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Demonstrations of cachestat, the Linux ftrace version.
Here is some sample output showing file system cache statistics, followed by
the workload that caused it:
# ./cachestat -t
Counting cache functions... Output every 1 seconds.
TIME HITS MISSES DIRTIES RATIO BUFFERS_MB CACHE_MB
08:28:57 415 0 0 100.0% 1 191
08:28:58 411 0 0 100.0% 1 191
08:28:59 362 97 0 78.9% 0 8
08:29:00 411 0 0 100.0% 0 9
08:29:01 775 20489 0 3.6% 0 89
08:29:02 411 0 0 100.0% 0 89
08:29:03 6069 0 0 100.0% 0 89
08:29:04 15249 0 0 100.0% 0 89
08:29:05 411 0 0 100.0% 0 89
08:29:06 411 0 0 100.0% 0 89
08:29:07 411 0 3 100.0% 0 89
[...]
I used the -t option to include the TIME column, to make describing the output
easier.
The workload was:
# echo 1 > /proc/sys/vm/drop_caches; sleep 2; cksum 80m; sleep 2; cksum 80m
At 8:28:58, the page cache was dropped by the first command, which can be seen
by the drop in size for "CACHE_MB" (page cache size) from 191 Mbytes to 8.
After a 2 second sleep, a cksum command was issued at 8:29:01, for an 80 Mbyte
file (called "80m"), which caused a total of ~20,400 misses ("MISSES" column),
and the page cache size to grow by 80 Mbytes. The hit ratio during this dropped
to 3.6%. Finally, after another 2 second sleep, at 8:29:03 the cksum command
was run a second time, this time hitting entirely from cache.
Instrumenting all file system cache accesses does cost some overhead, and this
tool might slow your target system by 2% or so. Test before use if this is a
concern.
This tool also uses dynamic tracing, and is tied to Linux kernel implementation
details. If it doesn't work for you, it probably needs fixing.
Use -h to print the USAGE message:
# ./cachestat -h
USAGE: cachestat [-Dht] [interval]
-D # print debug counters
-h # this usage message
-t # include timestamp
interval # output interval in secs (default 1)
eg,
cachestat # show stats every second
cachestat 5 # show stats every 5 seconds
See the man page and example file for more info.
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