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Re: Scheduler

	How is the priority of process  decided for process running in SCHED_FIFO & SCHED_RR.
	Thanks for helping me.

On Wed, Feb 14, 2001 at 11:17:16PM -0600, Kondaiah (IE10) wrote:
> Hi folks,
> The Schduler scans the list of process in  the list of process in the
> 'ready'
> state, and uses several criteria to choose which process to execute.The
> linux scheduler has three different scheduling policies:one for 'normal'
> processes, two for 'real time' processes.
> Each process has an associated process type, a fixed priority, and a
> variable priority.The process types can be:
> 	SCHED_FIFO	 for an a unalterable 'real time' process.
> 	SCHED_RR	 for an alterable 'real time' process.
> 	SCHED_OTHER for a 'classical' process.
> The scheduler policy depends upon the type of the processes in the'ready'
> list.
> When a process of the type SCHED_FIFO becomes ready,it is executed
> immediatel.The scheduler nominates the process with highest priority and
> executes it.This process is normally pre-emptible,that is , it has the
> processor resources, and the system will not interrupt its execution except
> in three situations:
> 1.Another process of the type SCHED_FIFO having higher priority becomes
> ready,and is then executed,
> 2.The process becomes suspended while waiting for an event, such as a input
> or output.
> 3.The process voluntarily gives up the processor following a call to the
> sched_yield.The process the passes the 'ready'  state,and other processor
> are executed.
> .When a process of the type SCHED_RR becomes ready, it is executed
> immediately.Its behavior is thus 
> similar to SCHED_FIFO, with one exception: when the process is executed, a
> time sot is attributed to it.When this time period has elapsed, processes of
> the types SCHED_FIFO OR SCHED_RR and having an equal or higher priority to
> the current process, may be selected and executed.
> Process of the type SCHED_OTHER can only be executed when no 'real time'
> processes in the ready state exist. The process to be excutedis then
> selected  after considering the dynamic priorities.The dynamic priority of
> the process is decidedpartly the level specified by the usersystem calls
> 'nice' and setpriority, and partly by a factor calculated by the
> system.Inorder to take of aging activity, the priority of a processes which
> take several clock cycles to execute diminishes during execution and it may
> become of lower priority than processes which are not executing and whose
> priority is not modified.
> process->counter=(procee->conuter/Int value)+nice_priority specifed by the
> user.
> Implementation of scheduler can be read at kernel/sched.c.
> Thanks
> Kondaiah Dasari
> > -----Original Message-----
> > From:	Sambasiva Rao.B [SMTP:samba@xxxxxxxxxxxxxx]
> > Sent:	Tuesday, April 15, 1997 8:33 PM
> > To:	linux-india-programmers@xxxxxxxxxxxxxxxxxxxxx
> > Subject:	Re: [LIP] Scheduler
> > 
> > 
> > On Tue, 13 Feb 2001 jk76_in@xxxxxxxxx wrote:
> > 
> > > Hi,
> > > 	Can any one explain how scheduling is done in linux
> > 
> > 	Each proceess was given priority(p_priority) based on user
> > priority. For every clock cycle, the counter value decremented for the
> > current running process. When counter reaches to zero swtch() called. To
> > take care of aging, the prority is computed as
> > 	p->counter = (p->counter/2) + p_nice
> > 
> > 	The process having max(p->counter) will be scheduled.
> > 
> > 	It is more efficint than BSD implementation
> > 
> > --samba
> > 
> > 
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