NAME¶

dcstat - Directory entry cache (dcache) stats. Uses Linux eBPF/bcc.

SYNOPSIS¶

dcstat [interval [count]]

DESCRIPTION¶

The Linux directory entry cache (dcache) improves the performance of file and directory name lookups. This tool provides per-second summary statistics of dcache performance.

This uses kernel dynamic tracing of kernel functions, lookup_fast() and d_lookup(), which will need to be modified to match kernel changes.

Since this uses BPF, only the root user can use this tool.

REQUIREMENTS¶

CONFIG_BPF and bcc.

EXAMPLES¶

Print summaries each second:

# dcstat

Print output every five seconds, three times:

# dcstat 5 3

FIELDS¶

REFS/s

Number dcache lookups (references) per second.

SLOW/s

Number of dcache lookups that failed the lookup_fast() path and executed the lookup_slow() path instead.

MISS/s

Number of dcache misses (failed both fast and slow lookups).

HIT%

Percentage of dcache hits over total references.

OVERHEAD¶

The overhead depends on the frequency of file and directory name lookups. While the per-event overhead is low, some applications may make over 100k lookups per second, and the low per-event overhead will begin to add up, and could begin to be measurable (over 10% CPU usage). Measure in a test environment.

SOURCE¶

This is from bcc.

https://github.com/iovisor/bcc

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.

OS¶

Linux

AUTHOR¶

Brendan Gregg

SEE ALSO¶

dcsnoop(8)