Straight from the Core M Datasheet:
"When operating in turbo mode, the processor monitors its own power and adjusts the turbo frequencies to maintain the average power within limits over a thermally significant time period."
"Turbo Time Parameter: An averaging constant used for PL1 exponential weighted moving average power calculation"
If PL3 runs significant amount of time in order to affect longer-running benchmark times, it would be reflected in power use, and thus thermals. Regardless of whether its running at PL2 or PL3 it all needs to be not exceeding TDP, or PL1. Therefore its inaccurate to say its a "15W CPU" when the TDP is at 4.5W, since it'll have to go back down there anyway. Now Intel might be lying through their teeth and it really goes over set TDP limits but if they act according to their own Datasheets what you described can't happen and its irrelevant.
Sure there is!
Even at PL2 there's a maximum duration it can stay at that time before it has to come BACK DOWN. You can cool it using LN2 and connect to a 5000W power supply, but after some time it'll have to go back down to PL1.
And again, Haswell has PL3 too. In that case, with Core M, it has TDP of 4.5/6W and PL3 of 15W. That means Haswell Y is at 11.5W TDP and 40-60W PL3. So what's your point?
First off all, lets understand what TDP is and that
TDP is configurable in Core-M.
From Intel Core-M 14nm Datasheet
http://www.intel.com/content/www/us/en/processors/core/core-m-processor-family-datasheet-vol-1.html
5.0 Thermal Management
The thermal solution provides both component-level and system-level thermal
management. To allow for the optimal operation and long-term reliability of Intel
processor-based systems, the system/processor thermal solution should be designed
so that the processor:
• Remains below the maximum junction temperature (Tj
Max
) specification at the
maximum thermal design power (TDP).
• Conforms to system constraints, such as system acoustics, system skin-
temperatures, and exhaust-temperature requirements.
Caution:
Thermal specifications given in this chapter are on the component and
package level and apply specifically to the processor. Operating the processor outside
the specified limits may result in permanent damage to the processor and potentially
other components in the system.
5.1 Thermal Considerations
The processor TDP is the maximum sustained power that should be used for design of
the processor thermal solution. TDP is a power dissipation and junction temperature
operating condition limit, specified in this document, that is validated during
manufacturing for the base configuration when executing a near worst case
commercially available workload as specified by Intel for the SKU segment. TDP may
be exceeded for short periods of time or if running a "power virus" workload.
The processor integrates multiple processing and graphics cores and PCH on a single
package.This may result in differences in the power distribution across the die and
must be considered when designing the thermal solution.
Intel
®
Turbo Boost Technology 2.0 allows processor cores and processor graphics
cores to run faster than the guaranteed frequency. It is invoked opportunistically and
automatically as long as the processor is conforming to its temperature, power
delivery, and current specification limits. When Intel Turbo Boost Technology 2.0 is
enabled:
• Applications are expected to run closer to TDP more often as the processor will
attempt to maximize performance by taking advantage of available TDP headroom
in the processor package.
• The processor may exceed the TDP for short durations to use any available
thermal capacitance within the thermal solution. The duration and time of such
operation can be limited by platform runtime configurable registers within the
processor.
• Thermal solutions and platform cooling that are designed to less than thermal
design guidance may experience thermal and performance issues since more
applications will tend to run at or near TDP for significant periods of time.
Note:
Intel Turbo Boost Technology 2.0 availability may vary between the different SKUs.
1. TDP is the maximum sustained power that should be used for design of
the processor thermal solution. TDP is a power dissipation and junction temperature operating condition limit.
2. The processor may exceed the TDP for short durations to use any available
thermal capacitance within the thermal solution. The duration and time of such
operation can be limited by platform (a)runtime configurable registers within the
processor.
2(a) refers to PLs
Now lets have a look at the PLs
5.3.1
Package Power Control
The package power control settings of PL1, PL2, and PL3 Tau allow the designer to
configure Intel Turbo Boost Technology 2.0 to match the platform power delivery and
package thermal solution limitations.
3. PL1, PL2 and PL3
can be adjusted by the designer
to match platform power delivery and thermal solution limitations.
5.3.1 (continue)
• Power Limit 1 (PL1): A threshold for average power that will not exceed -
recommend to set to equal TDP power. PL1 should not be set higher than thermal
solution cooling limits.
• Power Limit 2 (PL2): A threshold that if exceeded, the PL2 rapid power limiting
algorithms will attempt to limit the spike above PL2.
• Power Limit 3 (PL3): A threshold that if exceeded, the PL3 rapid power limiting
algorithms will attempt to limit the duty cycle of spikes above PL3 by reactively
limiting frequency. This is an optional setting
• Turbo Time Parameter (Tau): An averaging constant used for PL1 exponential
weighted moving average (EWMA) power calculation.
Notes:
• Implementation of Intel
®
Turbo Boost Technology 2.0 only requires configuring
PL1, PL1 Tau and PL2.
• See the Turbo Implementation guide and BIOS Writers Guide (BWG) for additional
details on use in your system (see related documents section).
• PL3 is disabled by default.
4: PL3 :
A threshold that if exceeded the PL3 algorithm will attempt to limit the duty cycle of
spikes above PL3 by reactively limiting frequency (throttle down)
5.3.2 Turbo Time Parameter
Turbo Time Parameter is a mathematical parameter (units in seconds) that controls
the Intel Turbo Boost Technology 2.0 algorithm using moving average of energy
usage. During a maximum power turbo event of about 1.25 x TDP, the processor
could sustain PL2 for up to approximately 1.5 times the Turbo Time Parameter. If the
power value and/or Turbo Time Parameter is changed during runtime, it may take
approximately 3 to 5 times the Turbo Time Parameter for the algorithm to settle at the
new control limits. The time varies depending on the magnitude of the change and
other factors. There is an individual Turbo Time Parameter associated with Package
Power Control.
----------------
Now, lets see the back plate of that Tablet again. Remember that back plate acts as a heat-sink.
Quote from the tt article
This device is actually cooled through its back plate.
The backplate is high grade aluminum and acts as a giant heat sink.
-------------
Now, from
1,2,3,4
The processor will be able to overcome TDP and even
spike OVER PL3, then it will throttle down.
It can operate at PL3 for up to 10ms
if the Thermal and current limits are not breached.
Then it will revert to lower PL2 until thermal or current limits are lowered. Then it can again reach PL3 for another up to 10ms, if thermal and current limits allow it.
Then it will revert to PL2 until thermal or current limits are lowered again so it can revert to PL3 again.
The time that it will be able to revert to PL3 state(that is for how many times it will be able to go to PL3 then PL2 then PL3 etc) depends on the Heat-sink and Ambient temperature.
With that Aluminum Backplate the lower the ambient temperature the higher the ability to operate at PL3 for more consecutive time. Also, that Aluminum backplate has higher thermal capacity allowing the processor to reach PL3 state for extended times.
ps: I never said the CPU is 15W TDP. But with that heat-sink it will allow it to sustain long periods of PL3 and PL2.