cluster/dht: rebalance perf enhancement

Problem: Throttle settings "normal" and "aggressive" for rebalance
did not have performance difference.

normal mode spawns $(no. of cores - 4)/2 threads and aggressive
spawns $(no. of cores - 4) threads. Though aggressive mode has twice
the number of threads compared to that of normal mode, there was no
performance gain when switched to aggressive mode from normal mode.

RCA:
During the course of debugging the above problem, we tried assigning
migration job to migration threads spawned by rebalance, rather than
synctasks(as there is more overhead associated to manage the task
queue and threads). This gave us a significant improvement over rebalance
under synctasks. This patch does not really gurantee that there will be a
clear performance difference between normal and aggressive mode, but this
patch certainly maximized the disk utilization for 1GBfiles run.

Results:

Test enviroment:
Gluster Config:
Number of Bricks: 2 (one brick per disk(RAID-6 12 disk))
Bricks:
Brick1: server1:/brick/test1/1
Brick2: server2:/brick/test1/1
Options Reconfigured:
performance.readdir-ahead: on
server.event-threads: 4
client.event-threads: 4

1000 files with 1GB each were created/renamed such that all files will have
server1 as cached and server2 as hashed, so that all files will be migrated.

Test machines had 24 cores each.

Results  with/without synctask based migration:
-----------------------------------------------

mode                    normal(10threads)          aggressive(20threads)

timetaken               0:55:30 (h:m:s)            0:56:3 (h:m:s)
withsynctask

timetaken
with migrator           0:38:3 (h:m:s)             0:23:41 (h:m:s)
threads

From above table it can be seen that, there is a clear 2x perf gain between
rebalance with synctask vs rebalance with migrator threads.

Additionally this patch modifies the code so that caller will have the exact error
number returned by dht_migrate_file(earlier the errno meaning was overloaded). This
will help avoiding scenarios where migration failure due to ENOENT, can result in
rebalance abort/failure.

Change-Id: I8904e2fb147419d4a51c1267be11a08ffd52168e
BUG: 1420166
Signed-off-by: Susant Palai <spalai@redhat.com>
Reviewed-on: https://review.gluster.org/16427
Smoke: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: N Balachandran <nbalacha@redhat.com>
Reviewed-by: Raghavendra G <rgowdapp@redhat.com>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>

0 files changed, 0 insertions, 0 deletions