PerfView GC Trigger Reasons

Question

I have been investigating some garbage collection issues in a c# server app. I'm currently using PerfView to do this. After collecting some data and getting a load of GC Stats I'm a little confused about one of the columns 'Trigger Reason'. I'm getting two values 'AllocLarge' and 'AllocSmall'. I have searched through the help and google and can't find what exactly these two terms mean.

Answer 1

The .NET GC treats objects larger than 85K (a large object) very differently than other objects (small objects). In particular large objects are only collected in 'Generation 2' (the most expensive kind of GC). 'AllocLarge' means a GC was triggered while allocating a large object (and thus must have provoked a Gen 2 GC). 'AllocSmall' means a GC happened in responce to a allocation of an 'ordinary' object.

Note that in general it is bad to have short lived large objects (since these force expensive GCs). You can see everywhere you allocated a large object by looking at the 'GC Alloc Stats' view and looking for the pseudo-frame 'LargeObject'. Double click on that (which brings you to the 'callers' view, and yoiu will see where you are allocating large objects.