deallocate_block() used to call MADV_FREE_REUSABLE / MADV_FREE /
MADV_DONTNEED inline on every freed block. With sweep typically
freeing many blocks per GC, the cumulative syscall cost shows up
as real GC pause time.
Move the work onto a single global "decommit worker" thread:
- deallocate_block now just poisons the slot and pushes it onto a
per-allocator m_freshly_freed queue. No syscalls.
- allocate_block prefers m_freshly_freed over m_blocks, so a slot
that's recycled before the worker sees it skips the
REUSABLE/REUSE pair entirely. This is the main payoff.
- Heap::sweep_dead_cells kicks the worker at the end of sweep.
The worker sleeps 50 ms after each kick to give the JS thread
breathing room, then drains each registered allocator's
m_freshly_freed, madvises slots in batches of 64 with
sched_yield between batches, and splices them onto m_blocks.
- Per-allocator refcount + condvar lets ~BlockAllocator wait
until the worker has dropped its reference before our storage
goes away. (Chunks themselves remain leaked: type-isolated VM
is permanent, so we never tear them down.)
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