dcsnoop.bt(8) System Manager's Manual dcsnoop.bt(8)
NAME
dcsnoop.bt - Trace directory entry cache (dcache) lookups. Uses bpf-
trace/eBPF.
SYNOPSIS
dcsnoop.bt
DESCRIPTION
By default, this traces every dcache lookup, and shows the process per-
forming the lookup and the filename requested.
The output of this tool can be verbose, and is intended for further in-
vestigations 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 sub-
ject 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 dc-
stat(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)
USER COMMANDS 2018-09-08 dcsnoop.bt(8)
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