NAME¶

dcsnoop.bt - Trace directory entry cache (dcache) lookups. Uses bpftrace/eBPF.

SYNOPSIS¶

dcsnoop.bt

DESCRIPTION¶

By default, this traces every dcache lookup, and shows the process performing the lookup and the filename requested.

The output of this tool can be verbose, and is intended for further investigations of dcache performance beyond dcstat(8), which prints per-second summaries.

This uses kernel dynamic tracing of the d_lookup() function, and will need and will need updating to match any changes to this function.

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

REQUIREMENTS¶

CONFIG_BPF and bcc.

EXAMPLES¶

Trace all dcache lookups:

# dcsnoop.bt

FIELDS¶

TIME(ms)

Time of lookup, in milliseconds.

PID

Process ID.

COMM

Process name.

T

Type: R == reference, M == miss. A miss will print two lines, one for the reference, and one for the miss.

FILE

The file name component that was being looked up. This contains trailing pathname components (after '/'), which will be the subject of subsequent lookups.

OVERHEAD¶

File name lookups can be frequent (depending on the workload), and this tool prints a line for each failed lookup, and with -a, each reference as well. The output may be verbose, and the incurred overhead, while optimized to some extent, may still be from noticeable to significant. This is only really intended for deeper investigations beyond dcstat(8), when absolutely necessary. Measure and quantify the overhead in a test environment before use.

SOURCE¶

This is from bpftrace.

https://github.com/iovisor/bpftrace

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

This is a bpftrace version of the bcc tool of the same name. The bcc tool may provide more options and customizations.

https://github.com/iovisor/bcc

OS¶

Linux

STABILITY¶

Unstable - in development.

AUTHOR¶

Brendan Gregg

SEE ALSO¶

dcstat(8)