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diff --git a/cddl/contrib/dtracetoolkit/Notes/cputimes_notes.txt b/cddl/contrib/dtracetoolkit/Notes/cputimes_notes.txt new file mode 100644 index 000000000000..cdf7ecdac5a6 --- /dev/null +++ b/cddl/contrib/dtracetoolkit/Notes/cputimes_notes.txt @@ -0,0 +1,138 @@ +************************************************************************** +* The following are additional notes on the cputimes command. +* +* $Id: cputimes_notes.txt 44 2007-09-17 07:47:20Z brendan $ +* +* COPYRIGHT: Copyright (c) 2007 Brendan Gregg. +************************************************************************** + + +* How cputimes works + +cputimes measures time consumed by the kernel, idle therads and processes, +by tracking the activity of the schedular. In particular we track on-cpu +and off-cpu events for kernel therads, measuring the timestamps at each event. + + +* Why cputimes? + +If you are interested in how much time processes are consuming, the data +given by "prstat" or "prstat -m" is fine. However there is no easy way to +see kernel consumed time, which is the idea behind cputimes. + + +* What does it mean? + +The output shows categories of threads by the sum of time, in nanoseconds. + +A nanosecond is 10^-9, or 0.000000001 of a second. This program uses +nanoseconds as units, but does not have nanosecond accuracy. It would be +reasonable to assume that this has microsecond accuracy (10^-6), so in +practise ignore the last three digits of the times. + +The sections reported are, + +PROCESSES - the sum of all the process time on the CPU. +KERNEL - the sum of the time spent in the kernel. +IDLE - the time the kernel spent in the idle thread, waiting for some work. + +If your system isn't doing much, then the idle time will be quite large. If +your system is running many applications, then there may be no idle time +at all - instead most of the time appearing under processes. + + +* When is there a problem? + +Expect to see most of the time in processes or idle, depending on how busy +your server is. Seeing a considerable amout of time in kernel would +definately be interesting. + +The kernel generally doesn't use much CPU time, usually less than 5%. +If it were using more, that may indicate heavy activity from an interrupt +thread, or activity caused by DTrace. + +For example, + + # cputimes 1 + 2005 Apr 27 23:49:32, + THREADS TIME (ns) + IDLE 28351679 + KERNEL 436022725 + PROCESS 451304688 + +In this sample the kernel is using a massive amount of the CPUs, around 47%. +This sample was taken during heavy network utilisation, the time consumed +by the TCP/IP and network driver threads (and DTrace). The "intrstat" command +could be used for further analysis of the interrupt threads responsible +for servicing the network interface. + + +* Problems with cputimes + +The way cputimes measures schedular activity turns out to be a lot of work. +There are many scheduling events per second where one thread steps onto a +CPU and another leaves. It turns out that cputimes itself causes some degree +of kernel load. + +Here we run 1 cputimes, + + # cputimes 1 + 2005 May 15 12:00:41, + THREADS TIME (ns) + KERNEL 12621985 + PROCESS 982751579 + 2005 May 15 12:00:42, + THREADS TIME (ns) + KERNEL 12267577 + PROCESS 983513765 + [...] + +Now a second cputimes is run at the same time, + + # cputimes 1 + 2005 May 15 12:02:06, + THREADS TIME (ns) + KERNEL 17366426 + PROCESS 978804165 + 2005 May 15 12:02:07, + THREADS TIME (ns) + KERNEL 17614829 + PROCESS 978671601 + [...] + +And now a third, + + # cputimes 1 + 2005 May 15 12:03:09, + THREADS TIME (ns) + KERNEL 21303089 + PROCESS 974925124 + 2005 May 15 12:03:10, + THREADS TIME (ns) + KERNEL 21222992 + PROCESS 975152727 + [...] + +Each extra cputimes is consuming an extra 4 to 5 ms of the CPU as kernel time. +Around 0.5%. This can be used as an estimate of the kernel load caused by +running cputimes, and a similar strategy could be used to measure the kernel +load of other DTrace scripts. + +However the following CPU characteristics must be taken into consideration, + + # psrinfo -v + Status of virtual processor 0 as of: 05/15/2005 12:06:05 + on-line since 04/30/2005 13:32:32. + The i386 processor operates at 867 MHz, + and has an i387 compatible floating point processor. + +as well as the type of activity that was also running on the system, which +cputimes was monitoring (frequency of scheduling events). + +A system with a slower CPU will use a larger proportion of kernel time to +perform the same tasks. Also, a system that is context switching more +(switching between different processes) is likely to consume more kernel time +as well. + + + |