Re: scheduler/SCHED_FIFO behaviour

[Arun Srinivas]

The 2 processes which I  am measuring  are parent and child processes.I want 
both of'em to be scheduled at the same time.My code simply does the 
following:

1) From main(i.e., parent) create a shared memory seg. using shmget() and 
shmat(). This is for communication between parent and child. I am trying to 
use this as a locking mechanism to make them tightly coupled so that one 
does not race before the other.
2) create child by fork() and call shmat() to attach this segment to child 
too
3) In parent and child call ioctl() to pass their PID's from user space to 
kernel space...so that I can measure when the particular PID's are scheduled 
in the scheduler

I suppose shmget() dosent use a system call.So still confused about the 
occasional resechedule behavior.



> From: Steven Rostedt <rostedt@xxxxxxxxxxx>
> To: Arun Srinivas <getarunsri@xxxxxxxxxxx>
> CC: linux-kernel@xxxxxxxxxxxxxxx
> Subject: Re: scheduler/SCHED_FIFO behaviour
> Date: Sun, 01 May 2005 11:51:25 -0400
>
> On Sun, 2005-05-01 at 07:36 +0530, Arun Srinivas wrote:
> > hi
> >
> >   I spkoe to you some days ago regarding scheduling two processes 
> together
> > on a HT.As I told you before I run them as SCHED_FIFO processes.I 
> understood
> > the theory you told me in your previous reply as to why both of 
> SCHED_FIFO
> > processes get scheduled only once and then run till completion.
> >
> > But, sometimes a see a occasional reschedulei.e., the 2 processes get
> > scheduled one more time after they are scheduled for the 1st time. I ran 
> my
> > code 100 times and observed this behavior 8 out of  100 times. What 
> could be
> > the reason?
> > (As I said i want my 2 processes to run together without any reschedule
> > after they are scheduled for the first time).
>
>  The only way a real time priority process of SCHED_FIFO gets
> rescheduled, is if the process voluntarily calls schedule (you call a
> system call that calls schedule), or a higher priority process gets
> scheduled.  I don't know what your program is doing, or what priorities
> that they are running with to know if something like this has occurred.
>
> Also, if the programs you are running haven't been locked into memory
> (they exist partially on the hard drive still), then it will take time
> to map the code into memory when that code is called, and a schedule
> will occur then as well.
>
> -- Steve

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