What is the mutator?

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? -- Jason Dusek