[tptacek]

Contra "exDM69", there's nothing unreasonable or "unrealistic" about allocating 8 bytes at a time; it's in fact extremely convenient to be able to do that. I think it's telling that someone would call that workload unrealistic; it indicates to me that they've never even really considered alternatives to malloc.

It's a little like those C programmers who try to use 256 byte static arrays for all their strings because they don't quite grok malloc/realloc.

[jbooth]

Well, personally, if I'm using C, it's typically for a very narrowly defined, performance-sensitive problem. Both times I've done this in the last couple years, I found myself doing a couple big mallocs at the start and then running some tight loops over that memory with no further mallocs -- for use cases with more allocation, I'll just use whatever other language is more convenient, preferably one with a GC.

But if I were writing a bigger application entirely in C/C++, I could totally see lots of small allocations happening at different points (and getting into wackiness with custom pooled allocators and auto_ptr).

[tptacek]

That's essentially what I do in much of my C code: implement pool allocators. And that's essentially what the post we're commenting on is talking about.

Incidentally, good common case for 8-byte allocations that is actually common in real-world C code: 32 bit linked list nodes (4 bytes nextptr, 4 bytes dataptr).

[cdman]

Hmm, are you sure that this (linked lists with small elements) is a good example which you should tell people about?

Linked lists are bad because they have big overhead (8 bytes for every element in your case - which dominates by a large margin if you store an int in each node for example) and they are really bad for CPU caches (they have very little spatial locality), thus slowing down your code.

I much rather prefer the "growing array" approach.

[jbooth]

Yeah, but they have some unique characteristics related to inserting/deleting from the middle or beginning of the list. They're actually used in a few places in the linux kernel, for performance reasons, in spite of cache locality issues.

[vonmoltke]

That is similar to the signal processing application I worked on a couple years ago. It was a real-time application with fixed sizes at each point in the algorithm, so we knew maximum memory usage at initialization. Thus, we allocated all memory up-front and continuously reused the buffers until we were shut down. Under the operational architecture we were moving to when I left, our application never actually called malloc at all; the control program that ran the signal processor and which started us malloced massive segments of memory on each machine and gave it to us, which we then "allocated" to our own buffers.