How much GCHandle pinned memory/objects would make Garbage Collector slows down?

Question

I'm sure this answer will depends the user machine, but there must be some best practices on pinning data.

I need to hold like 5 arrays of bytes containing 1.048.576 bytes each. Normally I would prefer to use GCHandle (managed) memory, but some people says it would slow down the GC. I know that can happen, but how much memory/objects need to be pinned to start to really affect the GC?

Here are the options that I have:

1. GCHandle.Alloc GCHandleType.Pinned (managed). It will slow down GC??
2. Marshal.AllocHGlobal (unmanaged acess). unsafe code
3. use a Bitmap to hold data in Scan0 (unmanaged acess). unsafe code

Answer 1

This is a hopelessly unanswerable question. Pinned objects do not slow down the GC that much, it is just a rock in the road when the GC compacts the heap. Easy enough to work around that rock by simply skipping the pinned section of the heap.

The far worse outcome is that it will have a lasting effect on the code that runs after the collection is completed. Since the heap isn't compacted that well, the locality of references is not as good so the processor won't be able to get as much mileage from the CPU caches. Quantifying that slow-down isn't possible, it greatly depends on the kind of code that runs afterwards. Only that it is worse and lasts for a while, until the next GC.

The only good advice is that, if you have to pin, then do so for as short amount of time as possible. And to avoid scenarios where a collection may occur while an object is pinned. Which, roughly, means that you avoid allocating memory while holding the pin. Not always practical if the program is running multiple threads, making the <gcServer> element in the .config file attractive. Which selects a different GC strategy that uses a lot more memory but gives threads their own GC heap segments. There is no simple guidance to determine when to do this, profiling with realistic data sets is required.

Neither Marshal.AllocHGlobal nor Bitmap have any notable affect on the GC heap, they allocate from unmanaged memory heaps.