[Bugs] [Bug 1659327] New: 43% regression in small-file sequential read performance

[bugzilla at redhat.com]

https://bugzilla.redhat.com/show_bug.cgi?id=1659327

            Bug ID: 1659327
           Summary: 43% regression in small-file sequential read
                    performance
           Product: GlusterFS
           Version: mainline
            Status: NEW
         Component: open-behind
          Assignee: bugs at gluster.org
          Reporter: rgowdapp at redhat.com
                CC: bugs at gluster.org
  Target Milestone: ---
    Classification: Community



Description of problem:

With fixes [1][2] small-file workload [3] showed around 54% of regression in
read performance:

with patches [1][2]:
1856

without patches [1][2]:
3053.8 files/sec

Note that with patches [1][2], even though open-behind is present in the stack,
opens sent before read are actually sent to bricks as we turn off anonymous-fds
and set read-after-open to true. Since every read is witnessed by open-behind
(as its a parent of quick-read and reads are absorbed only by quick-read),
open-behind does an actual open on bricks. Without patches [1][2] none of open,
read, flush and release were sent to bricks. With patches [1][2] open, flush,
release are sent to bricks (reads are absorbed by quick-read). This extra fops
has caused the performance regression.

One fix I can think of is to change the relative order of open-behind and
quick-read (quick read as parent of open-behind), so that reads are not
witnessed by open-behind. Preliminary test runs show the regression indeed is
fixed by this solution (keeping read-after-open on and use-anonymous-fd no).
I'll post the average numbers after tests complete

[1] https://review.gluster.org/r/Ifa52d8ff017f115e83247f3396b9d27f0295ce3f
[2] https://review.gluster.org/r/Iaf130db71ce61ac37269f422e348a45f6ae6e82c
[3] https://github.com/distributed-system-analysis/smallfile

Version-Release number of selected component (if applicable):


How reproducible:
always

Steps to Reproduce:
1.
2.
3.

Actual results:


Expected results:


Additional info:

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