Re: [HELP] FUSE writeback performance bottleneck

[Joanne Koong <joannelkoong@gmail.com>]

On Thu, Aug 22, 2024 at 10:00 AM Joanne Koong <joannelkoong@gmail.com> wrote:
>
> On Tue, Jun 4, 2024 at 3:02 AM Miklos Szeredi <miklos@szeredi.hu> wrote:
> >
> > On Tue, 4 Jun 2024 at 11:32, Bernd Schubert <bernd.schubert@fastmail.fm> wrote:
> >
> > > Back to the background for the copy, so it copies pages to avoid
> > > blocking on memory reclaim. With that allocation it in fact increases
> > > memory pressure even more. Isn't the right solution to mark those pages
> > > as not reclaimable and to avoid blocking on it? Which is what the tmp
> > > pages do, just not in beautiful way.
> >
> > Copying to the tmp page is the same as marking the pages as
> > non-reclaimable and non-syncable.
> >
> > Conceptually it would be nice to only copy when there's something
> > actually waiting for writeback on the page.
> >
> > Note: normally the WRITE request would be copied to userspace along
> > with the contents of the pages very soon after starting writeback.
> > After this the contents of the page no longer matter, and we can just
> > clear writeback without doing the copy.
> >
> > But if the request gets stuck in the input queue before being copied
> > to userspace, then deadlock can still happen if the server blocks on
> > direct reclaim and won't continue with processing the queue.   And
> > sync(2) will also block in that case.
>
> Why doesn't it suffice to just check if the page is being reclaimed
> and do the tmp page allocation only if it's under reclaim?

Never mind, Josef explained it to me. I misunderstood what the
PG_reclaim flag does.
>
> >
> > So we'd somehow need to handle stuck WRITE requests.   I don't see an
> > easy way to do this "on demand", when something actually starts
> > waiting on PG_writeback.  Alternatively the page copy could be done
> > after a timeout, which is ugly, but much easier to implement.
> >
> > Also splice from the fuse dev would need to copy those pages, but that
> > shouldn't be a problem, since it's just moving the copy from one place
> > to another.
> >
> > Thanks,
> > Miklos