It's been noted that z3fold doesn't scale well when it's run in a large
number of threads on many cores, which can be easily reproduced with fio
'randrw' test with --numjobs=32.  E.g.  the result for 1 cluster (4 cores)
is:

Run status group 0 (all jobs):
   READ: io=244785MB, aggrb=496883KB/s, minb=15527KB/s, ...
  WRITE: io=246735MB, aggrb=500841KB/s, minb=15651KB/s, ...

While for 8 cores (2 clusters) the result is:

Run status group 0 (all jobs):
   READ: io=244785MB, aggrb=265942KB/s, minb=8310KB/s, ...
  WRITE: io=246735MB, aggrb=268060KB/s, minb=8376KB/s, ...

The bottleneck here is the pool lock which many threads become waiting
upon.  To reduce that spin lock contention, z3fold can operate only on
the lists local to the current CPU whenever possible.  Due to the nature
of z3fold unbuddied list handling (it only takes the first entry off the
list on a hot path), if the z3fold pool is big enough and balanced well
enough, limiting search to only local unbuddied list doesn't lead to a
significant compression ratio degrade (2.57x vs 2.65x in our
measurements).

This patch also introduces two worker threads: one for async in-page
object layout optimization and one for releasing freed pages.  This is
done to speed up z3fold_free() which is often on a hot path.

The fio results for 8-core case are now the following:

Run status group 0 (all jobs):
   READ: io=244785MB, aggrb=1568.3MB/s, minb=50182KB/s, ...
  WRITE: io=246735MB, aggrb=1580.8MB/s, minb=50582KB/s, ...

So we're in for almost 6x performance increase.

Link: http://lkml.kernel.org/r/20170806181443.f9b65018f8bde25ef990f9e8@gmail.com


Signed-off-by: Vitaly Wool <vitalywool@gmail.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>