Messages in this thread | | | Subject | Re: [PATCH 11/32] nohz/cpuset: Don't turn off the tick if rcu needs it | From | Steven Rostedt <> | Date | Tue, 27 Mar 2012 21:35:35 -0400 |
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On Tue, 2012-03-27 at 20:19 -0500, Christoph Lameter wrote: > On Tue, 27 Mar 2012, Steven Rostedt wrote: > > > > Obviously compiling a kernel with preemptiong introduces additional > > > overhead to guarantee more deterministic behavior. Additional overhead > > > increases latencies generated by the OS in general. Compile a kernel > > > without preemption and it will run faster and thus have lower latencies. > > > > I call that "lower overhead". > > Good marketing but it does not change the facts.
I see we are mixing the paint for the bike shed.
> > > "Latency is a measure of time delay experienced in a system, the precise > > definition of which depends on the system and the time being measured. > > Latencies may have different meaning in different contexts." > > > > That last sentence is key. So lets avoid the term "latency" as it > > obviously has a different meaning to the both of us. > > > > Instead, lets use "determinism" (what we call latency in the realtime > > world) and "overhead" (what you seem to see as latency caused by the > > kernel). > > I sure wish you would be using the term determinism instead of "latency". > > Overhead causes latency and the definition that you quoted is what I am > talking about. Latencies are the delays in processing experienced by the > application through the speed of system calls and by interruptions of > a user space process by the kernel for various reasons.
I could also argue that a non-preempt kernel has a large latency as well. Although it may have good through put for one task, another task may suffer from a large latency waiting for a lower priority task to get out of a system call.
You say tomAYto I say tomAHto.
Read the article: http://en.wikipedia.org/wiki/Latency_%28engineering%29
Especially the section about: Computer hardware and operating system latency
You'll see that it describes latency much closer to my definition than yours.
Heck, google "operating system latency" and you'll see a lot of talk about reaction times and how fast the hardware can do its job. I don't see anything about the time a system call takes.
-- Steve
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