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cmd/trace: add basic documentation to main page
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This change adds rudimentary explanation of the various
visualizations to main page of the trace server.
There is clearly a vast amount one could write here,
especially in the form of tutorials, but I've tried to
restrict it to just basic conceptual overview.

Change-Id: Id4dfe9d47f9b31ed5f8fe39f8b3a7c60c0ae8d5a
Reviewed-on: https://go-review.googlesource.com/c/go/+/412876
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
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@adonovan
adonovan committed Jun 22, 2022
1 parent b004c73 commit be0b2a3
Showing 1 changed file with 183 additions and 11 deletions.
194 changes: 183 additions & 11 deletions src/cmd/trace/main.go
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Original file line number Diff line number Diff line change
Expand Up @@ -185,23 +185,195 @@ func httpMain(w http.ResponseWriter, r *http.Request) {

var templMain = template.Must(template.New("").Parse(`
<html>
<style>
/* See https://github.com/golang/pkgsite/blob/master/static/shared/typography/typography.css */
body {
font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Helvetica, Arial, sans-serif, 'Apple Color Emoji', 'Segoe UI Emoji';
font-size: 1rem;
line-height: normal;
max-width: 9in;
margin: 1em;
}
h1 { font-size: 1.5rem; }
h2 { font-size: 1.375rem; }
h1,h2 {
font-weight: 600;
line-height: 1.25em;
word-break: break-word;
}
p { color: grey85; font-size:85%; }
</style>
<body>
<h1>cmd/trace: the Go trace event viewer</h1>
<p>
This web server provides various visualizations of an event log gathered during
the execution of a Go program that uses the <a href='https://pkg.go.dev/runtime/trace'>runtime/trace</a> package.
</p>
<h2>Event timelines for running goroutines</h2>
{{if $}}
<p>
Large traces are split into multiple sections of equal data size
(not duration) to avoid overwhelming the visualizer.
</p>
<ul>
{{range $e := $}}
<a href="{{$e.URL}}">View trace ({{$e.Name}})</a><br>
<li><a href="{{$e.URL}}">View trace ({{$e.Name}})</a></li>
{{end}}
<br>
</ul>
{{else}}
<a href="/trace">View trace</a><br>
<ul>
<li><a href="/trace">View trace</a></li>
</ul>
{{end}}
<a href="/goroutines">Goroutine analysis</a><br>
<a href="/io">Network blocking profile</a> (<a href="/io?raw=1" download="io.profile">⬇</a>)<br>
<a href="/block">Synchronization blocking profile</a> (<a href="/block?raw=1" download="block.profile">⬇</a>)<br>
<a href="/syscall">Syscall blocking profile</a> (<a href="/syscall?raw=1" download="syscall.profile">⬇</a>)<br>
<a href="/sched">Scheduler latency profile</a> (<a href="/sche?raw=1" download="sched.profile">⬇</a>)<br>
<a href="/usertasks">User-defined tasks</a><br>
<a href="/userregions">User-defined regions</a><br>
<a href="/mmu">Minimum mutator utilization</a><br>
<p>
This view displays a timeline for each of the GOMAXPROCS logical
processors, showing which goroutine (if any) was running on that
logical processor at each moment.
Each goroutine has an identifying number (e.g. G123), main function,
and color.
A colored bar represents an uninterrupted span of execution.
Execution of a goroutine may migrate from one logical processor to another,
causing a single colored bar to be horizontally continuous but
vertically displaced.
</p>
<p>
Clicking on a span reveals information about it, such as its
duration, its causal predecessors and successors, and the stack trace
at the final moment when it yielded the logical processor, for example
because it made a system call or tried to acquire a mutex.
Directly underneath each bar, a smaller bar or more commonly a fine
vertical line indicates an event occuring during its execution.
Some of these are related to garbage collection; most indicate that
a goroutine yielded its logical processor but then immediately resumed execution
on the same logical processor. Clicking on the event displays the stack trace
at the moment it occurred.
</p>
<p>
The causal relationships between spans of goroutine execution
can be displayed by clicking the Flow Events button at the top.
</p>
<p>
At the top ("STATS"), there are three additional timelines that
display statistical information.
