tools/cachestat_example.txt

Demonstrations of cachestat, the Linux eBPF/bcc version.


cachestat shows hits and misses to the file system page cache. For example:

# cachestat
    HITS   MISSES  DIRTIES HITRATIO   BUFFERS_MB  CACHED_MB
    1132        0        4  100.00%          277       4367
     161        0       36  100.00%          277       4372
      16        0       28  100.00%          277       4372
   17154    13750       15   55.51%          277       4422
      19        0        1  100.00%          277       4422
      83        0       83  100.00%          277       4421
      16        0        1  100.00%          277       4423
^C       0      -19      360    0.00%          277       4423
Detaching...

While tracing, there was a burst of misses in the fourth second, bringing
the hit ration down to 55%.


This shows a 1 Gbyte uncached file that is read twice:

(root) ~ # ./cachestat.py 
    HITS   MISSES  DIRTIES HITRATIO   BUFFERS_MB  CACHED_MB
       1        0        0  100.00%            5        191
     198    12136        0    1.61%            5        238
       1    11007        3    0.01%            5        281
       0     6384        0    0.00%            5        306
       1    14464        0    0.01%            5        363
       0    11776        0    0.00%            5        409
       1    11712        0    0.01%            5        454
      32    13184        0    0.24%            5        506
       0    11232        0    0.00%            5        550
       1    13056        0    0.01%            5        601
      16    14720        0    0.11%            5        658
      33     9920        0    0.33%            5        697
       0    13248        0    0.00%            5        749
       4    14144        0    0.03%            5        804
       0     9728        0    0.00%            5        842
       1    10816        0    0.01%            5        885
     808    13504        1    5.65%            5        938
       0    11409        0    0.00%            5        982
       0    11520        0    0.00%            5       1027
       0    15616        0    0.00%            5       1088
       1     9792        0    0.01%            5       1126
       0     8256        0    0.00%            5       1158
       1     9600        0    0.01%            5       1196
     599     4804        0   11.09%            5       1215
       1        0        0  100.00%            5       1215
       0        0        0    0.00%            5       1215
       3        1        0   75.00%            5       1215
   79536       34        0   99.96%            5       1215
   87693      274        4   99.69%            6       1214
   89018     3546        0   96.17%            7       1227
   33531      201        4   99.40%            7       1228
      22       44        0   33.33%            8       1228
       0        0        0    0.00%            8       1228
      73       21        2   77.66%            8       1228

It took 24 seconds to read the 1 Gbyte file the first time, shown in the output
by the high MISSES rate and low HITRATIO. The second time it took 4 seconds,
and the HITRATIO was around 99%.


This output shows a 1 Gbyte file being created and added to the page cache:

(root) ~ # ./cachestat.py 
    HITS   MISSES  DIRTIES HITRATIO   BUFFERS_MB  CACHED_MB
       1        0        0  100.00%            8        209
       0        0   165584    0.00%            8        856
       0        0    96505    0.00%            8       1233
       0        0        0    0.00%            8       1233

Note the high rate of DIRTIES, and the CACHED_MB size increases by 1024 Mbytes.


USAGE message:

# cachestat -h
usage: cachestat.py [-h] [-T] [interval] [count]

Count cache kernel function calls

positional arguments:
  interval         output interval, in seconds
  count            number of outputs

optional arguments:
  -h, --help       show this help message and exit
  -T, --timestamp  include timestamp on output