Article: Llano Hot Chips Update

Article: Llano at Hot Chips
By: iz (i.delete@this.z.x), September 1, 2011 2:07 am
Room: Moderated Discussions
David Kanter ( on 8/31/11 wrote:
>As always, we encourage feedback and discussions.

The article mentions:

A large frequency range is essential for the DVFS and good performance for consumer products. However it is undesirable for the frequency to be very coarse-grained without smaller intermediate steps. In many lightly loaded scenarios the CPU cores will often switch back to the base frequency to keep under the power and thermal limits. These frequency adjustments cost both performance and power, so it is better to have a variety of steady state frequencies available and avoid transitions. For example, 5-10% frequency steps would yield much better results.

But I don't think that is actually very true. Looking at
my laptop, which has fine grained control between 600MHz
and 1400MHz, the cpu ends up at 1400MHz for 22% and at 600
for 77%. CPU loads are very spiky in nature. The transition
latencies are low enough that a 50% CPU load looks like an
alternating sequence of 0% and 100% CPU loads.

Only thing that makes sense for fine grained frequency
control is when the CPU does it automatically when waiting
for memory to catch up. But OS/BIOS-level frequency control
has no use for fine grained intermediate steps, because
going straight to max speed and back to sleep saves more
power and has better performance. Power saving between
different frequency steps is too small and the latencies
for changing between the steps too high to keep up with
the machine load.

The only useful case I can think of is when a lot of IO
is happening which doesn't need much CPU processing time,
but causes interrupts often enough that going to sleep
would add too much latency. Or other, more predictable
steady-state loads. But those seem likely to show up at
server loads.
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TopicPosted ByDate
Article: Llano Hot Chips UpdateDavid Kanter2011/08/31 12:32 PM
  Article: Llano Hot Chips UpdateMS2011/08/31 01:22 PM
    Article: Llano Hot Chips UpdateDavid Kanter2011/08/31 06:07 PM
      Article: Llano Hot Chips UpdateMS2011/09/01 08:16 AM
        Article: Llano Hot Chips UpdateDavid Kanter2011/09/01 01:52 PM
    Article: Llano Hot Chips UpdateGionatan Danti2011/09/01 02:53 AM
      Links are okDavid Kanter2011/09/01 04:02 AM
  Article: Llano Hot Chips Updateiz2011/09/01 02:07 AM
    Article: Llano Hot Chips UpdateGionatan Danti2011/09/01 03:01 AM
    Article: Llano Hot Chips UpdateDavid Kanter2011/09/01 04:08 AM
      Article: Llano Hot Chips Updateiz2011/09/02 01:04 AM
  Article: Llano Hot Chips Updateiz2011/09/01 02:08 AM
  More fusion, how?Moritz2011/09/04 12:33 AM
    More fusion, how?Rohit2011/09/04 09:27 AM
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