alt Hacker NewsI've read some allocator walkthroughs before but I thought that this line stood out:
"to get individual free() working, the allocator needs to remember something about every allocation it handed out. and that’s the moment metadata stops being optional."
That's just a very nice distillation of an important concept.
Replies
geocar • today at 6:44 AM
Oh? How do you think munmap does it?
If you can convince the caller to keep track of that metadata themselves you obviously don’t need to. That can be important.
Something I noticed is that _very often_ the code that is calling malloc(n) is keeping track of n somehow for its own reasons (bounds checking, grow/gap pointers, etc) so merging the value halves stack churn and it’s an easy win.
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nathaah3 • last Monday at 1:06 PM
glad you found it useful :)
This becomes particularly important when the allocation and metadata cannot (or should not) be stored in the same address space. An example of this is allocating memory on GPUs.
The conventional approach for allocating memory on GPUs for games and other applications is to use a real-time allocator such as TLSF. However, it is not usually discussed that TLSF is real-time because it stores metadata in-band. It is possible to create a variant of TLSF that preserves its real-time properties while storing metadata out-of-band, but this requires careful consideration.