[NEW] Defrag - Global handling for reallocations during defrag cycle

[zvi-code]

The problem/use-case that the feature addresses
Current defrag implementation uses explicit allocatorDefragAlloc/allocatorDefragFree functions that bypass tcache. The motivation to bypass tcache is so that when we free a memory ptr because of defrag we want to avoid reallocating it from tcache in next allocation, bypassing the tcache frees the ptr directly to the arena.

The dedicated API for defrag alloc introduce a challange for modules where we do not have full control over the memory allocations (and we might not want to have this control).

In these cases we would want, ideally, to defrag by just reallocating the object(if that's required).

raised in the context of the discussion #1210 (comment)

Description of the feature

The proposal is to achieve this by embedding the special defrag specific allocatorDefragAlloc/allocatorDefragFree API into the zmalloc zmalloc\zfree api for the defragging thread(s).

To achieve this we would need to introduce zmalloc_thread_start_defrag() and zmalloc_thread_stop_defrag(). These would be called on begin\end of defrag cycle and when started, the zmalloc\zfree of the defrag thread would use the allocatorDefragAlloc/allocatorDefragFree instead of the normal alloc.

Alternatives you've considered

Other alternative is to give modules access to allocatorDefragAlloc/allocatorDefragFree

Additional information

This feature can be combined with additional improvement to utilize user defined tcache, to keep some of the performance benefit of tcache during defrag while keeping the alloc-free separation (force ptrs recycle through the arena before realloc)