Intel introduced a handful of new,
lower-power Y-series Ivy Bridge CPUs designed to fit into thinner and lighter
Ultrabooks and tablets. The slides in Intel's keynote called these "7
watt" Ivy Bridge CPUs, and the company compared them directly to the 17
watt U-series chips in wide use today.
The actual thermal design power (TDP) of those processors is in fact 13 watts—still lower than before, but less
miraculous than had previously been implied. A new measurement, scenario design
power (SDP), was actually being used to achieve that 7 watt figure.
There was one particular element of that
write-up not entirely accurate: based on our conversations with Intel reps, we
thought SDP was purely a marketing ploy, a measurement of the amount of power
the processor would use on average. It turns out there is an element of
marketing to these new 7 watt CPUs, but there's a technical element, too—Intel
is simply giving a name to and publicizing a measurement previously left
behind-the-scenes. We talked with an Intel engineer to get a better
explanation.
TDP explained
To fully understand what's going on here, you
need to understand TDP. In Intel's case, a specified chip's TDP has less to do
with the amount of power a chip needs to use (or can use) and more to do with
the amount of power the computer's fan and heatsink needs to be able to
dissipate while the chip is under sustained load. Actual power usage can be
higher or (much) lower than TDP, but the figure is intended to give guidance to
engineers designing cooling solutions for their products.
In laptops especially, balancing power usage
and performance is all about sliding clock speeds, active CPU cores, and other
values up and down dynamically depending on what the computer is doing. Older mobile
processors could simply ramp their clock speeds down when idle to save power,
but newer Turbo Boost-equipped CPUs can actually exceed their stated clock
speeds (and their TDP) for a short amount of time. The idea is that the sooner
the CPU can get its work done, the sooner it can go back into a power-sipping
idle state. If the CPU needs to work hard for an extended period of time and
the laptop gets warmer, it will slowly ramp down its speed until it's operating
at its stated TDP.
There are two OEM-configurable "power
level" states that define how quick the CPU can be in these situations:
PL2 tells the processor how much power it's allowed to use when it needs a
short burst of speed, and PL1 defines how quickly the processor can run under sustained
load. Both of these PL states are given in terms of watts, and generally
speaking PL1 is set to the CPU's maximum TDP value.
Here's where we start to get closer to what
Intel is doing with its Y-series CPUs: for its U-series 17 watt TDP CPUs, Intel
actually tests them and validates them for usage at three different PL1 values:
17 watts (the max TDP), 20 watts, and 14 watts. The latter two values aren't
officially defined anywhere on the CPUs' official product pages, but they're
there for OEMs whose cooling solutions either exceed or undershoot the TDP
value.
Under what Intel considers to be
"normal" workloads—short bursts of activity followed by a relatively
prompt return to an idle state—the same CPU will perform at exactly the same
level regardless of the PL1 value set by the OEM. It's under sustained heavy
workloads—things like gaming, video and photo editing, crunching big databases,
and so on—that the PL1 value makes a difference. Depending on what it's set at,
you might have two different laptops using the same processor that actually
perform differently under load.
SDP: Taking it one
step further
This is at the heart of what Intel is doing
with the Y-series processors: their maximum TDP has been lowered four watts,
from 17 to 13. Intel is also validating them for use at two lower PL1 values:
10 watts and 7 watts. This is where the marketing we discussed earlier comes
in—rather than keeping these values under the covers as it has so far been
content to do, Intel has taken that lowest value, put it on its product pages,
and called it SDP.
Intel told me that these SDP values are only
intended for use with current and future Y-series processors. U-series,
M-series, and desktop class processors will continue to use Intel's TDP
ratings, at least for the foreseeable future, and while they may be validated
for use at lower and higher values it won't be a core part of those CPUs'
specifications as it is with the Y-series chips.
For systems that do use these processors,
they will enable thinner and longer-lasting laptops and tablets, but they make
it more difficult to tell exactly how fast a given PC will be when under
sustained load. Short bursts of CPU activity will run equally quickly on a Core
i5-3339Y whether the PC OEM sets its PL1 value to the SDP of 7 watts or the
full TDP of 13 watts, but for long-running CPU-heavy activities like gaming and
video encoding there's a chance that two different PCs running the same
processor may perform the same task at different speeds. The near-50-percent
drop in TDP is also likely to cause a much more noticeable performance hit than
a 17 watt processor with a PL1 value set to 14 watts.
Is any of this deal-breaking? Almost
certainly not—even when running at the 7 watt PL1 value, the systems will still
be much quicker than anything based on Atom and ARM would be. The increased
efficiency will also enable better performance and battery life to even smaller
and slimmer devices than we currently have. Even if OEMs stick to the max TDP
of 13 watts, you're still saving power over the older 17 watt processors while
maintaining performance similar to that found in Sandy Bridge-era Ultrabooks.
This is a testament to both the increasing maturity of Intel's 22nm
manufacturing process and processor binning, which Intel assured me it was
doing in order to find the most power-efficient processors.
It also helps the tasks that will expose
speed discrepancies between systems with disparate PL1 settings are usually
best left to beefier systems. Serious gamers, professional video editors, and
others to whom top-flight performance is vital will likely want to have larger
but faster laptops and desktops to speed those tasks along.
What these processors do unfortunately add to the mix is
another value the technically savvy need to be aware of when they buy their
systems—one that we'll need to be particularly vigilant about in reviews of PCs
that use them. Intel isn't being specific about the clock speeds or performance
levels we can expect from PCs with differing PL1 values. Instead, we were told,
essentially, that we would need to benchmark systems as we received them and
find this out ourselves. Especially in PCs using Y-series processors, Ars will
be careful to do this in our reviews going forward. [Arstechnica]
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