
> > On Mon,  8 Feb 2021 19:30:05 -0700 Jens Axboe <axboe@xxxxxxxxx> wrote:
> >
> >> Hi,
> >>
> >> For v1, see:
> >>
> >> https://lore.kernel.org/linux-fsdevel/20210208221829.17247-1-axboe@xxxxxxxxx/
> >>
> >> tldr; don't -EAGAIN IOCB_NOWAIT dio reads just because we have page cache
> >> entries for the given range. This causes unnecessary work from the callers
> >> side, when the IO could have been issued totally fine without blocking on
> >> writeback when there is none.
> >>
> >
> > Seems a good idea.  Obviously we'll do more work in the case where some
> > writeback needs doing, but we'll be doing synchronous writeout in that
> > case anyway so who cares.
>
> Right, I think that'll be a round two on top of this, so we can make the
> write side happier too. That's a bit more involved...
>
> > Please remind me what prevents pages from becoming dirty during or
> > immediately after the filemap_range_needs_writeback() check?  Perhaps
> > filemap_range_needs_writeback() could have a comment explaining what it
> > is that keeps its return value true after it has returned it!
>
> It's inherently racy, just like it is now. There's really no difference
> there, and I don't think there's a way to close that. Even if you
> modified filemap_write_and_wait_range() to be non-block friendly,
> there's nothing stopping anyone from adding dirty page cache right after
> that call.
>

Jens, do you have some numbers before and after your patchset is applied?

And kindly a test "profile" for FIO :-)?

Thanks.

- Sedat -




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