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Add docs on UNMANAGED_CODE_TIME and CPU_TIME #1613

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Apr 25, 2022
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Add docs on UNMANAGED_CODE_TIME and CPU_TIME #1613

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Add docs on UNMANAGED_CODE_TIME and CPU_TIME
Apr 18, 2022
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move documentation and address feedback
Apr 19, 2022
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2 changes: 1 addition & 1 deletion documentation/TraceEvent/TraceEventProgrammersGuide.md
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Expand Up @@ -935,4 +935,4 @@ Unfortunately when a provider registers with the operating system (the Win32 `Ev

## On the fly filtering AND Logging using `ETWReloggerTraceEventSource`

TODO
TODO
30 changes: 28 additions & 2 deletions src/PerfView/SupportFiles/HtmlReportUsersGuide.htm
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Expand Up @@ -251,8 +251,34 @@ <h3>
TypeLoad information is only available in .NET 5 and above.

Otherwise, the data captured will be restricted to JIT and assembly load operations. The /RuntimeLoading switch may also be used.
<hr />
<p>&nbsp;</p>
<hr />
<h3>
<a id="UnderstandingEventPipeThreadTime">Understanding EventPipe Thread Time</a>
</h3>
<p>
EventPipe is a technology in .NET Core 3.1 on that allows the collection of events and CPU
sampling on all platforms. The CPU sampling performed by EventPipe is only aware of managed
code, which means that transitions into native code, e.g., P/Invokes, won't appear in the
trace. This means that stacks that containing native code will end with the last managed frame.
</p>
<p>
For example, if an application has a sequence of methods <tt>Main->A->B->C->MyNativeFunction</tt>,
where <tt>MyNativeFunction</tt> P/Invokes into native code, then samples collected while that native
code is on the stack will only contain <tt>Main->A->B->C->MyNativeFunction</tt>. Any native functions
after the P/Invoke method won't appear in the trace.
</p>
<p>
During a typical CPU usage investigation, you may want to ask about on-CPU vs off-CPU time to determine
when your code is blocked waiting or actively doing work. Due to the only knowing about
managed frames, CPU samples collected via EventPipe can't give exact on/off-CPU information. Instead,
the TraceEvent library uses a heuristic to add pseudo-frames to the trace that indicate whether there
are additional native frames on the stack or not. When a nettrace file is opened in the "Thread Time"
view or is exported to another format, e.g., SpeedScope, the heuristic will insert either <tt>UNMANAGED_CODE_TIME</tt>
or <tt>CPU_TIME</tt> onto the stacks. <tt>UNMANAGED_CODE_TIME</tt> represents stacks where there are one or more native
frames after the last managed frame and that these frames may be blocked waiting or actively on the CPU.
<tt>CPU_TIME</tt> represents stacks where the last managed frame is the function currently on the CPU and doing work.
</p>
<hr />
<p>&nbsp;</p>
<p>&nbsp;</p>
<p>&nbsp;</p>
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