Changed log lock from 'quick' to 'basic' because this is an I/O lock. #124
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We cannot use a 'quick' lock (i.e. lock spinning on the CPU) for the log lock because it can wait a lot on I/Os. Using a 'quick' lock leads to eating the CPU for no good reason. Example of 'pidstat' output when using various locks for log_lock: - 'quick' lock and slow log file system (tail -f on the log file on XFS on RHEL 8) 04:15:11 PM UID TGID TID %usr %system %CPU CPU Command 04:15:21 PM 998 16431 - 100.00 4.20 100.00 2 unbound 04:15:21 PM 998 - 16431 31.00 1.00 32.00 2 |__unbound 04:15:21 PM 998 - 16432 31.30 0.80 32.10 0 |__unbound 04:15:21 PM 998 - 16433 30.20 1.40 31.60 1 |__unbound 04:15:21 PM 998 - 16434 30.70 1.00 31.70 3 |__unbound - 'quick' lock and log file system being fast 04:15:40 PM UID TGID TID %usr %system %CPU CPU Command 04:15:50 PM 998 16431 - 10.00 1.60 11.60 1 unbound 04:15:50 PM 998 - 16431 2.50 0.50 3.00 1 |__unbound 04:15:50 PM 998 - 16432 2.30 0.40 2.70 3 |__unbound 04:15:50 PM 998 - 16433 2.70 0.30 3.00 0 |__unbound 04:15:50 PM 998 - 16434 2.60 0.40 3.00 2 |__unbound - 'basic' lock (this commit) and slow log file system (tail -f on the log file on XFS on RHEL 8) 04:29:48 PM UID TGID TID %usr %system %CPU CPU Command 04:29:58 PM 998 11632 - 7.10 14.10 21.20 3 unbound 04:29:58 PM 998 - 11632 1.70 3.20 4.90 3 |__unbound 04:29:58 PM 998 - 11633 1.60 3.30 4.90 1 |__unbound 04:29:58 PM 998 - 11634 2.00 4.10 6.10 1 |__unbound 04:29:58 PM 998 - 11635 1.90 3.50 5.40 1 |__unbound We can see in the above example, when 'basic' lock is used, that CPU isn't consumed when log file system is slow. Another reproducer scenario: put the log file on a NFS share with 'sync' option.
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That look like it improves CPU usage, the lower CPU usage is nice for high throughput (and thus also slower log wait times I guess) situations, so I think this is a very good fix to have. Thanks for the measurements and the suggested fix! |
wcawijngaards
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Dec 3, 2019
- Merge pull request #124 from rmetrich: Changed log lock from 'quick' to 'basic' because this is an I/O lock.
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Thank you, I appreciate. For your information, this was reported by one of Red Hat customers and I filed RHBZ https://bugzilla.redhat.com/show_bug.cgi?id=1775708 for that. |
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We cannot use a 'quick' lock (i.e. lock spinning on the CPU) for the log
lock because it can wait a lot on I/Os. Using a 'quick' lock leads to
eating the CPU for no good reason.
Example of 'pidstat' output when using various locks for log_lock:
04:15:11 PM UID TGID TID %usr %system %CPU CPU Command
04:15:21 PM 998 16431 - 100.00 4.20 100.00 2 unbound
04:15:21 PM 998 - 16431 31.00 1.00 32.00 2 |__unbound
04:15:21 PM 998 - 16432 31.30 0.80 32.10 0 |__unbound
04:15:21 PM 998 - 16433 30.20 1.40 31.60 1 |__unbound
04:15:21 PM 998 - 16434 30.70 1.00 31.70 3 |__unbound
04:15:40 PM UID TGID TID %usr %system %CPU CPU Command
04:15:50 PM 998 16431 - 10.00 1.60 11.60 1 unbound
04:15:50 PM 998 - 16431 2.50 0.50 3.00 1 |__unbound
04:15:50 PM 998 - 16432 2.30 0.40 2.70 3 |__unbound
04:15:50 PM 998 - 16433 2.70 0.30 3.00 0 |__unbound
04:15:50 PM 998 - 16434 2.60 0.40 3.00 2 |__unbound
04:29:48 PM UID TGID TID %usr %system %CPU CPU Command
04:29:58 PM 998 11632 - 7.10 14.10 21.20 3 unbound
04:29:58 PM 998 - 11632 1.70 3.20 4.90 3 |__unbound
04:29:58 PM 998 - 11633 1.60 3.30 4.90 1 |__unbound
04:29:58 PM 998 - 11634 2.00 4.10 6.10 1 |__unbound
04:29:58 PM 998 - 11635 1.90 3.50 5.40 1 |__unbound
We can see in the above example, when 'basic' lock is used, that CPU
isn't consumed when log file system is slow.
Another reproducer scenario: put the log file on a NFS share with 'sync'
option.