- calling the System.Runtime.GCSettings.IsServerGC property
- attaching to the process using WinDbg and checking how many GC threads you have using the command "!sos.threads" without the quotes and (according to the below criteria)...
- If your app is non-hosted on a multi-proc machine, you will get the Workstation GC - Concurrent by default.
- If your app is hosted on a multi-proc machine, you will get the ServerGC by default.
Workstation GC
- Uni-processor machine
- Always suspends threads
- 1 Ephemeral GC Heap (SOH), 1 LOH GC Heap
- Runs on thread that triggered GC
- Thread priority is the same as the thread that triggered GC
Workstation GC - Concurrent
- Only runs concurrent in Gen2/LOH (full collection)
- Mutually exclusive with Server Mode
- Slightly larger working set
- GC Thread expires if not in use after a while
- 1 Ephemeral GC Heap (SOH), 1 LOH GC Heap
- Has a dedicated GC Thread
- Thread priority is Normal
Server GC
- Larger segment sizes
- Faster than Workstation GC
- Always suspends threads
- 1 Ephemeral GC Heap (SOH) for each logical processor (this includes hyperthreaded), 1 LOH GC Heap for each logical processor (this includes hyperthreaded)
- Has dedicated GC Threads
- Thread priority is THREAD_PRIORITY_HIGHEST
A seldom known fact is that even if you try to set the Server mode of GC, you might not be running in Server GC; the GC ultimately determines which mode will be optimal for your app and WILL override your settings if it determines your ServerGC setting will negatively impact your application. Also, any hosted CLR app will have any manual GC settings overridden.
In CLR 4.0, things change just a little bit
- Concurrent GC is now Background GC
- Background GC only applies to Workstation GC
- Old (Concurrent GC):
- During a Full GC Allowed allocations up to end of ephemeral segment size
- Otherwise, suspends all other threads
- New (Background GC):
- Allows for ephemeral GC’s simultaneously with Background GC if necessary
- Performance is much faster
- Server GC always blocks threads for collection of any generation
In CLR 4.5, things change just a little bit...again
- Background Server GC
- Server GC no longer blocks. Instead, it uses dedicated background GC threads that can run concurrently with user code - see MSDN: Background Server GC
.NET 4.7.1 GC Improvements
.NET Framework 4.7.1 brings in changes in Garbage Collection (GC) to improve the allocation performance, especially for Large Object Heap (LOH) allocations. This is due to an architectural change to split the heap’s allocation lock into 2, for Small Object Heap (SOH) and LOH. Applications that make a lot of LOH allocations, should see a reduction in allocation lock contention, and see better performance. These improvements allow LOH allocations while Background GC (BGC) is sweeping SOH. Usually the LOH allocator waits for the whole duration of the BGC sweep process before it can satisfy requests to allocate memory. This can hinder performance. You can observe this problem in PerfView’s GCStats where there is an ‘LOH allocation pause (due to background GC) > 200 msec Events’ table. The pause reason is ‘Waiting for BGC to thread free lists’. This feature should help mitigate this problem.
