[Bugs] [Bug 1694943] New: parallel-readdir slows down directory listing

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1694943

            Bug ID: 1694943
           Summary: parallel-readdir slows down directory listing
           Product: GlusterFS
           Version: mainline
            Status: NEW
         Component: core
          Assignee: bugs at gluster.org
          Reporter: nbalacha at redhat.com
                CC: bugs at gluster.org
  Target Milestone: ---
    Classification: Community



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 at server2 nithya]# time ll |wc -l
20001

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

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




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

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

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

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

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