.TH dcsnoop 8  "2018-09-08" "USER COMMANDS"
.SH NAME
dcsnoop.bt \- Trace directory entry cache (dcache) lookups. Uses bpftrace/eBPF.
.SH SYNOPSIS
.B dcsnoop.bt
.SH 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.
.SH REQUIREMENTS
CONFIG_BPF and bcc.
.SH EXAMPLES
.TP
Trace all dcache lookups:
#
.B dcsnoop.bt
.SH FIELDS
.TP
TIME(ms)
Time of lookup, in milliseconds.
.TP
PID
Process ID.
.TP
COMM
Process name.
.TP
T
Type: R == reference, M == miss. A miss will print two
lines, one for the reference, and one for the miss.
.TP
FILE
The file name component that was being looked up. This contains trailing
pathname components (after '/'), which will be the subject of subsequent
lookups.
.SH 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.
.SH SOURCE
This is from bpftrace.
.IP
https://github.com/iovisor/bpftrace
.PP
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.
.IP
https://github.com/iovisor/bcc
.SH OS
Linux
.SH STABILITY
Unstable - in development.
.SH AUTHOR
Brendan Gregg
.SH SEE ALSO
dcstat(8)