[dmd-concurrency] priority messages

[Michel Fortin]

Le 2010-01-25 à 11:36, Kevin Bealer a écrit :

> Are you also considering multiple levels of priority?  In the reader /
> processor / writer case, I can think of several priorities the processor
> might want:
> 
> 1. Normal -- here is another buffer to write.
> 2. Medium -- the file read failed, stop with this file.
> 3. Medium#2 -- too much information too fast, please stop sending until
> further notice.
> 4. High -- The system is going down, pack up and get out of dodge
> 
> I'm not sure if #2 and #3 need to be at different levels, but it seems like
> 1, (2 or 3), and 4 should be distinct.  E.g. you don't care about either
> Medium if there is a High in the pipe and you want to process Normals after
> any High or Medium is handled.
> 
> This argues for a many-level approach, which might as well be an integer or
> short.

I think only one queue where you can put messages directly on the front is enough. The more priorities you have, the more overhead it adds.

Seriously, why should 4 be distinct from 2? First, they both should be rare enough that it won't make a difference in which order you execute them. And second: what 2 makes the thread do (stopping writing a file) you must do it for 4 anyway, so it's not like you're really wasting time by handling 2 before 4.

And 3 should be handled by setting a maximum capacity for queues. Even if you needed a message for that, you could just make it a priority message too. Stopping sending messages should be pretty quick to handle.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/