c# - .NET Free memory usage (how to prevent overallocation / release memory to the OS)

Question

I'm currently working on a website that makes large use of cached data to avoid roundtrips. At startup we get a "large" graph (hundreds of thouthands of different kinds of objects). Those objects are retrieved over WCF and deserialized (we use protocol buffers for serialization) I'm using redgate's memory profiler to debug memory issues (the memory didn't seem to fit with how much memory we should need "after" we're done initializing and end up with this report

Now what we can gather from this report is that:

1) Most of the memory .NET allocated is free (it may have been rightfully allocated during deserialisation, but now that it's free, i'd like for it to return to the OS)

2) Memory is fragmented (which is bad, as everytime i refresh the cash i need to redo the memory hungry deserialisation process and this, in turn creates large object that may throw an OutOfMemoryException due to fragmentation)

3) I have no clue why the space is fragmented, because when i look at the large object heap, there are only 30 instances, 15 object[] are directly attached to the GC and totally unrelated to me, 1 is a char array also attached directly to the GC Heap, the remaining 15 are mine but are not the cause of this as i get the same report if i comment them out in code.

So my question is, what can i do to go further with this? I'm not really sure what to look for in debugging / tools as it seems my memory is fragmented, but not by me, and huge amounts of free spaces are allocated by .net , which i can't release.

Also please make sure you understand the question well before answering, i'm not looking for a way to free memory within .net (GC.Collect), but to free memory that is already free in .net , to the system as well as to defragment said memory.

Note that a slow solution is fine, if it's possible to manually defragment the large heap i'd be all for it as i can call it at the end of RefreshCache and it's ok if it takes 1 or 2 second to run.

Thanks for your help!

A few notes i forgot: 1) The project is a .net 2.0 website, i get the same results running it in a .net 4 pool, idem if i run it in a .net 4 pool and convert it to .net 4 and recompile.

2) These are results of a release build, so debug build can not be the issue.

3) And this is probably quite important, i do not get these issues at all in the webdev server, only in IIS, in the webdev i get memory consumption rather close to my actual consumption (well more, but not 5-10X more!)

Answer 1

I know this isn't the answer you'd like to hear, but you can't forcefully release the memory back to the OS. However, for what reason do you want to do so? .NET will free its heap back to the OS once you're running low on physical memory. But if there's an ample amount of free physical memory, .NET will keep its heap to make future allocation of objects faster. If you really wanted to force .NET to release its heap back to the OS, I suppose you could write a C program which just mallocs until it runs out of memory. This should cause the OS to signal .NET to free its unused portion of the heap.

It's better that unused memory be reseved for .NET so that your application will have better allocation performance (since the runtime knows what memory is free and what isn't, allocation can just use the free memory without having to syscall into the OS to get more memory).

The garbage collector is in charge of defragmenting the heap. Every so often (usually during collection runs), it will move objects around the heap if it determines this needs to be done. (This is why C++/CLI has the pin_ptr construct for "pinning" objects).

Fragmentation usually isn't a big issue though with memory, since it provides fast random access.

As for your OutOfMemoryException, I don't have a good answer for. Ordinarily I'd suspect that your old object graph isn't being collected (some object somewhere is holding a reference onto it, a "memory leak"). But since you're using a profiler, I don't know then.