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Date:   Tue, 23 Feb 2021 20:27:42 +0000
From:   Matthew Wilcox <willy@infradead.org>
To:     Steve French <smfrench@gmail.com>
Cc:     Jeff Layton <jlayton@redhat.com>,
        David Howells <dhowells@redhat.com>,
        Trond Myklebust <trondmy@hammerspace.com>,
        Anna Schumaker <anna.schumaker@netapp.com>,
        Steve French <sfrench@samba.org>,
        Dominique Martinet <asmadeus@codewreck.org>,
        CIFS <linux-cifs@vger.kernel.org>, ceph-devel@vger.kernel.org,
        linux-cachefs@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-mm <linux-mm@kvack.org>, linux-afs@lists.infradead.org,
        v9fs-developer@lists.sourceforge.net,
        Christoph Hellwig <hch@lst.de>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        David Wysochanski <dwysocha@redhat.com>,
        LKML <linux-kernel@vger.kernel.org>,
        William Kucharski <william.kucharski@oracle.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>, Chao Yu <yuchao0@huawei.com>,
        linux-f2fs-devel@lists.sourceforge.net
Subject: Re: [PATCH 00/33] Network fs helper library & fscache kiocb API [ver
 #3]
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On Mon, Feb 15, 2021 at 11:22:20PM -0600, Steve French wrote:
> On Mon, Feb 15, 2021 at 8:10 PM Matthew Wilcox <willy@infradead.org> wrote:
> > The switch from readpages to readahead does help in a couple of corner
> > cases.  For example, if you have two processes reading the same file at
> > the same time, one will now block on the other (due to the page lock)
> > rather than submitting a mess of overlapping and partial reads.
> 
> Do you have a simple repro example of this we could try (fio, dbench, iozone
> etc) to get some objective perf data?

I don't.  The problem was noted by the f2fs people, so maybe they have a
reproducer.

> My biggest worry is making sure that the switch to netfs doesn't degrade
> performance (which might be a low bar now since current network file copy
> perf seems to signifcantly lag at least Windows), and in some easy to understand
> scenarios want to make sure it actually helps perf.

I had a question about that ... you've mentioned having 4x4MB reads
outstanding as being the way to get optimum performance.  Is there a
significant performance difference between 4x4MB, 16x1MB and 64x256kB?
I'm concerned about having "too large" an I/O on the wire at a given time.
For example, with a 1Gbps link, you get 250MB/s.  That's a minimum
latency of 16us for a 4kB page, but 16ms for a 4MB page.

"For very simple tasks, people can perceive latencies down to 2 ms or less"
(https://danluu.com/input-lag/)
so going all the way to 4MB I/Os takes us into the perceptible latency
range, whereas a 256kB I/O is only 1ms.

So could you do some experiments with fio doing direct I/O to see if
it takes significantly longer to do, say, 1TB of I/O in 4MB chunks vs
256kB chunks?  Obviously use threads to keep lots of I/Os outstanding.