Document locking behavior of the default GC

[dlangBugzillaToGithub]

Andrej Mitrovic (@AndrejMitrovic) reported this on 2023-06-21T10:00:16Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=24006

Description

As Steven mentions here: https://forum.dlang.org/post/u6tgl4$1v07$1@digitalmars.com

> The D GC has a single global lock to allocate memory -- even memory that might be on a free list. So the threads are all bottlenecked on waiting their turn for the lock.

This behavior isn't documented on https://dlang.org/spec/garbage.html

But it is mentioned in passing on this page: https://dlang.org/articles/d-array-article.html#caching

Specifically "Doing this means an O(lg(n)) lookup in the GC's memory pool for every append (not to mention acquiring the global GC lock)."