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From:   Daire Byrne <daire@dneg.com>
Date:   Tue, 8 Feb 2022 18:48:49 +0000
Message-ID: <CAPt2mGP2guMMf1C9VoQ0AvZ819jPuz0vDoEzJJhtL8q5DJ300A@mail.gmail.com>
Subject: Re: parallel file create rates (+high latency)
To:     "J. Bruce Fields" <bfields@fieldses.org>
Cc:     NeilBrown <neilb@suse.de>, Patrick Goetz <pgoetz@math.utexas.edu>,
        linux-nfs <linux-nfs@vger.kernel.org>
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On Wed, 26 Jan 2022 at 02:57, J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Wed, Jan 26, 2022 at 11:02:16AM +1100, NeilBrown wrote:
> > On Wed, 26 Jan 2022, J. Bruce Fields wrote:
> > > On Tue, Jan 25, 2022 at 03:15:42PM -0600, Patrick Goetz wrote:
> > > > So the directory is locked while the inode is created, or something
> > > > like this, which makes sense.
> > >
> > > It accomplishes a number of things, details in
> > > https://www.kernel.org/doc/html/latest/filesystems/directory-locking.html
> >
> > Just in case anyone is interested, I wrote this a while back:
> >
> > http://lists.lustre.org/pipermail/lustre-devel-lustre.org/2018-November/008177.html
> >
> > it includes a patch to allow parallel creates/deletes over NFS (and any
> > other filesystem which adds support).
> > I doubt it still applies, but it wouldn't be hard to make it work if
> > anyone was willing to make a strong case that we would benefit from
> > this.

Well, I couldn't resist quickly testing Neil's patch. I found it
applied okay to v5.6.19 with minimal edits.

As before, without the patch, parallel file creates in a single
directory with 1000 threads topped out at an aggregate of 3 creates/s
over a 200ms link. With the patch it jumps up to 1,200 creates/s.

So a pretty dramatic difference. I can't say if there are any other
side effects or regressions as I only did this simple test.

It's great for our super niche workloads and use case anyway.

Daire


> Neato.
>
> Removing the need to hold an exclusive lock on the directory across
> server round trips seems compelling to me....
>
> I also wonder: why couldn't you fire off the RPC without any locks, then
> wait till you get a reply to take locks and update your local cache?
>
> OK, for one thing, calls and replies and server processing could all get
> reordered.  We'd need to know what order the server processed operations
> in, so we could process replies in the same order.
>
> --b.