MEDIUM: lua-thread: use atomics for memory accounting
Let's switch memory accounting to atomics so that the allocator function
may safely be used from concurrent Lua states.
Given that this function is extremely hot on the call path, we try to
optimize it for the most common case, which is:
- no limit
- there's enough memory
The accounting is what is particuarly expensive in threads since all
CPUs compete for a cache line, so when the limit is not used, we don't
want to use accounting. However we need to preserve it during the boot
phase until we may parse a "tune.lua.maxmem" value. For this, we turn
the unlimited "0" value to ~0 at the end of the boot phase to mark the
definite end of accounting. The function then detects this value and
directly jumps to realloc() in this case.
When the limit is enforced however, we use a CAS to check and reserve
our share of memory, and we roll back on failure. The CAS is used both
for increments and decrements so that a single operation is enough to
update the counters.
1 file changed