On Sun, 15 May 2011, Greg KH wrote:
> On Sun, May 15, 2011 at 12:41:30PM -0400, Robert P. J. Day wrote:
> >
> > given that ubuntu ships with the kernel configured for tickless
> > behaviour, and that RHEL6 also ships tickless, is it safe to say that
> > tickless is now the standard configuration? is there a compelling
> > reason to *not* run tickless with the latest 2.6 kernels?
>
> Yes it is the "standard" and no, there is not any reason to not enable
> it. Unless you like burning extra power for no reason.
ok, good to know. i assume, then, that it's fairly pointless to use
the value of "jiffies" for anything that requires even moderate
accuracy. i was poking around the timer code, and i can see this in
kernel/time/timekeeping.c:
/*
* The 64-bit jiffies value is not atomic - you MUST NOT read it
* without sampling the sequence number in xtime_lock.
* jiffies is defined in the linker script...
*/
void do_timer(unsigned long ticks)
{
jiffies_64 += ticks;
update_wall_time();
calc_global_load(ticks);
}
so obviously jiffies_64 can jump up an arbitrary number of ticks on
any call. i can also see where that routine is called in
kernel/time/tick-sched.c:
static void tick_do_update_jiffies64(ktime_t now)
{
unsigned long ticks = 0;
ktime_t delta;
... snip ...
}
do_timer(++ticks);
etc, etc.
i may read more just to fill out the missing bits, unless there's a
decent online explanation of how jiffies works in the context of
tickless kernels. thanks.
rday
--
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Robert P. J. Day Ottawa, Ontario, CANADA
http://crashcourse.ca
Twitter: http://twitter.com/rpjday
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