On Friday, 15 January 2021 at 14:24:40 UTC, welkam wrote:
No. And it will never will. Currently DMD uses custom allocator
for almost everything. It works as follows. Allocate a big
chunk(1MB) of memory using malloc. Have a internal pointer that
points to the beginning of unallocated memory. When someone ask
for memory return that pointer and increment internal pointer
with the 16 byte aligned size of allocation. Meaning the new
pointer is pointing to unused memory and everything behind the
pointer has been allocated. This simple allocation strategy is
called bump the pointer and it improved DMD performance by ~70%.
You can use GC with D compiler by passing -lowmem flag. I didnt
measure but I heard it can increase compilation time by 3x.
https://github.com/dlang/dmd/blob/master/src/dmd/root/rmem.d#L153
Actually druntime uses map (Linux) and VirtualAlloc (Windows) to
break out more memory. C-lib malloc is an option but not used in
most platforms and this option also is very inefficient in terms
of waste of memory because of alignment requirements.
Bump the pointer is a very fast way to allocate memory but what
is more interesting is what happens when you return the memory.
What does the allocator do with chunks of free memory? Does it
put it in a free list, does it merge chunks? I have a feeling
that bump the pointer is not the complete algorithm that D uses
because of that was the only one, D would waste a lot of memory.
As far as I can see, it is simply very difficult to create a
completely lockless allocator. Somewhere down the line there will
be a lock, even if you don't add one in druntime (the lock will
be in the kernel instead when breaking out memory). Also merging
chunks can be difficult without locks.
