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Kernel events without kevents

Kernel events without kevents

Posted Mar 17, 2007 18:35 UTC (Sat) by mikov (subscriber, #33179)
In reply to: Kernel events without kevents by pphaneuf
Parent article: Kernel events without kevents

64 is a pretty low number of objects, I'd say, but it depends on the design. It seems more oriented to a "one thread per connection" design, where a single connection could be handling a few things (waiting for an answer from a database, but also watching the socket for disconnection, say). Also, the way it can easily lead to starvation seems like a major design problem with it. Careless ordering of the list of handles could lead to a stuck process! Odd, that.

Well, I've never had to use WaitForMultipleObjects() with more than a few handles. I think that for a single-threaded IO server design (which admittedly I haven't done), I'd use the ReadFileEx family of functions, which register a completion callback (APC) - this elliminates the 64 handle problem. Then I'd use WaitForMultipleObjectsEx() if I have to let the callbacks run _and_ wait for semaphores and stuff.

Addmitedly, since Win32 has always had well integrated threads, the need to shoehorn everything into a single thread has never been very strong.

It's a kind strange, comparing the two APIs. I feel that the Win32 is a bit more integrated, yes, having had this stuff for a longer time (everything is an object with a handle, that can be given to WaitForMultipleObjects()), but somehow doesn't feel like that (reading from a socket with ReadFile() seems a bit odd).

I guess it is force of habit. The other methods are also available - recv(),read(), etc. BTW, for some things the Win32 API can be a major PITA. For example serial communication - this is where you don't want overlapped operations and WaitForMultipleObjects - instead you really want select(). Alas, in Win32 select() works only on sockets ...

If your UI is giving you grief, the answer isn't necessarily to stop using messages, but to have another thread, so that there's more execution contexts to do the work. Ideally, it would all be so uniform that UI code could run on any thread as well, so things would just get done as quickly as possible, no matter what it is, nothing having the edge over the other.

But that's exactly it. In the main thread of a GUI application you need messages to drive the GUI. However if you create another thread (let's say for IO), there is no point at all in using Windows messages there. None of the APIs generate them, so you'd have to write code to send them yourself, make a message loop to handle them, etc. What's the point ? If you really needed some sort of of message queue in your design, there is zero reason to use the Windows GUI one - you are much better off coding something custom.

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