Description of problem:
While running tests with the upstream master (HEAD at commit dfa255ae7f2dab4fb3d84c67a0452c5b32455877), I noticed that enabling parallel-readdir seems to increase the time taken for a directory listing:


Numbers from a pure distribute 3 brick volume:

Volume Name: pvol
Type: Distribute
Volume ID: c39c8c16-82d3-4b0b-8050-9c3d22c800ea
Status: Started
Snapshot Count: 0
Number of Bricks: 3
Transport-type: tcp
Bricks:
Brick1: server1:/mnt/bricks/fsgbench0002/brick-0
Brick2: server1:/mnt/bricks/fsgbench0003/brick-0
Brick3: server1:/mnt/bricks/fsgbench0004/brick-0
Options Reconfigured:
transport.address-family: inet
nfs.disable: on



The volume was mounted on /mnt/nithya and I created 10K directories and 10K files in the volume root:


With readdir-ahead enabled:
----------------------------
[root@server2 nithya]# time ll |wc -l
20001

real	0m11.434s
user	0m0.116s
sys	0m0.241s
[root@server2 nithya]# time ll |wc -l
20001

real	0m6.825s
user	0m0.111s
sys	0m0.265s




With readdir-ahead and parallel-readdir enabled:
------------------------------------------------

[root@server2 nithya]# time ll |wc -l
20001

real	0m15.609s
user	0m0.148s
sys	0m0.379s
[root@server2 nithya]# time ll |wc -l
20001

real	0m9.930s
user	0m0.107s
sys	0m0.295s