Lew wrote: > As Dr. Jon pointed out upthread, one can write decent code with mutable > shared > state. It is also true that mutable state presents a lot of problems - > potential problems, ones that can be solved, but not ones that can be > solved > thoughtlessly. On the flip side, one can write a tremendous amount of > effective multi-threaded code involving shared mutable state with > attention to a few rules of thumb, like always synchronize access and > don't use different monitors to do so. > > Unlike some environments (e.g., database management systems), Java's tools > to > manage concurrency are explicit and low level. The programmer's job is to > make sure those tools are used correctly to avoid problems. As long as > they do that, then there is no special problem with shared mutable state. > > There is, however, a cost. Certain things must happen slower when you > share mutable state, than when you share immutable state or don't share > state. Certain things must happen when you share mutable state, regardless > of speed, > because without them your code doesn't work. For some reason, concurrent > programming is an area often not well understood by a significant > percentage > of workaday programmers. When problems do arise, they tend to be > probabilistic in nature and vary widely with system characteristics like > attempted load. > > So the meeting ground is, yes, concurrent mutable state can present > problems > if not properly managed. Properly managing such is not necessarily a huge > burden, but it must be borne. When done properly, shared mutable state > will not present problems in production.
I agree entirely but my statements were about parallelism and not concurrency. Parallel and concurrent programming have wildly different characteristics and solutions. I don't believe shared mutable state is overly problematic in the context of parallelism. Indeed, I think it is usually the best solution in that context. -- Dr Jon D Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?u -- http://mail.python.org/mailman/listinfo/python-list
