EventPipe rundown happening at the end of a session

[vaind]

I'm trying to implement continuous profiler with Diagnostics IPC StartEventPipeSession(). The idea is to start a profiler when the app launches and only do sample-profiling of select parts of the application. O

Originally, I've tried doing this by creating a new EventPipeSession each time I needed to profile some time span but the startup is pretty heavy (about 100 ms to start when the Microsoft-DotNETCore-SampleProfiler provider is included in the list). Therefore, I've switched to starting the session at the beginning and subscribing to the SampleProfilerTraceEventParser.ThreadSample only for the time needed. This way, the overall overhead is reasonable (below 5 % from my testing) and the long session startup time doesn't affect actual operations, only the app startup (and can be made in the background).

Now to the issue at hand

As the samples are captured, I'm trying to assign modules and methods. These are, however, only provided by the rundown provider when the session is being stopped. In my use-case, this would mean when the application is shut down. The relevant code is: https://github.com/dotnet/runtime/blob/1d5c424487c476831d9f98b1ca78b4fda6d18066/src/native/eventpipe/ep.c#L587-L604

I wonder what the reason for this is and whether there is a way to trigger rundown right at the beginning of the session and then rely on runtime provider during the application lifetime to get updates.

[ghost]

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Issue Details

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I'm trying to implement continuous profiler with Diagnostics IPC `StartEventPipeSession()`. The idea is to start a profiler when the app launches and only do sample-profiling of select parts of the application. O Originally, I've tried doing this by creating a new `EventPipeSession` each time I needed to profile some time span but the startup is pretty heavy (about 100 ms to start when the `Microsoft-DotNETCore-SampleProfiler` provider is included in the list). Therefore, I've switched to starting the session at the beginning and subscribing to the `SampleProfilerTraceEventParser.ThreadSample` only for the time needed. This way, the overall overhead is reasonable (below 5 % from my testing) and the long session startup time doesn't affect actual operations, only the app startup (and can be made in the background). ### Now to the issue at hand As the samples are captured, I'm trying to assign modules and methods. These are, however, only provided by the **rundown** provider when the session is being stopped. In my use-case, this would mean when the application is shut down. The relevant code is: https://github.com/dotnet/runtime/blob/1d5c424487c476831d9f98b1ca78b4fda6d18066/src/native/eventpipe/ep.c#L587-L604 I wonder what the reason for this is and whether there is a way to trigger **rundown** right at the beginning of the session and then rely on **runtime** provider during the application lifetime to get updates.

Author:	vaind
Assignees:	-
Labels:	`area-Diagnostics-coreclr`
Milestone:	-

[tommcdon]

@davmason @noahfalk

[davmason]

Hi @vaind,

To answer the question of why rundown behaves this way, it's because it was designed for the scenario where you take a trace and then do analysis later. It wasn't designed to do live processing and analysis of traces. This dates back many years to the desktop framework.

Having rundown happen at the beginning of the session is something we have thought about but so far other work items have taken priority.

For your scenario, a trace will continue to emit method and module events as they are loaded, so you could always work around by starting two sessions and immediately stopping one. The stopped session will give you rundown and then you can get future events from the ongoing session.

[vaind]

Thanks @davmason, that's what I've assumed. I've also thought about that workaround but wasn't sure it would be alright and wouldn't miss anything important. Thanks for the confirmation it should work that way.

As you're saying you've thought about having rundown at the beginning of the session (or maybe it could be triggered by an EventPipeSession method instead to provide greater flexibility), I guess this issue can stay open as a feature request?

[tommcdon]

After careful consideration, we do not plan to action this particular item in this release. We will continue to evaluate it for future releases. Ideally, we would like to fix every issue and implement every idea people submit. Realistically, we cannot address every item.

[vaind]

Thanks for the update. Looking forward to this, whenever it does make the cut.

[brianrob]

Given the streaming scenario for EventPipe, it would be good to keep this around and implement it in the future. I don't imagine that it would take too much work to make it happen - likely just updating the callbacks in the runtime to allow EventPipe sources to trigger rundown (both start and stop). Not sure what the corresponding change in Mono would be, but @lateralusX will know.

[bruno-garcia]

@brianrob with the move towards NativeAOT, do we need to change Mono too?

.NET 8 is out, any chance this can make it to .NET 9?

[brianrob]

I anticipate that if we're going to make this change in CoreCLR, we'd want to do so in Mono too if for no other reason than consistency. I don't think that NativeAOT affects anything here.

In terms of getting this into .NET 9, I will defer to @davmason and @tommcdon who own this space for CoreCLR.