Assembly loader event tracing is very expensive (10x application slowdown)

[jkotas]

Repro

1. dotnet new console in .NET 7 SDK, copy&paste the following program (repro extracted from actual app where the issue was observed)

using System.Diagnostics;

var sw = new Stopwatch();
for (;;)
{
    sw.Restart();
    for (int i = 0; i < 100000; i++) Type.GetType("System.String, System.Runtime");
    Console.WriteLine(sw.ElapsedMilliseconds);
}

2. dotnet run
3. Download latest perfview and start a global collection session with default settings (Select "Collect / Collect" from menu and press "Start Collection" button)

Actual result

Program runs more than 10x slower when the perfview collection session is active

Expected result

Default perfview collection settings should not slow down the program by more than 10%

---

The problem is caused by very slow assembly loader tracing. The fix should be to either make assembly loader tracing a lot faster and lighter weight for cases like this one and/or assembly loader tracing should not be enabled for default (perfview) event collection settings.

I couldn't figure out the best area label to add to this issue. If you have write-permissions please help me learn by adding exactly one area label.

Tagging subscribers to this area: @vitek-karas, @agocke, @VSadov
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Issue Details

---

Repro

1. dotnet new console in .NET 7 SDK, copy&paste the following program (repro extracted from actual app where the issue was observed)

using System.Diagnostics;

var sw = new Stopwatch();
for (;;)
{
    sw.Restart();
    for (int i = 0; i < 100000; i++) Type.GetType("System.String, System.Runtime");
    Console.WriteLine(sw.ElapsedMilliseconds);
}

2. dotnet run
3. Download latest perfview and start a global collection session with default settings (Select "Collect / Collect" from menu and press "Start Collection" button)

Actual result

Program runs more than 10x slower when the perfview collection session is active

Expected result

Default perfview collection settings should not slow down the program by more than 10%

---

The problem is caused by very slow assembly loader tracing. The fix should be to either make assembly loader tracing a lot faster and lighter weight for cases like this one and/or assembly loader tracing should not be enabled for default (perfview) event collection settings.

Author:	jkotas
Assignees:	-
Labels:	area-AssemblyLoader-coreclr, untriaged
Milestone:	-

[vitek-karas]

I don't think detailed assembly load tracing should be enabled by default - it was not designed for that case. We specifically "Didn't care much" about perf since the goal was to provide on demand diagnostic tool.

@elinor-fung could you please look into disabling this as part of the default collection?

[elinor-fung]

Agreed - it is intended for targeted assembly loading investigation and should not be enabled by default.

I'll look at updating perfview / TraceEvent defaults.

[elinor-fung]

It looks like some of the VS tools also enable it - I'll get in contact for those as well.