[RFT] malloc: reduce largebin granularity to reduce fragmentation

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Eric,

I agree that limiting size classes will help with fragmentation. However
you need more than this: when it finds a block that is larger than requested
it will always split off the end even if it is a much smaller block. As a result
large blocks get smaller over time. Additionally allocations that can't be
satisfied by the free lists are taken from the large remainder of the arena,
thus interleaving allocations of all sizes rather than grouping similar sizes
together in separate regions like modern allocators.

The tcache and smallbin freelist also keeps freed block listed as 'allocated',
thus blocking merging of free blocks into larger ones - this is only done
every now and again during a consolidation pass. Threads that free blocks
but never allocate a large block again may never really free their freed
blocks.

So to reduce fragmentation you also have to address all these issues too.
My general feeling is that this allocator is way too ancient and hacked up to
compete with modern allocators like jemalloc or mimalloc.

Cheers,
Wilco