Re: [PATCH 00/33] Network fs helper library & fscache kiocb API [ver #3]

[Matthew Wilcox]

On Mon, Feb 15, 2021 at 06:40:27PM -0600, Steve French wrote:
> It could be good if netfs simplifies the problem experienced by
> network filesystems on Linux with readahead on large sequential reads
> - where we don't get as much parallelism due to only having one
> readahead request at a time (thus in many cases there is 'dead time'
> on either the network or the file server while waiting for the next
> readpages request to be issued).   This can be a significant
> performance problem for current readpages when network latency is long
> (or e.g. in cases when network encryption is enabled, and hardware
> offload not available so time consuming on the server or client to
> encrypt the packet).
> 
> Do you see netfs much faster than currentreadpages for ceph?
> 
> Have you been able to get much benefit from throttling readahead with
> ceph from the current netfs approach for clamping i/o?

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.

We're not there yet on having multiple outstanding reads.  Bill and I
had a chat recently about how to make the readahead code detect that
it is in a "long fat pipe" situation (as opposed to just dealing with
a slow device), and submit extra readahead requests to make best use of
the bandwidth and minimise blocking of the application.

That's not something for the netfs code to do though; we can get into
that situation with highly parallel SSDs.