CRT Heap Fragmentation in Windows

Heap Fragmentation

Fragmenting a heap is something I haven’t worried about for years. When you allocate and deallocate memory in certain patterns you can leave areas of unallocated memory stranded inamongst allocated memory. This can lead to the situation where you have, say, 10Mb of memory free, but yet an allocation for 256 bytes fails as although you have all this free memory, none if it is in a big enough continuous lump to give you your 256 bytes. Of course on modern PCs the allocation won’t actually fail, it will just [a] take a while as the OS looks through its list of free blocks for a 256 byte space, and then [b] allocate even more memory from the system, possibly hitting virtual memory. Not great — but not terrible, you’d think.


I started my programming career as a computer games programmer. On the consoles we worked on, processor time and memory were always at a premium. In the very early days, dynamic memory allocation was a no-no: everything was statically allocated. Memory allocation routines used simple allocate-only heaps which were reset at the end of a level. This made optimal use of the available memory, but meant the heap couldn’t be used for dynamic objects at run-time. With no way to free the memory, allocating new objects would mean eventually the memory would become exhausted. Only with the advent of the Dreamcast, Xbox and PlayStation 2 would developers consider using a general heap for allocation. Even then, allocators were hand-written for speed and frugality of memory overhead, and many different heaps were used for varying types of object.

For Red Dog, I used a trusty memory allocation routine I’d written in my Acorn days. The allocator was based loosely on one of Donald Knuth‘s Fundamental Algorithms, and performed excellently. It was also aggressive at coalescing adjacent free blocks in the heap, which made freeing objects and then reallocating new ones both fast and space efficient. Memory fragmentation was still an issue with this allocator — one of the reasons we had multiple heaps for different objects was to help alleviate this problem. We tended to have one for the enemies’ virtual machine stacks, one for particle systems, another for dynamic models, and so on.

This allocator made it to later games like SWAT too. Games were now being written in C++, and this made the memory allocator that much more important. C++ does have a tendency to lean heavily on the allocator — especially when you use modern concepts like interface-based design and pImpled objects. In fairness I believe we let the built-in malloc/free-based allocator manage the majority of objects in SWAT, only using my custom allocator in critical areas. This still meant using it for a fair few STL objects, writing an STL allocator object that wrapped the memory allocator, not easy to get working across two disparate tool sets: Xbox and PS2.

The problem

So why this explanation of console memory management strategies? Nowadays, with advanced operating systems able to manage fragmentation by physically mapping memory about (albeit in 4k pages), and processor speeds and memory sizes so great you’d not expect it to be an issue anymore. Well, very recently I was investigating some performance issues in StyleManager. StyleManager has to preprocess C/C++ source code in order to perform its tasks; and given that #include-ing windows.h adds about a million symbols to your preprocessor, there’s a fair amount of memory allocations going on. After much general optimising, I had a great performing preprocessor, but then I spotted a strange issue. If I ran the preprocessor multiple times, it steadily slowed down, in one case slowing down by around 40% each invocation.

At first I was convinced my optimisations had introduced a memory leak. After a few hours’ investigation this proved not to be the case. I then steadily worked through all the persistent state of my application, looking for anything left behind after a preprocessor run that could explain the situation. I was drawing a total blank. In sheer desperation, assuming it was something relating to my memory allocation patterns, I replaced the allocator with a simple 1Gb lump of memory I malloc() at startup and take each allocation from in turn, ignoring all deallocations. Apart from doubling the speed of the application (quite shockingly!), the incremental slowdown vanished. So then I knew it was something to do with the memory usage — but what?

The solution

If you’ve read this far, you’ve almost certainly guessed what it is — the heap was getting fragmented. Even though every single allocation I was making was getting freed at the end of each run, somehow the OS heap manager wasn’t dealing very well with the allocation patterns I was using and so each time I added a new preprocessed token the allocator slowed. I finally proved it to myself (after some lucky hits on google) by finding a Windows XP-only setting for the heap which sets it up as a ‘Low-fragmentation Heap‘ (LFH). Disabling my custom null memory allocator and simply adding:

ULONG HeapFragValue = 2;
    &HeapFragValue, sizeof(HeapFragValue));

…to the top of my unit test’s main() function not only sped the application up to nearly as fast as my null allocator, but the incremental slowdown disappeared too. As far as I can explain, it has to be the fragmentation causing the issue. I was utterly shocked by this; my understanding of memory management systems was that they could handle anything you threw at them nowadays, especially on say a 3.2GHz 2Gb machine! Clearly not!

Unfortunately the LFH is an XP- and above-only feature, so I haven’t been able to use it generally for StyleManager. Instead I’ve written a quick and dirty block allocator which I’ve added to our main memory (ab)using classes, specifically in this case the list of preprocessed tokens. Even though I only spent a few hours honing the allocator it already vastly outperforms the standard heap routines and doesn’t suffer from any fragmentation issues.


Even in today’s computing environment you can’t ignore memory allocation routines. They can be responsible for a lot of your program’s wasted performance. Specifically on the Windows CRT heaps, fragmentation can cause issues even in circumstances where you’d expect them not to. Microsoft seems to have addressed this by adding support for Low-fragmentation Heaps in Windows XP and above. In some cases, writing just one or two custom allocation routines can hugely improve your program’s performance.

Subsequent notes

Since writing this article I’ve found another way to speed up heap allocation in my specific application — calling _set_sbh_threshold (set small object threshold) with a fairly small number (I used 128). This had a similar speed-up to the other techniques mentioned, and is cross-Windows compatible. Interestingly, when I traced into the CRT code, it would appear having a non- zero sbh_threshold makes the CRT use a ‘Visual C 6’ heap instead of its newer OS-level Heap. Can it really be that the older heap technologies were actually better at handling StyleManager’s type of situation than the fancy new ones? Makes you think, anyway — and I’ll be sticking with my own allocator for now.

Thanks to Malcolm Rowe for his proof-reading skills and editorial comments.

Filed under: Coding
Posted at 13:57:46 GMT on 7th December 2005.

About Matt Godbolt

Matt Godbolt is a C++ developer working in Chicago in the finance industry.