"Goroutines" is a time series of the count of existing goroutines;
clicking on it displays their breakdown by state at that moment:
running, runnable, or waiting.
"Heap" is a time series of the amount of heap memory allocated (in orange)
and (in green) the allocation limit at which the next GC cycle will begin.
"Threads" shows the number of kernel threads in existence: there is
always one kernel thread per logical processor, and additional threads
are created for calls to non-Go code such as a system call or a
function written in C.
</p>
<p>
Above the event trace for the first logical processor are
traces for various runtime-internal events.
The "GC" bar shows when the garbage collector is running, and in which stage.
Garbage collection may temporarily affect all the logical processors
and the other metrics.
The "Network", "Timers", and "Syscalls" traces indicate events in
the runtime that cause goroutines to wake up.
</p>
<p>
The visualization allows you to navigate events at scales ranging from several
seconds to a handful of nanoseconds.
Consult the documentation for the Chromium <a href='https://www.chromium.org/developers/how-tos/trace-event-profiling-tool/'>Trace Event Profiling Tool<a/>
for help navigating the view.
</p>
<ul>
<li><a href="/goroutines">Goroutine analysis</a></li>
</ul>
<p>
This view displays information about each set of goroutines that
shares the same main function.
Clicking on a main function shows links to the four types of
blocking profile (see below) applied to that subset of goroutines.
It also shows a table of specific goroutine instances, with various
execution statistics and a link to the event timeline for each one.
The timeline displays only the selected goroutine and any others it
interacts with via block/unblock events. (The timeline is
goroutine-oriented rather than logical processor-oriented.)
</p>
<h2>Profiles</h2>
<p>
Each link below displays a global profile in zoomable graph form as
produced by <a href='https://go.dev/blog/pprof'>pprof</a>'s "web" command.
In addition there is a link to download the profile for offline
analysis with pprof.
All four profiles represent causes of delay that prevent a goroutine
from running on a logical processor: because it was waiting for the network,
for a synchronization operation on a mutex or channel, for a system call,
or for a logical processor to become available.
</p>
<ul>
<li><a href="/io">Network blocking profile</a> (<a href="/io?raw=1" download="io.profile">⬇</a>)</li>
<li><a href="/block">Synchronization blocking profile</a> (<a href="/block?raw=1" download="block.profile">⬇</a>)</li>
<li><a href="/syscall">Syscall blocking profile</a> (<a href="/syscall?raw=1" download="syscall.profile">⬇</a>)</li>
<li><a href="/sched">Scheduler latency profile</a> (<a href="/sche?raw=1" download="sched.profile">⬇</a>)</li>
</ul>
<h2>User-defined tasks and regions</h2>
<p>
The trace API allows a target program to annotate a <a
href='https://pkg.go.dev/runtime/trace#Region'>region</a> of code
within a goroutine, such as a key function, so that its performance
can be analyzed.
<a href='https://pkg.go.dev/runtime/trace#Log'>Log events</a> may be
associated with a region to record progress and relevant values.
The API also allows annotation of higher-level
<a href='https://pkg.go.dev/runtime/trace#Task'>tasks</a>,
which may involve work across many goroutines.
</p>
<p>
The links below display, for each region and task, a histogram of its execution times.
Each histogram bucket contains a sample trace that records the
sequence of events such as goroutine creations, log events, and
subregion start/end times.
For each task, you can click through to a logical-processor or
goroutine-oriented view showing the tasks and regions on the
timeline.
Such information may help uncover which steps in a region are
unexpectedly slow, or reveal relationships between the data values
logged in a request and its running time.
</p>
<ul>
<li><a href="/usertasks">User-defined tasks</a></li>
<li><a href="/userregions">User-defined regions</a></li>
</ul>
<h2>Garbage collection metrics</h2>
<ul>
<li><a href="/mmu">Minimum mutator utilization</a></li>
</ul>
<p>
This chart indicates the maximum GC pause time (the largest x value
for which y is zero), and more generally, the fraction of time that
the processors are available to application goroutines ("mutators"),
for any time window of a specified size, in the worst case.
</p>
</body>
</html>
`))
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