biolatency_example.txt

Demonstrations of biolatency, the Linux BPF/bpftrace version.


This traces block I/O, and shows latency as a power-of-2 histogram. For example:

# ./biolatency.bt
Attaching 3 probes...
Tracing block device I/O... Hit Ctrl-C to end.
^C

@usecs:
[256, 512)             2 |                                                    |
[512, 1K)             10 |@                                                   |
[1K, 2K)             426 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[2K, 4K)             230 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@                        |
[4K, 8K)               9 |@                                                   |
[8K, 16K)            128 |@@@@@@@@@@@@@@@                                     |
[16K, 32K)            68 |@@@@@@@@                                            |
[32K, 64K)             0 |                                                    |
[64K, 128K)            0 |                                                    |
[128K, 256K)          10 |@                                                   |

While tracing, this shows that 426 block I/O had a latency of between 1K and 2K
usecs (1024 and 2048 microseconds), which is between 1 and 2 milliseconds.
There are also two modes visible, one between 1 and 2 milliseconds, and another
between 8 and 16 milliseconds: this sounds like cache hits and cache misses.
There were also 10 I/O with latency 128 to 256 ms: outliers. Other tools and
instrumentation, like biosnoop.bt, can shed more light on those outliers.


There is another version of this tool in bcc: https://github.com/iovisor/bcc
The bcc version provides options to customize the output.