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118 lines
3.3 KiB
118 lines
3.3 KiB
# lws detailed latency
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## Introduction
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lws has the capability to make detailed latency measurements and
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report them in realtime to a specified callback.
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A default callback is provided that renders the data as text in
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space-separated format suitable for gnuplot, to a specified file.
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## Configuring
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Enable `LWS_WITH_DETAILED_LATENCY` at cmake.
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Create your context with something similar to this
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```
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#if defined(LWS_WITH_DETAILED_LATENCY)
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info.detailed_latency_cb = lws_det_lat_plot_cb;
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info.detailed_latency_filepath = "/tmp/lws-latency-results";
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#endif
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```
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`lws_det_lat_plot_cb` is provided by lws as a convenience to convert
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the stuct data provided at the callback interface to space-separated
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text data that is easy to process with shell commands and gnuplot.
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## `lws_det_lat_plot_cb` format
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```
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728239173547 N 23062 0 0 23062 0 0 0
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728239192554 C 18879 0 0 18879 0 0 0
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728239217894 T 25309 0 0 25309 0 0 0
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728239234998 r 0 0 0 0 271 172 256
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728239250611 r 0 0 0 0 69 934 4096
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728239255679 w 19 122 18 159 20 80 80
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728239275718 w 20 117 15 152 18 80 80
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728239295578 w 10 73 7 90 7 80 80
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728239315567 w 9 67 5 81 7 80 80
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728239335745 w 23 133 9 165 14 80 80
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...
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```
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Each event is shown in 9 columns
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- unix time in us
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- event type
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- N = Name resolution
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- C = TCP Connection
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- T = TLS negotiation server
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- t = TLS negotiation client
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- r = Read
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- w = Write
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- us duration, for w time client spent waiting to write
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- us duration, for w time data spent in transit to proxy
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- us duration, for w time proxy waited to send data
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- as a convenience, sum of last 3 columns above
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- us duration, time spent in callback
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- last 2 are actual / requested size in bytes
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## Processing captured data with ministat
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Eg, to summarize overall latencies on all captured writes
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```
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$ cat /tmp/lws-latency-results | grep " w " | cut -d' ' -f6 | ministat
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...
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N Min Max Median Avg Stddev
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x 1000 43 273 141 132.672 32.471693
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```
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## Processing captured data with gnuplot
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### Gnuplot plotting script
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Create a gnuplot script, eg myscript.gp
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```
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reset
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set term pngcairo enhanced nocrop font "OpenSans, 12" size 800,600#output terminal and file
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set output "lws-latency.png"
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#set yrange [0:10000]
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#to put an empty boundary around the
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#data inside an autoscaled graph.
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set offset graph 0.05,0.05,0.05,0.0
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set style fill transparent solid 0.5 #fillstyle
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set tics out nomirror
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set xlabel "event"
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set ylabel "latency (us)"
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set format x ""
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set title "Write latency"
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set key invert reverse Right inside nobox
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set key autotitle columnheader
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set style data histogram
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set style histogram rowstacked
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set style fill solid border -1
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set boxwidth 0.75
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set style fill solid 1.00 noborder
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set tic scale 0
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set grid ytics lc rgb "#505050"
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unset border
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unset xtics
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plot '/tmp/1' \
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using ($3 + $4 + $5):xtic(1) w boxes lt rgbcolor "blue" title 'prox wr wait', \
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'' using ($3 + $4):xtic(1) w boxes lt rgbcolor "green" title 'txfr to prox', \
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'' using 3:xtic(1) w boxes lt rgbcolor "red" title 'cli wri wait'
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```
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### gnuplot invocation
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```
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$ cat /tmp/lws-latency-results | grep " w " \>/tmp/1 ; gnuplot myscript.gp && eog lws-latency.png
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```
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