On 3/19/2011 1:09 PM, Michel Fortin wrote:
On 2011-03-19 12:03:51 -0400, Andrei Alexandrescu <seewebsiteforem...@erdani.org> said:* "Most of this module completely subverts..." Vague characterizations ("most", "completely", "some") don't belong in a technical documentation. (For example there's either subversion going on or there isn't.) Also, std.concurrency and std.parallelism address different needs so there's little competition between them. Better: "Unless explicitly marked as $(D @trusted) or $(D @safe), artifacts in this module are not provably memory-safe and cannot be used with SafeD. If used as documented, memory safety is guaranteed."Actually, I think this is a bad description of what it subverts. What it subverts isn't the memory-safety that SafeD provides, but the safety against low-level races that even unsafe D protects against unless you cast shared away. For instance: void main() { int sum = 0; foreach (int value; taskPool.parallel([0,2,3,6,1,4,6,3,3,3,6])) { sum += value; } writeln(sum); } The "+=" would need to be an atomic operation to avoid low-level races. I think that ParallelForeach's opApply should only accept a shared delegate. I define shared delegate as a delegate that does not reference any non-shared variables of its outer scope. The problem is that DMD currently doesn't know how to determine whether a delegate literal is shared or not, thus a delegate literal is never shared and if ParallelForeach's opApply asked a shared delegate as it should it would just not work. Fix DMD to create shared delegate literals where appropriate and everything can be guarantied race-free.
If you want pedal-to-metal parallelism without insane levels of verbosity, you can't have these safety features. I thought long and hard about this issue before submitting this lib for review and concluded that any solution would make std.parallelism so slow, so limited and/or such a PITA to use that I'd rather it just punt these issues to the programmer. In practice, parallel foreach is used with very small, performance-critical snippets that are fairly easy to reason about even without any language-level race safety. This is a fundamental design decision that will not be changing. If it's unacceptable then:
1. I give up. 2. I wish someone had told me earlier.
