.NET Garbage Collector - what is its thread priority?

Question

I have found some great articles (Maoni, Richter #1, Richter #2) giving many details as to the theory and practice of the GC, yet I cannot find anything that states how the GC's thread priority is set.

The closest I've found is this one that states that the Finalizer thread "runs asynchronously to the application and at a high priority."

I always thought that it was a "low-priority" thread, but reading more and more about it that seems to be wrong (since the GC has to block all your other threads, and you don't want to have your app depend on a low-priority thread in order to resume in a timely fashion).

Does anybody know for sure know what the actual priority is supposed to be?

Answer 1

The GC thread runs at a normal priority. The finalizer thread runs at "Highest" priority.

You can see this by turning on the Debug "Thread" window, and breaking anywhere in a managed application. The threads are all listed (although they're not named), with their priorities. It takes a bit to decipher which is which, but there will be an extra "Normal" and "Highest" priority thread, which correspond to the GC and the Finalizer thread.

Answer 2

In CLR via C#, Richter explains that:

A special high-priority CLR thread is dedicated to calling Finalize methods

(see the "Finalization Internals" heading of chapter 20)

This is the only context in which he talks about a garbage collector thread. A little earlier in the chapter, he explains that garbage collection is started in response to one of the following events:

• Generation 0 is full
• Call to GC.Collect
• Windows is reporting low memory conditions
• The CLR is unloading an AppDomain
• The CLR is shutting down

...which suggests that the only thread created by the garbage collector is this single, "high-priority" finalizer thread.

Edit: He then goes on, in "Concurrent Collection", to explain that:

On a multiprocessor system running the workstation version of the execution engine, the garbage collector has an additional background thread to collect objects concurrently while the application runs. [...] The garbage collector has a normal priority background thread that marks unreachable objects.

Answer 3

the garbage collector runs at lower priority than critical threads

Answer 4

The GC thread priority is an implementation detail, right now they are running in normal priority, however the finalizer thread is running as a high priority and has a timeout as well.