[Bugs] [Bug 1489513] New: read-ahead underperrforms expectations
bugzilla at redhat.com
bugzilla at redhat.com
Thu Sep 7 15:10:56 UTC 2017
https://bugzilla.redhat.com/show_bug.cgi?id=1489513
Bug ID: 1489513
Summary: read-ahead underperrforms expectations
Product: GlusterFS
Version: mainline
Component: read-ahead
Severity: medium
Priority: medium
Assignee: csaba at redhat.com
Reporter: csaba at redhat.com
CC: bugs at gluster.org, rgowdapp at redhat.com
The following test case -- originally used to verify the fix of bug 1393419
(read-ahead not working if open-behind is turned on) -- shows that using
read-ahead we get
worse performance than without it, in a scenario (sequential reading of a large
file) that is typically expected to be benefited by read-ahead.
# Test setup
Reading a 2Gb sparse file. For each setup (ie. with or without
patch https://review.gluster.org/15811) and setting (of referred vol options)
we did alltogether 200 iteations, on 2 VM-s (100 + 100, ie. one VM with the
patch, one without, and switching after first 100 which has the patched setup).
read-ahead-page-count open-behind without patch (MiB/s) with patch (MiB/s)
1 on 128.08 98.56
4 on 128.98 88.71
16 on 127.64 81.55
1 off 108.55 108.52
4 off 98.67 99.93
16 off 94.03 94.99
# Results with read-ahead off
Note that the patch changes xlator order even in this case.
• xlator order without patch:
1. debug/io-stats
2. performance/io-threads
3. performance/md-cache
4. performance/open-behind
5. performance/quick-read
6. performance/io-cache
7. performance/readdir-ahead
8. performance/write-behind
9. cluster/distribute
10. protocol/client
• xlator order with patch:
1. debug/io-stats
2. performance/io-threads
3. performance/md-cache
4. performance/quick-read
5. performance/io-cache
6. performance/readdir-ahead
7. performance/open-behind
8. performance/write-behind
9. cluster/distribute
10. protocol/client
However, the above order change does not seem to make a difference.
open-behind without patch (MiB/s) with patch (MiB/s)
on 132.87 133.51
off 139.70 139.77
# Conclusion
Performance ordering of settings (approximate MiB/s):
• no o/b & no r/a: 139
• o/b only: 133
• o/b only (r/a formally on but effectively disabled by o/b): 128
• r/a only: 108
• o/b & r/a (both working due to patch): 98
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