Messages in this thread | | | Subject | Re: [PATCH] clocksource: document some basic concepts | From | john stultz <> | Date | Mon, 15 Nov 2010 11:48:37 -0800 |
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On Mon, 2010-11-15 at 11:48 +0100, Peter Zijlstra wrote: > On Mon, 2010-11-15 at 11:33 +0100, Linus Walleij wrote: > > +sched_clock() > > +------------- > > + > > +In addition to the clock sources and clock events there is a special weak > > +function in the kernel called sched_clock(). This function shall return the > > +number of nanoseconds since the system was started. An architecture may or > > +may not provide an implementation of sched_clock() on its own. > > + > > +As the name suggests, sched_clock() is used for scheduling the system, > > +determining the absolute timeslice for a certain process in the CFS scheduler > > +for example. It is also used for printk timestamps when you have selected to > > +include time information in printk for things like bootcharts. > > + > > +Compared to clock sources, sched_clock() has to be very fast: it is called > > +much more often, especially by the scheduler. If you have to do trade-offs > > +between accuracy compared to the clock source, you may sacrifice accuracy > > +for speed in sched_clock(). It however require the same basic characteristics > > +as the clock source, i.e. it has to be monotonic. > > Not so, we prefer it be synchronized and monotonic, but we don't require > so, see below. > > > +The sched_clock() function may wrap only on unsigned long long boundaries, > > +i.e. after 64 bits. Since this is a nanosecond value this will mean it wraps > > +after circa 585 years. (For most practical systems this means "never".) > > Currently true, John Stultz was going to look into ammending this by > teaching the kernel/sched_clock.c bits about early wraps (and a way for > architectures to specify this)
I'd like to, although at the moment I don't have much space on my plate to do this, so in the mean time, if someone has time and interest into looking at this, ping me and I can lay out the basics of what likely should be done.
thanks -john
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