thread context switches: detection, prevention Post: 302177806

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Top Forums UNIX for Advanced & Expert Users thread context switches: detection, prevention Post 302177806 by fabulous2 on Sunday 23rd of March 2008 11:35:56 AM

03-23-2008

Registered User

Quote:

Originally Posted by jim mcnamara

Raise the base priority of the process.

Use realtime processing in sched.h if your system has it. I do not know if java supports this or not.

Thanks for the pointer about sched.h.

According to The Linux Kernel Archives the latest stable version of the Linux kernel is 2.6.24.3. Unfortunately, I was not able to find an online listing for sched.h for that kernel version. If anyone knows of a good source for this stuff lemme know. (Altho maybe it does not matter: while you will get 2.6.24 with, say, ubuntu 8.04 when released, currently you get 2.6.2 with ubuntu 7.10) The closest version to it that I found was linux-2.6.4/include/linux/sched.h

That version has a few occurences of "real", but none seemed to be relevant. Feel free to point out anything that I may have missed.

I have never done any kernel hacking before. If I modify sched.h, would I then have to recompile the kernel and somehow reboot into the new version or something?

Let me ask a general question. Suppose that you have a multi-core CPU. Does a single process (or thread?)--in linus, solaris, windows--get the entire CPU at once, or can multiple processes/threads simultaneously be on the CPU?

I would have thought the latter is true, but when I began thinking about how a context switch is done, which includes saving all the CPU registers, it seems like it might be the case that just a single process/thread should occupy the CPU at once, else it might be difficult/impossible to know which registers belong to what? Or is this a non-issue? I would love to be educated.

Also, is the linux interrupt system responsible for periodic involuntary process contexct switches? According to p.5 of
Chapter 7: Scheduling
there is the claim that

Quote:

An interrupt occurs every k clock ticks, thus causing the hardware clock's con-troller to execute the logical equivalent of an yield() to invoke the interrupt
handler. The interrupt handler invokes the scheduler to reschedule the CPU.

How often do interrupts in linux occur? Is there any way to disable this, or at least prevent my hot thread from being context switched out of its core when an interrupt occurs?

fabulous2

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LEARN ABOUT DEBIAN

sched_4bsd

SCHED_4BSD(4)						   BSD Kernel Interfaces Manual 					     SCHED_4BSD(4)

NAME

     sched_4bsd -- 4.4BSD scheduler

SYNOPSIS

     options SCHED_4BSD

DESCRIPTION

     The sched_4bsd scheduler is the traditional system scheduler, providing both high throughput and solid interactive response in the presence
     of load.

     The following sysctls are relevant to the operation of sched_4bsd:

     kern.sched.name
	     This read-only sysctl reports the name of the active scheduler.

     kern.sched.quantum
	     This read-write sysctl reports or sets the length of the quantum (in micro-seconds) granted to a thread.

     kern.sched.ipiwakeup.enabled
	     This read-write sysctl sets whether or not the scheduler will generate an inter-processor interrupt (IPI) to an idle CPU when a
	     thread is woken up.  Otherwise, idle CPUs will wait until the next clock tick before looking for new work.

     kern.sched.preemption
	     This read-only sysctl reports whether or not the kernel is configured to support preemption, which reduces the latency to run lower
	     priority threads on wakeup.

     Some sysctls will be available only on systems supporting SMP.

SEE ALSO

     sched_ule(4), sysctl(8)

HISTORY

     The sched_4bsd scheduler has been present, in various forms, since the inception of BSD.

BUGS

     While a highly robust and time-tested scheduler, sched_4bsd lacks specific knowledge of how to schedule advantageously in non-symmetric pro-
     cessor configurations, such as hyper-threading.

BSD
								 January 21, 2008							       BSD