bcc-funclatency - Man Page

Time functions and print latency as a histogram.


funclatency [-h] [-p PID] [-i INTERVAL] [-d DURATION] [-T] [-u] [-m] [-F] [-r] [-v] pattern


This tool traces function calls and times their duration (latency), and shows the latency distribution as a histogram. The time is measured from when the function is called to when it returns, and is inclusive of both on-CPU time and time spent blocked.

This tool uses in-kernel eBPF maps for storing timestamps and the histogram, for efficiency.

Currently nested or recursive functions are not supported properly, and timestamps will be overwritten, creating dubious output. Try to match single functions, or groups of functions that run at the same stack layer, and don't ultimately call each other.

WARNING: This uses dynamic tracing of (what can be many) functions, an activity that has had issues on some kernel versions (risk of panics or freezes). Test, and know what you are doing, before use.

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


CONFIG_BPF and bcc.


pattern Function name or search pattern. Supports "*" wildcards. See Examples. You can also use -r for regular expressions.


Print usage message.

-p PID

Trace this process ID only.


Print output every interval seconds.


Total duration of trace, in seconds.


Set the level of nested or recursive functions.


Include timestamps on output.


Output histogram in microseconds.


Output histogram in milliseconds.


Print a separate histogram per function matched.


Use regular expressions for the search pattern.


Print the BPF program (for debugging purposes).


Time the do_sys_open() kernel function, and print the distribution as a histogram:

# funclatency do_sys_open

Time the read() function in libc across all processes on the system:

# funclatency c:read

Time vfs_read(), and print the histogram in units of microseconds:

# funclatency -u vfs_read

Time do_nanosleep(), and print the histogram in units of milliseconds:

# funclatency -m do_nanosleep

Time libc open(), and print output every 2 seconds, for duration 10 seconds:

# funclatency -i 2 -d 10 c:read

Time vfs_read(), and print output every 5 seconds, with timestamps:

# funclatency -mTi 5 vfs_read

Time vfs_read() for process ID 181 only:

# funclatency -p 181 vfs_read

Time both vfs_fstat() and vfs_fstatat() calls, by use of a wildcard:

# funclatency 'vfs_fstat*'

Time both vfs_fstat* calls, and print a separate histogram for each:

# funclatency -F 'vfs_fstat*'



Nanosecond range


Microsecond range


Millisecond range


How many calls fell into this range


An ASCII bar chart to visualize the distribution (count column)


This traces kernel functions and maintains in-kernel timestamps and a histogram, which are asynchronously copied to user-space. While this method is very efficient, the rate of kernel functions can also be very high (>1M/sec), at which point the overhead is expected to be measurable. Measure in a test environment and understand overheads before use. You can also use funccount to measure the rate of kernel functions over a short duration, to set some expectations before use.


This is from bcc.


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




Unstable - in development.


Brendan Gregg, Sasha Goldshtein

See Also



2015-08-18 USER COMMANDS