FFM API allocation shootout

[Maurizio Cimadamore]

On 01/08/2023 17:19, mail at smogura.eu wrote:
> * Should all memory allocations even with custom allocators be tracked - this can be a little bit error prone.

Custom allocators/arenas came up twice.

I don't think custom arenas would defeat the counting - I'd expect most 
custom arenas (like the PoolArena used in the benchmark) to just 
delegate to a "true" confined arena under the hood - so, some "tracked 
allocation would still occur.

The only case where this would not work is if you use an arena that 
bypasses Unsafe (e.g. MallocArena), using Linker. In that case no 
NativeMemoryTracking, and no JMX.

Re. custom counters, I believe that a custom arena has freedom to use 
its own counter, if it so wishes.

So perhaps that's an argument _against_ tracking memory in special ways 
(and/or setting limits). Now that Arena is the star of the show, and can 
be customized (via delegation), it is possible for custom subclasses to 
do things such as track memory usage, reject allocation requests over a 
certain limit, etc.

If that was the case, a possible plan could be to disentangle arenas 
from Bits::reserveMemory completely, and maybe, over time, figure out 
what kind of "common counters" application need.

Maurizio