[PATCH 2/5] Add single-threaded path to malloc/realloc/calloc/memalloc

[Wilco Dijkstra]

DJ Delorie wrote:
> Does SINGLE_THREAD_P return true if a multi-threaded app joins all its
> threads, so that there's only one thread remaining?  If so, there will
> still be more arenas in play that might be usable, and the remaining
> thread might be better served by whatever arena it had been using, due
> to data locality on NUMA machines.

No that doesn't happen, so it's not a problem today.

> I also wonder if there's an upper limit to the main arena, where we'd
> have to use other arenas to fully use available VM.

Not as far as I can tell, there is the traditional stack and heap that grow
towards each other via sbrk. I don't believe you could reduce that since
mmaps in the middle would not be allowed.

> If either of these are issues, it might be better to conditionalize the
> lock calls than to duplicate the code, so we keep the alt-arena code in
> play.

That would mean sorting out all the locking in the arena code as well...
If we want to retry a different arena then the code that fails to allocate
more memory should do that (to avoid adding the retry in the fast path).

Wilco