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Top Forums UNIX for Advanced & Expert Users Causes of high runq-sz and cswch/s output from sar Post 302241909 by jim mcnamara on Tuesday 30th of September 2008 02:13:44 PM
Old 09-30-2008
I/O may cause a process to lose current context. ie. switch context.

What I/O scheduler are you using: AS, CFQ?

If your %iowait is high try increasing quantum.

It is possible to see 100% iowait. What iowait really measures is the percent of time at least one process is in an iowait state. I would bet your system shows a high values during the context switch rush.

cfq is able to dedicate a time slice to each process that uses a block device. You can adjust the time slice. Consider looking into that.

Since you are not incurring a cpu hit, all you are doing is increasing wasted cpu time in context switches. Rather than something more useful. IMO.
 

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tapset::scheduler(3stap)												  tapset::scheduler(3stap)

NAME
tapset::scheduler - systemtap scheduler tapset DESCRIPTION
scheduler.cpu_off Process is about to stop running on a cpu See probe::scheduler.cpu_off(3stap) for details. scheduler.cpu_on Process is beginning execution on a cpu See probe::scheduler.cpu_on(3stap) for details. scheduler.tick Schedulers internal tick, a processes timeslice accounting is updated See probe::scheduler.tick(3stap) for details. scheduler.balance A cpu attempting to find more work. See probe::scheduler.balance(3stap) for details. scheduler.ctxswitch A context switch is occuring. See probe::scheduler.ctxswitch(3stap) for details. scheduler.kthread_stop A thread created by kthread_create is being stopped See probe::scheduler.kthread_stop(3stap) for details. scheduler.kthread_stop.return A kthread is stopped and gets the return value See probe::scheduler.kthread_stop.return(3stap) for details. scheduler.wait_task Waiting on a task to unschedule (become inactive) See probe::scheduler.wait_task(3stap) for details. scheduler.wakeup Task is woken up See probe::scheduler.wakeup(3stap) for details. scheduler.wakeup_new Newly created task is woken up for the first time See probe::scheduler.wakeup_new(3stap) for details. scheduler.migrate Task migrating across cpus See probe::scheduler.migrate(3stap) for details. scheduler.process_free Scheduler freeing a data structure for a process See probe::scheduler.process_free(3stap) for details. scheduler.process_exit Process exiting See probe::scheduler.process_exit(3stap) for details. scheduler.process_wait Scheduler starting to wait on a process See probe::scheduler.process_wait(3stap) for details. scheduler.process_fork Process forked See probe::scheduler.process_fork(3stap) for details. scheduler.signal_send Sending a signal See probe::scheduler.signal_send(3stap) for details. SEE ALSO
probe::scheduler.cpu_off(3stap), probe::scheduler.cpu_on(3stap), probe::scheduler.tick(3stap), probe::scheduler.balance(3stap), probe::scheduler.ctxswitch(3stap), probe::scheduler.kthread_stop(3stap), probe::scheduler.kthread_stop.return(3stap), probe::scheduler.wait_task(3stap), probe::scheduler.wakeup(3stap), probe::scheduler.wakeup_new(3stap), probe::scheduler.migrate(3stap), probe::scheduler.process_free(3stap), probe::scheduler.process_exit(3stap), probe::scheduler.process_wait(3stap), probe::scheduler.process_fork(3stap), probe::scheduler.signal_send(3stap), stap(1), stapprobes(3stap) IBM
--- tapset::scheduler(3stap)
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