Alan Alan - 9 days ago 6
C++ Question

How to solve Memory Fragmentation

We've occasionally been getting problems whereby our long-running server processes (running on Windows Server 2003) have thrown an exception due to a memory allocation failure. Our suspicion is these allocations are failing due to memory fragmentation.

Therefore, we've been looking at some alternative memory allocation mechanisms that may help us and I'm hoping someone can tell me the best one:

1) Use Windows Low-fragmentation Heap

2) jemalloc - as used in Firefox 3

3) Doug Lea's malloc

Our server process is developed using cross-platform C++ code, so any solution would be ideally cross-platform also (do *nix operating systems suffer from this type of memory fragmentation?).

Also, am I right in thinking that LFH is now the default memory allocation mechanism for Windows Server 2008 / Vista?... Will my current problems "go away" if our customers simply upgrade their server os?

Answer

First, I agree with the other posters who suggested a resource leak. You really want to rule that out first.

Hopefully, the heap manager you are currently using has a way to dump out the actual total free space available in the heap (across all free blocks) and also the total number of blocks that it is divided over. If the average free block size is relatively small compared to the total free space in the heap, then you do have a fragmentation problem. Alternatively, if you can dump the size of the largest free block and compare that to the total free space, that will accomplish the same thing. The largest free block would be small relative to the total free space available across all blocks if you are running into fragmentation.

To be very clear about the above, in all cases we are talking about free blocks in the heap, not the allocated blocks in the heap. In any case, if the above conditions are not met, then you do have a leak situation of some sort.

So, once you have ruled out a leak, you could consider using a better allocator. Doug Lea's malloc suggested in the question is a very good allocator for general use applications and very robust most of the time. Put another way, it has been time tested to work very well for most any application. However, no algorithm is ideal for all applications and any management algorithm approach can be broken by the right pathelogical conditions against it's design.

Why are you having a fragmentation problem? - Sources of fragmentation problems are caused by the behavior of an application and have to do with greatly different allocation lifetimes in the same memory arena. That is, some objects are allocated and freed regularly while other types of objects persist for extended periods of time all in the same heap.....think of the longer lifetime ones as poking holes into larger areas of the arena and thereby preventing the coalesce of adjacent blocks that have been freed.

To address this type of problem, the best thing you can do is logically divide the heap into sub arenas where the lifetimes are more similar. In effect, you want a transient heap and a persistent heap or heaps that group things of similar lifetimes.

Some others have suggested another approach to solve the problem which is to attempt to make the allocation sizes more similar or identical, but this is less ideal because it creates a different type of fragmentation called internal fragmentation - which is in effect the wasted space you have by allocating more memory in the block than you need.

Additionally, with a good heap allocator, like Doug Lea's, making the block sizes more similar is unnecessary because the allocator will already be doing a power of two size bucketing scheme that will make it completely unnecessary to artificially adjust the allocation sizes passed to malloc() - in effect, his heap manager does that for you automatically much more robustly than the application will be able to make adjustments.