Read Ahead Cache Stats

:warning: The readaheadstat.py tool was originally showcased in "BPF Performance Tools" book by Brendan Gregg. It has now been upstreamed in the official BCC repository and will be maintained there. you can still refer to the influxdb version here to see how it works.

This tool shows the performance of the read ahead mechanism in your FS, specifically higlighting ununsed pages in the cache and how long they have remained there. Cache misses in read-ahead cache reduces the performance of a FS call and can be an important metric in understanding how moden architectures affect such metrics in the system.

readaheadstat.bt

This tool requires bpftrace to be installed on the system

$ sudo ./readaheadstat.bt

Attaching 5 probes...
^C
Readahead unused pages: 15816

Readahead used page age (ms):
@age_ms:
[0]                 2216 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@                     |
[1]                   74 |@                                                   |
[2, 4)               152 |@@                                                  |
[4, 8)               553 |@@@@@@@                                             |
[8, 16)              116 |@                                                   |
[16, 32)              81 |@                                                   |
[32, 64)              79 |@                                                   |
[64, 128)             88 |@                                                   |
[128, 256)           601 |@@@@@@@@                                            |
[256, 512)           157 |@@                                                  |
[512, 1K)            107 |@                                                   |
[1K, 2K)             136 |@                                                   |
[2K, 4K)            3689 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@|
[4K, 8K)              82 |@                                                   |
[8K, 16K)           1170 |@@@@@@@@@@@@@@@@                                    |

readaheadstat.py

This tool requires BCC to be installed on your system

$ sudo ./readaheadstat.py

Tracing... Hit Ctrl-C to end.
^C
Read-ahead unused pages: 3630
Histogram of read-ahead used page age (ms)
==========================================
     ms                  : count     distribution
         0 -> 1          : 136      |*****************                       |
         2 -> 3          : 0        |                                        |
         4 -> 7          : 0        |                                        |
         8 -> 15         : 1        |                                        |
        16 -> 31         : 0        |                                        |
        32 -> 63         : 0        |                                        |
        64 -> 127        : 0        |                                        |
       128 -> 255        : 0        |                                        |
       256 -> 511        : 43       |*****                                   |
       512 -> 1023       : 0        |                                        |
      1024 -> 2047       : 312      |****************************************|