On Mon, Feb 15, 2021 at 8:10 PM Matthew Wilcox [off-list ref] wrote:

On Mon, Feb 15, 2021 at 06:40:27PM -0600, Steve French wrote:

quoted

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.

Do you have a simple repro example of this we could try (fio, dbench, iozone
etc) to get some objective perf data?

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.

-- 
Thanks,

Steve