Hi :) On Mon, Sep 26, 2011 at 09:16, Parmenides <mobile.parmenides@xxxxxxxxx> wrote: > Hi, > > It seems that Linux's scheduler tends to allocate longer timeslice > for processes with higher priority. "tends".....yes, at least theoritically..... but not really in every cases in reality.... >Actually, the CFS scheduler which > is a new scheduler in Linux kernel also does the same thing. But, I > think this way does not fit with scheduler's principle. remember the keyword you ask? "fairness"? that is being fair to all processes.... but since, there are always more processes than processors, unfairness always happen..... > The goal chased by a scheduler is low latency and high thoughput. > Normally, a I/O-bound process has higher priority, while a CPU-bound > process has lower priority. you are referring above as dynamic priority, I believe. As for static priority, quite likely, all I/O and CPU bound processes start at static prio level 0 > So, a I/O-bound process (which has enough > timeslice) can preempt a CPU-bound process easily. yes, because I/O bound one sleep quite a long time and its dynamic priority gets a bonus. But if it sleeps not quite long, the bonus won't be too significant to "defeat" current running one, so it has to wait longer to be run by processor. >This way ensures > lower latency. It is also necessary that CPU-bound processes are to be > allocated longer timeslice to improve throughput owing to less process > switch costs. That means lower priority processes (CPU-bound) should > be allocated longer timeslice, whichs obviously conflicts with the > actual practice taken by the Linux's scheduler. Any explanation? What you refer initially is the time when time slice assignment is strictly derived from the static/nice level. So e.g process with nice level 0 has lesser time slice that nice level -5. But as you can see, situation change dynamically during run time, thus static prio must be taken into dynamic priority. And dynamic priority itself, must take another factor for time slice calculation. Here, sleep time comes into play. -- regards, Mulyadi Santosa Freelance Linux trainer and consultant blog: the-hydra.blogspot.com training: mulyaditraining.blogspot.com _______________________________________________ Kernelnewbies mailing list Kernelnewbies@xxxxxxxxxxxxxxxxx http://lists.kernelnewbies.org/mailman/listinfo/kernelnewbies
