Why does .NET reserve so much memory for my application?

Question

When I run my application, in a profiler I see that is uses about 80MB of memory (total committed bytes, performance counter). But when I look at the size of the allocated memory, it is over 400MB!

So my question is, why is .NET reserving so much memory for my application? Is this normal?

Answer 1

you should read Memory Mystery. I had similar questions a while ago and stopped asking myself after reading this. I read other sources, but I cant find now, use keywords "unreasonable allocation of memory windows OS". In a nutshell, OS gives more than your app require depending upon physically available memory resources for e.g. if you are running your app on two machines with different RAM, it can be guaranteed that both these machines will have different memory allocations

Answer 2

It's fairly common for software to issue one large memory request to the underlying operating system, then internally manage its own use of the allocated memory block. So common, in fact, that Windows' (and other operating systems') memory manager explicitly supports the concept, called "uncommitted memory" -- memory that the process has requested but hasn't made use of yet. That memory really doesn't exist as far as bits taking up space on your DRAM chips until the process actually makes use of it. The preallocation of memory effectively costs nothing.

Applications do this for many reasons -- though it's primarily done for performance reasons. An application with knowledge of its own memory usage patterns can optimize its allocator for that pattern; similarly, for address locality reasons, as successive memory requests from the OS won't always be 'next' to each other in memory, which can affect the performance of the CPU cache and could even preclude you from using some optimizations.

.NET in particular allocates space for the managed heap ahead of time, for both of the reasons listed above. In most cases, allocating memory on the managed heap merely involves incrementing a top-of-heap pointer, which is incredibly fast --- and also not possible with the standard memory allocator (which has a more general design to perform acceptably in a fragmented heap, whereas the CLR's GC uses memory compaction to sharply limit the fragmentation of the managed heap), and also not possible if the managed heap itself is fragmented across the process address space due to multiple allocations at different points in time.

Answer 3

"Reserving" memory is by no means the same as "allocated" ram. Read the posts Steve and Krishna linked to.

The part your client needs to look at is Private Bytes. But even that isn't exactly a hard number as parts of your app may be swapped to the virtual disk.

In short, unless your Private Bytes section is pretty well out of control OR you have leaks (ie: undisposed unmanaged resources) you (and your client) should ignore this and let the OS manage what is allocated, what's in physical ram and what's swapped out to disk.

Answer 4

As you no doubt know, there is a massive difference between actual memory used and allocated. An application's allocated memory doesn't mean that it's actually being used anywhere; all it really means is that the OS has 'marked' a zone of virtual memory (which is exactly that - virtual) ready for use by the application.

The memory isn't necessarily being used or starving other processes - it just could if the app starts to fill it.

This allocated number, also, will likely scale based on the overall memory ecosystem of the machine. If there's plenty of room when an app starts up, then it'll likely grab a larger allocation than if there's less.

That principle is the same as the one which says it's good practise to create a List<T>, say, with a reasonable initial capacity that'll mean a decent number of items can be added before resizing needs to take place. The OS takes the same approach with memory usage.