GC.GetTotalMemory occasionaly returns negative values

[AndreyAkinshin]

Description

Reproduction Steps

No stable repro, but it happens from time to time in one of my applications.

Expected behavior

GC.GetTotalMemory should always return only non-negative values

Actual behavior

GC.GetTotalMemory occasionally returns negative values

Regression?

No response

Known Workarounds

No response

Configuration

Windows 11 x64, .NET SDK 8.0.204, net8.0

Other information

If anyone can advise on how to diagnose/debug this issue, I can collect additional information when it happens next time.

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[firerain-fd]

I have the same problem.

[kg]

Is this while using NativeAOT or regular execution mode?

[kg]

I audited the code and the internal GC total memory calculations are done per-heap using size_t instead of ssize_t/int64, so it makes sense that if we somehow got a very large calculated size, it would be negative. Getting a size that big should be impossible though, so that indicates some sort of rare bug in the underlying calculations. I couldn't spot any obvious cause, so it would be great if someone could offer more details on the reproduction scenario, even if you can't share an application with us.

i.e.:

• How big are your heaps when this isn't failing?
• Is your application heavily multithreaded?
• Is your application under load when you call this?

[AndreyAkinshin]

In my case, this problem occurs in a small terminal app based on Terminal.Gui. Its typical memory footprint is about 1-2 MB. The app has several threads: Terminal.Gui threads + some background tasks in the user code for handling long-running operations, but it does not involve heavy multithreading. The app performs very basic tasks with a small number of allocations.

[kg]

Test case I constructed fails instantly on NET9.

using System.Threading;
using System.Collections.Generic;

bool Running = true;

// See https://aka.ms/new-console-template for more information
Console.WriteLine("Running...");

var threads = new List<Thread>();
for (int i = 0; i < 8; i++) {
    var thread = new Thread(ThreadMain);
    thread.Start();
    threads.Add(thread);
}

while (true) {
    var tm = GC.GetTotalMemory(false);
    if (tm < 0)
        throw new Exception($"Got total memory of {tm}");
    Thread.Yield();
}

void ThreadMain () {
    while (Running) {
        var s = "x";
        for (int i = 0; i < 1024; i++)
            s += i.ToString();
        Thread.Yield();
    }
}