Performance regression with random IO pattern from the client

[Jan Kara <jack@suse.cz>]

Hello,

during our performance testing we have noticed that commit b6669305d35a
("nfsd: Reduce the number of calls to nfsd_file_gc()") has introduced a
performance regression when a client does random buffered writes. The
workload on NFS client is fio running 4 processed doing random buffered writes to 4
different files and the files are large enough to hit dirty limits and
force writeback from the client. In particular the invocation is like:

fio --direct=0 --ioengine=sync --thread --directory=/mnt/mnt1 --invalidate=1 --group_reporting=1 --runtime=300 --fallocate=posix --ramp_time=10 --name=RandomReads-128000-4k-4 --new_group --rw=randwrite --size=4000m --numjobs=4 --bs=4k --filename_format=FioWorkloads.\$jobnum --end_fsync=1

The reason why commit b6669305d35a regresses performance is the
filemap_flush() call it adds into nfsd_file_put(). Before this commit
writeback on the server happened from nfsd_commit() code resulting in
rather long semisequential streams of 4k writes. After commit b6669305d35a
all the writeback happens from filemap_flush() calls resulting in much
longer average seek distance (IO from different files is more interleaved)
and about 16-20% regression in the achieved writeback throughput when the
backing store is rotational storage.

I think the filemap_flush() from nfsd_file_put() is indeed rather
aggressive and I think we'd be better off to just leave writeback to either
nfsd_commit() or standard dirty page cleaning happening on the system. I
assume the rationale for the filemap_flush() call was to make it more
likely the file can be evicted during the garbage collection run? Was there
any particular problem leading to addition of this call or was it just "it
seemed like a good idea" thing?

Thanks in advance for ideas.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR