sutton036

In a recently-posted video , Herb Sutter (a prominent C++ expert) describes his favorite C++ 10-liner as “a thread-safe reference-counted object cache”: shared_ptr<widget> get_widget(int id) {   static map<int, weak_ptr<widget>> cache;   static mutex m;   lock_guard<mutex> hold(m);   auto sp = cache[id].lock();   if (!sp) cache[id] = sp = load_widget(id);   return sp; } This example is very interesting. Firstly, it manages to pull in reference counting, weak references and a mutex which are all very rare in modern programming. Secondly, it contains a memory leak that is difficult to fix in C++ because APIs are burdened with memory management details and this API is incapable of expressing deterministic cleanup because there is no facility for a widget's destructor to remove its entry in the map. Finally, the correct name for this data structure is a concurrent weak dictionary , specifically one with weak values. You'll find corre...

Since the 1950s there have been three main families of collectors: semi-space, mark-sweep and mark-compact. Almost all production GCs have been generational mark-sweep even though they exhibit pathological performance when the nursery is full of survivors because they are marked, (physically) copied and all references to them updated which is ~3x slower than necessary and is practically useful when filling a hash table with heap-allocated keys and/or values. The Beltway (2002) and Immix (2008) garbage collectors introduced the new family called the mark-region GCs. With a mark-region GC the entire heap is a collection of regions and so is the nursery generation so it can be logically aged by replacing it with another region when it is full of survivors. Sun's Hotspot JVM introduced the first mainstream mark-region GC with its G1 collector. The advent of multicore in 2005 has meant more emphasis on parallel and concurrent garbage collectors. The Staccato (2008) garbage collector ...

Amidst the excitement around the new support for asynchronous programming in C# 5, Alex Davies of Red Gate software writes: " Before C# 5, I think I was about the only person in the world who really cared about asynchronous programming " In fact, many of our 1,000 corporate clients have been developing concurrent programs on .NET over the past two years. The world's foremost supplier of electronic trading and order matching software for brokers, exchanges and traders in the energy industry recently rewrote their trading user interface using async. The UK's largest insurance company provide life insurance quotes on-line using async. They are all doing this using  F#  because it has supported async out-of-the-box since it was first released in Visual Studio 2010. Even if you intend to write your asynchronous programs in C# 5, we recommend taking a look at how people have been writing asynchronous programs on .NET using  F#  including the following F#.NET Journal...

Concurrent programming regards operations that appear to overlap and is primarily concerned with the complexity that arises due to non-deterministic control flow. The quantitative costs associated with concurrent programs are typically both throughput and latency. Concurrent programs are often IO bound but not always, e.g. concurrent garbage collectors are entirely on-CPU. The pedagogical example of a concurrent program is a web crawler. This program initiates requests for web pages and accepts the responses concurrently as the results of the downloads become available, accumulating a set of pages that have already been visited. Control flow is non-deterministic because the responses are not necessarily received in the same order each time the program is run. This characteristic can make it very hard to debug concurrent programs. Some applications are fundamentally concurrent, e.g. web servers must handle client connections concurrently. Erlang is a language designed specifically for ...

Don Syme, creator of the F# programming language, recently gave a superb lecture on parallel and concurrent programming using F# at QCon 2010. Video and slides hosted by InfoQ here . Parallel programming continues to be a hot topic in the face of multicore computing but, as Don points out, the world is also moving steadily towards more concurrent programming. Work continues on our forthcoming "Multicore .NET" book that studies parallel programming using C# and F# in detail...