On 19/01/11 16:14, Sam Vilain wrote: > In general, Java's basic types typically correspond with types that can > be dealt with atomically by processors, or are small enough to be passed > by value. This already makes things a lot easier. > > I've had another reason for the differences explained to me. I'm not > sure I understand it fully enough to be able to re-explain it, but I'll > try anyway. As I grasped the concept, the key to making VMs fully > threadable with shared state, is to first allow reference addresses to > change, such as via generational garbage collection. This allows you to > have much clearer "stack frames", perhaps even really stored on the > thread-local/C stack, as opposed to most dynamic language interpreters > which barely use the C stack at all. Then, when the long-lived objects > are discovered at scope exit time they can be safely moved into the next > memory pool, as well as letting access to "old" objects be locked (or > copied, in the case of Software Transactional Memory). Access to > objects in your own frame can therefore be fast, and the number of locks > that have to be held reduced.
Ref: http://java.sun.com/docs/books/jvms/second_edition/html/Concepts.doc.html#33308 and to a lesser extent, the note on http://java.sun.com/docs/books/jvms/second_edition/html/Threads.doc.html#22244 > Perhaps to support/refute this argument, in your JVM, how do you handle: > > - memory allocation: object references' timeline and garbage collection > - call stack frames and/or return continuations - the C stack or the heap? > - atomicity of functions (that's the "synchronized" keyword?) > - timely object destruction > > put it forward that the overall design of the interpreter, and > therefore what is possible in terms of threading, is highly influenced > by these factors. > > When threading in C or C++ for instance (and this includes HipHop-TBB), > the call stack frame is on the C stack, so shared state is possible so > long as you pass heap pointers around and synchronise appropriately. > The "virtual" machine is of a different nature, and it can work. For > JVMs, as far as I know references are temporary and again the nature of > the execution environment is different. > > For VMs where there is basically nothing on the stack, and everything on > the heap, it becomes a lot harder. To talk about a VM I know better, > Perl has about 6 internal stacks all represented on the heap; a function > call/return stack, a lexical scope stack to represent what is in scope, > a variable stack (the "tmps" stack) for variables declared in those > scopes and for timely destruction, a stack to implement local($var) > called the "save" stack, a "mark" stack used for garbage collection, ok > well only 5 but I think you get my point. From my reading of the PHP > internals so far there are similar set there too, so comparisons are > quite likely to be instructive. It's a bit hard figuring out everything > that is going on internally (all these internal void* types don't help > either), and whether or not there is some inherent property of reference > counting, or whether it just makes a shared state model harder, is a > question I'm not sure is easy to answer > Based on https://github.com/smarr/RoarVM/blob/98caf11d0/README.rst it can be seen that indeed it is a completely different architecture. From the first of the ACM papers' abstract: In addition to the cost of inter-core communication, two hardware characteristics influenced our design: the absence of hardware-provided cache-coherence, and the inability to move a single object from one core's cache to another's without changing its address. > In any case, full shared state is not required for a large set of useful > parallelism APIs, and in fact contains a number of pitfalls which are > difficult to explain, debug and fix. I'm far more interested in simple > acceleration of tight loops - to make use of otherwise idle CPU cores > (perhaps virtual as in hyperthreading) to increase throughput - and APIs > like "map" express this well. The idea is that the executor can start > up with no variables in scope, though hopefully shared code segments, > call some function on the data it is passed in, and pass the answers > back to the main thread and then set about cleaning itself up. You could probably support this with any paper on Erlang ;-) Sam -- PHP Internals - PHP Runtime Development Mailing List To unsubscribe, visit: http://www.php.net/unsub.php
