weblatency-nd.8

.TH weblatency-nd 8  "2015-01-30" "USER COMMANDS"
.SH NAME
weblatency-nd \- Trace HTTP GETs and their latency (sendto()->recvfrom()). Uses Linux SystemTap (non-debuginfo).
.SH SYNOPSIS
.B weblatency-nd.stp
.SH DESCRIPTION
This measures HTTP GET time-to-first-byte latency by tracing client syscalls,
and shows statistics including average latency, maximum latency, and latency
distribution, by both PID and URL. This can be useful to understand application
level latency when it makes requests to other services. The latency distribution
is useful for understanding the make-up of latency, and whether there are
multiple modes or queueing effects present, which can direct further performance
tuning efforts.

weblatency-nd.stp works by tracing the entry to the sendto() syscall, and
matching on the string "GET" at the start of the message, and then timing how
long it takes for a recvfrom() to complete on the same file descriptor. This
sequence matches the sample application it was written for, but won't match
all: applications can use a write()->read() sequence instead on a socket
file descriptor. Adjust the script as needed.

It's common to analyze this latency by taking a tcpdump(1), and post-processing.
An advantage of weblatency-nd.stp is that the data is filtered and summarized
in-kernel, lowering the overheads of doing this task.
.SH REQUIREMENTS
SystemTap.
.SH EXAMPLES
.TP
Measure HTTP GET time-to-first-byte latency, and show statistics by-URL on Ctrl-C:
#
.B weblatency-nd.stp
.SH FIELDS
.TP
value
The minimum value, in nanoseconds, for this histogram bucket. Read this value
as "HTTP GET latency was at least xxx".
.TP
count
How many GETs fell into this latency range.
.SH OVERHEAD
It depends on the rate of sendto()/recvfrom() syscalls, which for busy web
servers can be frequent, and the overhead of this tool may be noticeable.
Test first.
.SH SOURCE
This is from the SystemTap lwtools collection.
.IP
https://github.com/brendangregg/systemtap-lwtools
.PP
Also look under the examples directory for a text 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
tcpdump(1)