scheduler (sched_4bsd) questions

[Julian Elischer]

>
>
>On Sunday 19 September 2004 03:10 am, Julian Elischer wrote:
>  
>
>>John Baldwin wrote:
>>    
>>
>>>On Saturday 18 September 2004 01:42 pm, Stephan Uphoff wrote:
>>>      
>>>
>>>>On Fri, 2004-09-17 at 21:20, Julian Elischer wrote:
>>>>        
>>>>
>>>>>Stephan Uphoff wrote:
>>>>>          
>>>>>
>>>>>>If this is true kernel threads can be preempted while holding
>>>>>>for example the root vnode lock (or other important kernel
>>>>>>resources) while not getting a chance to run until there are no more
>>>>>>user processes with better priority.
>>>>>>            
>>>>>>
>>>>>This is also true,  though it is a slightly more complicated thing than
>>>>>that.
>>>>>Preempting threads are usually interrupt threads and are thus usually
>>>>>short lived,.
>>>>>          
>>>>>
>>>>But interrupt threads often wake up other threads ...
>>>>        
>>>>
>>>That are lower priority and thus won't be preempted to.  Instead, they
>>>run when the interrupt thread goes back to sleep after it finishes.
>>>      
>>>
>>though that may still be before the original preempted thread gets run..
>>
>>that reminds me..
>>John, we should add a flag to setrunqueue to add a preempted thread back at
>>the FRONT of it's runqueue.. So that it gets a chance to use the rest of
>>its slot..
>>    
>>
>
>The scheduler can already detect this by noting that curthread is being passed 
>to sched_add() when it has quantum left and then put it at the head of the 
>queue for its priority but only with the remaining quanta.  This only really 
>works for schedulers that actually track quanta, i.e., not 4BSD.  Given that, 
>I think the scheduler is free to implement this how it chooses and doesn't 
>require another flag to setrunqueue().
>

Just for kicks I implemented this in my p4 branch to see if it would help the problem
with atapi cdroms failing to work when premption is turned on..

but it didn't help..

(it's a very strange problem.. turning on preemption makes my test machine's cdrom
time out so much in boot that it takes 5 minutes to probe during boot.)