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writeback_example.txt
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Demonstrations of writeback, the Linux bpftrace/eBPF version. This tool traces when the kernel writeback procedure is writing dirtied pages to disk, and shows details such as the time, device numbers, reason for the write back, and the duration. For example: # ./writeback.bt Attaching 4 probes... Tracing writeback... Hit Ctrl-C to end. TIME DEVICE PAGES REASON ms 23:28:47 259:1 15791 periodic 0.005 23:28:48 259:0 15792 periodic 0.004 23:28:52 259:1 15784 periodic 0.003 23:28:53 259:0 18682 periodic 0.003 23:28:55 259:0 41970 background 326.663 23:28:56 259:0 18418 background 332.689 23:28:56 259:0 60402 background 362.446 23:28:57 259:1 18230 periodic 0.005 23:28:57 259:1 65492 background 3.343 23:28:57 259:1 65492 background 0.002 23:28:58 259:0 36850 background 0.000 23:28:58 259:0 13298 background 597.198 23:28:58 259:0 55282 background 322.050 23:28:59 259:0 31730 background 336.031 23:28:59 259:0 8178 background 357.119 23:29:01 259:0 50162 background 1803.146 23:29:02 259:0 27634 background 1311.876 23:29:03 259:0 6130 background 331.599 23:29:03 259:0 50162 background 293.968 23:29:03 259:0 28658 background 284.946 23:29:03 259:0 7154 background 286.572 [...] By looking a the timestamps and latency, it can be seen that the system was not spending much time in writeback until 23:28:55, when "background" writeback began, taking over 300 milliseconds per flush. If timestamps of heavy writeback coincide with times when applications suffered performance issues, that would be a clue that they are correlated and there is contention for the disk devices. There are various ways to tune this: eg, vm.dirty_writeback_centisecs.