Slow thread suspension in busy loop

[szehetner]

I have encountered a case where thread suspension before a GC takes multiple seconds (up to 70 seconds in the worst case). A minimal repro can be found here: https://gist.github.com/szehetner/47515ee0f28e2ca9d4990d60ac230a07

This starts a thread in a busy spin loop and triggers GCs. In a lot of iterations the GC takes multiple seconds. PerfView confirms that the time is spent during thread suspension:

Tested with .NET 7 and .NET 8 on x64 Windows - suspension times seem to be lower in .NET 8, but still significant.

My understanding is that it shouldn't be possible for user code to prevent thread suspension for such a long time. Is this an issue in the runtime? Or is there something I can do to speed up the suspension without giving up control of the thread (so without Thread.Yield() or Thread.Sleep())?

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Issue Details

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I have encountered a case where thread suspension before a GC takes multiple seconds (up to 70 seconds in the worst case). A minimal repro can be found here: https://gist.github.com/szehetner/47515ee0f28e2ca9d4990d60ac230a07

This starts a thread in a busy spin loop and triggers GCs. In a lot of iterations the GC takes multiple seconds. PerfView confirms that the time is spent during thread suspension:

Tested with .NET 7 and .NET 8 on x64 Windows - suspension times seem to be lower in .NET 8, but still significant.

My understanding is that it shouldn't be possible for user code to prevent thread suspension for such a long time. Is this an issue in the runtime? Or is there something I can do to speed up the suspension without giving up control of the thread (so without Thread.Yield() or Thread.Sleep())?

Author:	szehetner
Assignees:	-
Labels:	area-System.Threading, tenet-performance
Milestone:	-

[jkotas]

cc @VSadov

[VSadov]

Interesting. I will take a look.

[VSadov]

It looks a lot like a case of a tight loop with a very fast call in it.
JIT does not insert a poll, since there is a call, but catching the thread in the call is difficult.

I can reporduce this and behavior is also sensitive to OS (Win11 seems less affected than Win10) and also depends on the suspension implementation (NativeAOT seems more robust)

app prints milliseconds taken by a GC.Collect() call while there is a thread concurrently running a tight loop.

// all measured with
set DOTNET_TieredCompilation=0 
as debug code suspends easily. 

===== Windows10

=== CoreCLR
Arbitrary delays, sometimes the app hangs forever. (pauses for minutes, then I just stop it)
That is regardless of 8.0 or 7.0, so this is not a result of some recent change.

=== NativeAOT

713
66
1698
2327
22
3804
1593
3343
1766
6077
1515
1218
1082

[VSadov]

My understanding is that it shouldn't be possible for user code to prevent thread suspension for such a long time. Is this an issue in the runtime?

This should not be generally happening and this situation if fairly uncommon. The scenario here is one of the hardest cases for suspension. Since there is a call in the loop, suspension is supposed to catch the thread when it returns from the call, but since the call does literally nothing, there is an extremely short opportunity. Such loops typically do not run for long, since they are clearly wasting CPU, but this one does... Then depending on OS API latencies and how suspension performs the retries the problem could be amplified to take seconds or minutes to suspend.

Since NativeAOT performs better here, it will be worth looking into what is happening in CoreCLR.

Or is there something I can do to speed up the suspension without giving up control of the thread (so without Thread.Yield() or Thread.Sleep())?

As a temporary workaround making a polling call could help. There is no specific API to do a suspension poll, but many OS/interop services do a poll. Calling Thread.Yield() once in a few iterations would be a logical choice, but if that is undesirable, something like Environment.TickCount could work too. It is a cheap call and will do a poll, so the following would be more suspension friendly.

            int someCounter = 0;

            while (!_cts.IsCancellationRequested)
            {
                _idleStrategy.Idle(0);

                // call Thread.Yield() once in a few iterations.
                //
                // if (someCounter++ % 1024 == 0)
                //    Thread.Yield();

                // or call some cheap OS API for the sideeffect of suspension poll
                _ = Environment.TickCount;
            }

[szehetner]

Thanks for the explanation. Using Environment.TickCount looks promising, I will test this further.

To provide some background on the busy spinning loop: Our actual system uses https://github.com/AdaptiveConsulting/Aeron.NET for receiving messages. At the core of that is a loop to poll for incoming messages. Keeping this thread spinning is a deliberate choice to achieve lower latency at the cost of wasted CPU cycles.

[acaly]

Keeping this thread spinning is a deliberate choice to achieve lower latency at the cost of wasted CPU cycles.

In my understanding, if this is the case, you don't want the GC to suspend this thread, do you?

I think in such a latency sensitive scenario, it might be better to separate the application into busy state and idle state, disable GC completely in the first state, and only do GC in the second.