writeback_example.txt

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.