Understanding TDP with unRaid

[Kurkoko]

Hello,

 

I'm thinking about upgrade the server to use also with a gamin vm. 

 

I'm not sure about TDP becouse in my understanding some CPU are 35w,65w,95w,... And GPU also have its own wattage.

 

-Does 2 diferent CPU like 8700k and 8700T will idle at same watts?

-Just when needed the CPU will raise cores power to maximum, and then depending on cpu tdp Will be more or less green?

-When a VM is idle or off does the GPU use 0watts? Maybe 10watts for a 1080ti?

 

Thanks and sorry for my english

[Iciclebar]

Hey Kurkoko,

 

Just a heads up, TDP is not power draw.  The TDP is a measure of how much cooling a processor will need to run at it's rated speed.  So if the processor is 3.5 GHz without turbo boost and is listed at 65w TDP, the cooler / enclosure needs to be able to dissipate 65w of thermal energy to keep the processor cooled enough to not throttle at its rated speed.  This is a slight overestimation because some thermal energy is lost to the board and socket as well.  

 

Anandtech has an article on explaining TDP if you are further interested.

TDP Article

 

Quote

-Does 2 diferent CPU like 8700k and 8700T will idle at same watts?

8700k and 8700T will probably not idle at the same wattage becuase they do not run at the same speeds base or boost.  Depending on what the motherboard manufacturer specifies for power values (more information from that article on TDP) the power draws can vary wildly.  As a ruie though, the 8700k should have more performance while turboing more aggressively, while the 8700t should draw less power and turbo less aggressively.

 

Quote

-Just when needed the CPU will raise cores power to maximum, and then depending on cpu tdp Will be more or less green?

It will turbo up when the cpu has load.  However it depends on several factors as to how "green" it is.  It depends on the processor TDP Rating (PL1), the processor power limit (PL2), the length of time the processor is allowed to stay at boost clock (Tau).  Board manufacturers can mess with these limits as they are stored in the firmware and the processors basically turbo the entire time.  For instance, if they set PL2 and TAU to something ridiculous the processor will never throttle based on power draw, and it will boost practically forever as long as the temperature doesn't reach the limits on the TCase sensor and force it to slow down based on thermals.  This is why the 9900k can draw 180+ watts even though its a "95w" processor.  Someone had just mentioned a review on a supermicro board that adheared to the intel recommendations strictly and inside that power envelope the 9900k is a lot closer to the 8700k performance wise in that situation.  GamersNexus and Hardware unboxed both did videos on power/tdp violations on intel processors pretty recently.

 

Quote

-When a VM is idle or off does the GPU use 0watts? Maybe 10watts for a 1080ti?

The GPU still uses power even when the VM is turned off but depending on the card the idle power draw can be pretty low.  Tom's Hardware reported the idle power draw of the 1080ti to be 13.2 Watts.

Edited December 27, 2018 by Iciclebar

[Tybio]

That's a great summary @Iciclebar, the key take away is that the TDP for a processor is the /minimum/ TDP your cooler needs.  Most boards/CPUs /can/ go over that if the cooling is good enough, so don't skimp and you can get more performance out of a CPU.  Server boards tend to be a lot more strict in this, but IMHO, you will never go wrong with good cooling.

 

For reference, my E-2176G which is basically an i7-8700k repackaged for the workstation market sips power when it is idle....my full system runs at~70W (and is never really idle as I have 2 VMs and like 10 dockers running!).  Even with a parity rebuild I was only pushing 100W...so as long as you avoid the major pitfalls (Disks spinning all the time, addon cards that you can work around needing by using on-board functions etc) you can get crazy low power usage.

[JonathanM]

Also keep in mind that other parts of the server are kept active while waiting on the processor to complete a task. A lower TDP processor may keep all your drives spun up much longer than an unthrottled version, causing the overall power consumption to be higher. It depends on the task at hand, transcoding video with handbrake would be one such example.

 

The real way to compare overall processor power is primarily by manufacturer, generation and die type. Artificially limiting a processor is only useful to keep the heatsink requirements low.