From: tim Rowledge tim@rowledge.org Reply-To: The general-purpose Squeak developers listsqueak-dev@lists.squeakfoundation.org To: The general-purpose Squeak developers listsqueak-dev@lists.squeakfoundation.org Subject: Re: Alternative Thread Schedulers Date: Tue, 29 May 2007 13:48:07 -0700
I think you'll find that ProcessScheduler already does this.
I don't understand what you mean by this. What I wanted to try was to create an alternative scheduler but without replacing the existing one (at least until I'm positive it is an improvement).
Look in the MessageName browser for messages contaiing 'wait'. There's a gazillion.
I didn't see anything that would put the process to sleep for the specified amount of time or until the first yield situation (i.e. wake me on either of these conditions).
John gave a very nice write up of how the scheduling works. What I did not see was how to hook into it from Smalltalk. For example, if you call suspend, which calls primitiveSuspend the VM calls transferTo: and wakeHighestPriority but where does this come back into Smalltalk for handling?
What I want to do is from a higher priority process to take total control of the scheduling and try out an event driven style scheduler. To be more specific, I would have, to start say, 6 different priorities: 1 real-time and 5 "normal". Real-time runs any time it wants for as long as it wants, but the rest have a specific quantum. The highest normal priority might have e.g. 20ms or maybe even just 10. The lowest might have 250ms or maybe even 400 (I believe this is what sun gave their lowest priorties). Of course tunning would be required with this.
If no real-time processes want the CPU then the highest normal process is run with something like:
nextProcess resume. "I'm assuming this doesn't start him until I, the high high prio process sleep" self waitMilliseconds: (Process quantumsFor: nextProcess priority).
Then if the process yields the CPU in some way it goes up in priority. If it uses the entire quantum then it is dropped in priority. But for this to work I would need my new scheduler class to get involved any time there was some kind of context switch. It can't happen that when the wait is over my process comes alive to find a different process is current then what he expects.
So what's the point?
Well, I see people on the list always saying to use Comanche as little as possible because it is too slow, but I wonder why this is the case. Yaws (the Erlang web server, which spawns one new process per connection) beats Apache quite handily, I wonder why Comanche couldn't do the same.
I read on the Wiki that Comanche also forks a new process for each new connection, but from what I could tell it looks like the fork is at the same priority of the server. So what this would mean is if 30 clients connect right after each other, 29 fast clients and one that requires a lot of processing then the clients will connect and be serviced until the 1 long one hits, then the rest have to wait for him to finish.
With the event driven scheduler described above each thread would be started in the highest priority with the lowest quantum (since the server spends the vast majority of his life sleeping he will quickly be promoted to top normal priority). So the first quick processes connect, make their request and yield before using their quantum. The long process would run out his entire 10 or 20ms quantum and get demoted, allowing all the rest of the process to come in and be processed before he runs again.
Now the long process would actually take longer in this case, but more different clients are serviced in the same time making the server at least appear more responsive.
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