Update performance.md (#71)

Add some information about the new GC pacer

Co-authored-by: Alan Jeffrey <403333+asajeffrey@users.noreply.github.com>

docs/_pages/performance.md

@@ -147,3 +147,9 @@ In addition to a fast allocator, all frequently used structures in Luau have bee
 While the best performing code in Luau spends most of the time in the interpreter, performance of the standard library functions is critical to some applications. In addition to specializing many small and simple functions using the builtin call mechanism, we spend extra care on optimizing all library functions and providing additional functions beyond the Lua standard library that help achieve good performance with idiomatic code.
 
 For example, functions like `insert`, `remove` and `move` from the `table` library have been tuned for performance on array-like tables, achieving 3x and more performance compared to un-tuned versions, and Luau provides functions like `table.create` and `table.find` to achieve further speedup when applicable. We also use a carefully tuned dynamic string buffer implementation for internal `string` library to reduce garbage created during string manipulation.
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+## Improved garbage collector pacing
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+Luau uses an incremental garbage collector which does a little bit of work every so often, and at no point does it stop the world to traverse the entire heap. The runtime will make sure that the collector runs interspersed with the program execution as the program allocates additional memory, which is known as "garbage collection assists", and can also run in response to explicit garbage collection invocation via `lua_gc`. In interactive environments such as video game engines it's possible, and even desirable, to request garbage collection every frame to make sure assists are minimized, since that allows scheduling the garbage collection to run concurrently with other engine processing that doesn't involve script execution.
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+Inspired by excellent work by Austin Clements on Go's garbage collector pacer, we've implemented a pacing algorithm that uses a proportional–integral–derivative controller to estimate internal garbage collector tunables to reach a target heap size, defined as a percentage of the live heap data (which is more intuitive and actionable than Lua 5.x "GC pause" setting). Luau runtime also estimates the allocation rate making it easy (given uniform allocation rates) to adjust the per-frame garbage collection requests to do most of the required GC work outside of script execution.