Manual: dcsnoop.bt

dcsnoop.bt(8)					   System Manager's Manual				       dcsnoop.bt(8)

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 dc‐
       stat(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/bpftrace/bpftrace

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

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

	      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)