Here’s the latest section of my ongoing server-design article. I also changed some of the previous section (context switches) from how it appears in the log.
Allocating and freeing memory is one of the most common operations in many applications. Accordingly, many clever tricks have been developed to make general-purpose memory allocators more efficient. However, no amount of cleverness can make up for the fact that the very generality of such allocators inevitably makes them far less efficient than the alternatives in many cases. I therefore have three suggestions for how to avoid the system memory allocator altogether.
Suggestion #1 is simple preallocation. We all know that static allocation is bad when it imposes artificial limits on program functionality, but there are many other forms of preallocation that can be quite beneficial. Usually the reason comes down to the fact that one trip through the system memory allocator is better than several, even when some memory is “wasted” in the process. Thus, if it’s possible to assert that no more than N items could ever be in use at once, preallocation at program startup might be a valid choice. Even when that’s not the case, preallocating everything that a request handler might need right at the beginning might be preferable to allocating each piece as it’s needed; aside from the possibility of allocating multiple items contiguously in one trip through the system allocator, this often greatly simplifies error-recovery code. If memory is very tight then preallocation might not be an option, but in all but the most extreme circumstances it generally turns out to be a net win.
Suggestion #2 is to use lookaside lists for objects that are allocated and freed frequently. The basic idea is to put recently-freed objects onto a list instead of actually freeing them, in the hope that if they’re needed again soon they need merely be taken off the list instead of being allocated from system memory. As an additional benefit, transitions to/from a lookaside list can often be implemented to skip complex object initialization/finalization.
It’s generally undesirable to have lookaside lists grow without bound, never actually freeing anything even when your program is idle. Therefore, it’s usually necessary to have some sort of periodic “sweeper” task to free inactive objects, but it would also be undesirable if the sweeper introduced undue locking complexity or contention. A good compromise is therefore a system in which a lookaside list actually consists of separately locked “old” and “new” lists. Allocation is done preferentially from the new list, then from the old list, and from the system only as a last resort; objects are always freed onto the new list. The sweeper thread operates as follows:
- Lock both lists.
- Save the head for the old list.
- Make the (previously) new list into the old list by assigning list heads.
- Free everything on the saved old list at leisure.
Objects in this sort of system are only actually freed when they have not been needed for at least one full sweeper interval, but always less than two. Most importantly, the sweeper does most of its work without holding any locks to contend with regular threads. In theory, the same approach can be generalized to more than two stages, but I have yet to find that useful.
One concern with using lookaside lists is that the list pointers might increase object size. In my experience, most of the objects that I’d use lookaside lists for already contain list pointers anyway, so it’s kind of a moot point. Even if the pointers were only needed for the lookaside lists, though, the savings in terms of avoided trips through the system memory allocator (and object initialization) would more than make up for the extra memory.
Suggestion #3 actually has to do with locking, which we haven’t discussed yet, but I’ll toss it in anyway. Lock contention is often the biggest cost in allocating memory, even when lookaside lists are in use. One solution is to maintain multiple private lookaside lists, such that there’s absolutely no possibility of contention for any one list. For example, you could have a separate lookaside list for each thread. One list per processor can be even better, due to cache-warmth considerations, but only works if threads cannot be preempted. The private lookaside lists can even be combined with a shared list if necessary, to create a system with extremely low allocation overhead.