Re: What is the mutator?

Message: 1 Date: Thu, 6 Aug 2009 00:38:08 -0700 From: Jason Dusek Subject: What is the mutator? To: glasgow-haskell-users@haskell.org Message-ID: <42784f260908060038h53d7cc0dy9f80e43f269a2faf@mail.gmail.com> Content-Type: text/plain; charset=UTF-8

I've been reading a little about GC latency and have run across statements like this:

One solution to the GC synchronisation problem would be to implement a concurrent garbage collector. Typically, however, concurrent GC adds some overhead to the mutator, since it must synchronise with the collector.some thunks are never “black-holed”, so giving a potential performance win. Unfortunately, in the parallel setting, it substantially enlarges the time window in which two or more duplicate threads might evaluate the same think, and thus

-- Comparing and Optimising Parallel Haskell Implementations for Multicore Machines

What is the mutator?

Hi Jason, as Malcolm already said, the "mutator" in this text is the/a thread evaluating some Haskell expression. Just to add some more details to the picture... In general, a Haskell expression is a computation represented as a graph in the heap. Haskell evaluates lazily and does not have to fully evaluate every part of it for the program to finish. Unevaluated parts are "thunks". As soon as one of potentially several concurrent (mutator) threads starts to evaluate a thunk, it is replaced by a blackhole, which keeps other threads out of it until the node in the graph is evaluated (say, to a list cons (:), with probably unevaluated head and tail). Then the blackhole is updated with the new value. Other threads block on the blackhole in the meantime (so not necessarily an exception in the case of concurrent mutator threads) and are woken up by the update. The passage you quote above is about two separate aspects: 1. Garbage collection and mutator running concurrently: while they usually do, they do not _have_ to exclude each other, but not doing so means that the objects they are treating have to be locked. 2. About "Blackholing": in the sequential evaluation (where hitting a blackhole indeed means to have a loop), some better performance can be gained by not blackholing a thunk immediately, so this was done in GHC earlier. However, it increases the chance for 2 mutator threads to evaluate the same thunk (double work), and we got better performance by blackholing immediately. Cheers, Jost