cachestat - Measure page cache hits/misses. Uses Linux ftrace.
This tool provides basic cache hit/miss statistics for the Linux page cache.
Its current implementation uses Linux ftrace dynamic function profiling to
create custom in-kernel counters, which is a workaround until such counters
can be built-in to the kernel. Specifically, four kernel functions are
- mark_page_accessed() for measuring cache accesses
- mark_buffer_dirty() for measuring cache writes
- add_to_page_cache_lru() for measuring page additions
- account_page_dirtied() for measuring page dirties
It is possible that these functions have been renamed (or are different
logically) for your kernel version, and this script will not work as-is. This
was written for a Linux 3.13 kernel, and tested on a few others versions. This
script is a sandcastle: the kernel may wash some away, and you'll need to
This program's implementation can be improved in the future when other kernel
capabilities are made available. If you need a more reliable tool now, then
consider other tracing alternatives (eg, SystemTap). This tool is really a
proof of concept to see what ftrace can currently do.
WARNING: This uses dynamic tracing of kernel functions, and could cause kernel
panics or freezes. Test, and know what you are doing, before use. It also
traces cache activity, which can be frequent, and cost some overhead. The
statistics should be treated as best-effort: there may be some error margin
depending on unusual workload types.
Since this uses ftrace, only the root user can use this tool.
CONFIG_FUNCTION_PROFILER, which you may already have enabled and available on
recent kernels, and awk.
- Include extra fields for debug purposes (see script).
- Print usage message.
- Include timestamps in units of seconds.
- Output interval in seconds. Default is 1.
- Show per-second page cache statistics:
- # cachestat
- Time, in HH:MM:SS.
- Number of page cache hits (reads). Each hit is for one
memory page (the size depends on your processor architecture; commonly 4
Kbytes). Since this tool outputs at a timed interval, this field indicates
the cache hit rate.
- Number of page cache misses (reads from storage I/O). Each
miss is for one memory page. Cache misses should be causing disk I/O. Run
iostat(1) for correlation (although the miss count and size by the time
disk I/O is issued can differ due to I/O subsystem merging).
- Number of times a page in the page cache was written to and
thus "dirtied". The same page may be counted multiple times per
interval, if it is written to multiple times. This field gives an
indication of how much cache churn there is, caused by applications
- The ratio of cache hits to total cache accesses (hits +
misses), as a percentage.
- Size of the buffer cache, for disk I/O. From
- Size of the page cache, for file system I/O. From
This tool currently uses ftrace function profiling, which provides efficient
in-kernel counters. However, the functions profiled are executed frequently,
so the overheads can add up. Test and measure before use. My own testing
showed around a 2% loss in application performance while this tool was
This is from the perf-tools collection.
Also look under the examples directory for a text file containing example usage,
output, and commentary for this tool.
Unstable - in development.