| .TH dcsnoop 8 "2016-02-10" "USER COMMANDS" |
| .SH NAME |
| dcsnoop \- Trace directory entry cache (dcache) lookups. Uses Linux eBPF/bcc. |
| .SH SYNOPSIS |
| .B dcsnoop [\-h] [\-a] |
| .SH DESCRIPTION |
| By default, this traces every failed dcache lookup (cache miss), and shows the |
| process performing the lookup and the filename requested. A \-a option can be |
| used to show all lookups, not just failed ones. |
| |
| 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 OPTIONS |
| .TP |
| \-h |
| Print usage message. |
| .TP |
| \-a |
| Trace references, not just failed lookups. |
| .SH EXAMPLES |
| .TP |
| Trace failed dcache lookups: |
| # |
| .B dcsnoop |
| .TP |
| Trace all dcache lookups: |
| # |
| .B dcsnoop \-a |
| .SH FIELDS |
| .TP |
| TIME(s) |
| Time of lookup, in seconds. |
| .TP |
| PID |
| Process ID. |
| .TP |
| COMM |
| Process name. |
| .TP |
| T |
| Type: R == reference (only visible with \-a), 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 bcc. |
| .IP |
| https://github.com/iovisor/bcc |
| .PP |
| Also look in the bcc distribution for a companion _examples.txt file containing |
| example usage, output, and commentary for this tool. |
| .SH OS |
| Linux |
| .SH STABILITY |
| Unstable - in development. |
| .SH AUTHOR |
| Brendan Gregg |
| .SH SEE ALSO |
| dcstat(1) |
