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<div class="moz-cite-prefix">So I answer this here, and then I get a
visit in the office with the same question... :-)<br>
<br>
On 25/03/14 23:04, Elazar Leibovich wrote:<br>
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<div>(I'm talking now about MONOTONIC_CLOCK_RAW, not taking NTP
adjustment into account)</div>
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To my understanding, the basic time counting mechanism at the
Linux kernel, is the jiffies counter. The way it counts time, is
by leveraging a CPU interrupt happening at a certain known
frequency. Every time this interrupt occurs, the interrupt
handler would increment a counter. By multiplying this counter
with the IRQ's frequency, we can estimate how much time passed.
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<div>Now, to my understanding, the NO_HZ_IDLE=y configuration,
would prevent any interrupt on idle CPUs. <br>
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At least for the time being, this does not mean what you think it
means. See <a class="moz-txt-link-freetext" href="http://lwn.net/Articles/549580/">http://lwn.net/Articles/549580/</a><br>
<br>
Even in full tickless mode (properly referred to as "full" tickless
mode), the boot CPU is still on a counter. Full ticklessness is
still some way away.<br>
<br>
Even when we do achieve that lofty goal, most CPUs have a hardware
counter that counts the time. Just like NAPI for network moved from
an interrupt mode to polling mode for performance's sake, so we can
do here.<br>
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<div>So to my understanding, if all CPUs are idle, nothing is
going to run on any CPU.</div>
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No.<br>
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<div>Who would count jiffies then? How can we be sure how much
time passed with no hardware clock (as it is the case in some
systems), and all CPUs asleep?</div>
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When we finally move into the utopian full tickless mode, jiffies
will lose their original meaning. In full tickless mode, there is no
base frequency for the clock. As such, jiffies become an arbitrary
number based on which you can decide how much time has passed, and
can be set to whatever. How low to set them becomes a question of
power management, then.<br>
<br>
Shachar<br>
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