Why does 'Any CPU (prefer 32-bit)' allow me to allocate more memory than x86 under .NET 4.5?

Question

According to many SO answers and this widely cited blog post, a .NET 4.5 application built for 'Any CPU' with the 'prefer 32-bit' option selected will run as a 32-bit process on both 32-bit and 64-bit systems (unlike in .NET 4.0 and earlier). In other words, x86 and AnyCPU with 'prefer 32-bit' selected are equivalent (ignoring whether or not it can run on ARM).

However, my tests have shown that on a 64-bit system an 'AnyCPU prefer 32-bit' application (which I confirm runs 32-bit) can allocate more memory than an x86 one. I wrote a .NET 4.5 C# console app that allocates 10MB byte arrays in a loop (keeping the references of course) until it hits an OutOfMemoryException, and ran it on a 64-bit system with plenty of RAM. When built as x86 it falls over at about 1.2GB allocated. The same code built as 'Any CPU (prefer 32-bit)' gets up to 1.5GB.

Why the difference?

Gordongordy

Answer 1

It turns out that, in Visual Studio 2015, building as 'AnyCPU (prefer 32-bit)' sets the IMAGE_FILE_LARGE_ADDRESS_AWARE bit on the executable (equivalent to running editbin /LARGEADDRESSAWARE on it), whereas it does not for an x86 build. This can be confirmed with dumpbin /HEADERS and looking for the line "Application can handle large (>2GB) addresses".

This is not the case for Visual Studio 2013. The change is apparently undocumented.

In theory, this should give the CLR an additional 2GB to play with. I don't know why the allocatable memory only goes up by about 300MB.

Gordongordy

Assuming you're allocating the 10MB objects into an array or list itself, you're likely blowing up that array. bhrnjica.net/2012/07/22/… set gcAllowVeryLargeObjects and try. My guess is you trigger List resizing its internal array and resizes into an "OOM" – Mohenjodaro

@ChrisMarisic I'm creating a new 10MB byte array in each iteration, and adding a reference to that to a List (to prevent them being garbage collected). So the List itself is small, just a few hundred references. – Gordongordy