Trading off safety and performance in the kernel

Posted May 14, 2015 23:42 UTC (Thu) by dlang (guest, #313)
In reply to: Trading off safety and performance in the kernel by mjg59
Parent article: Trading off safety and performance in the kernel

>> and do the shallow C states really save that much power over a lower clock speed?

> Yes. Even the most shallow C state will unclock the core, and running at 0MHz is somewhat cheaper than running at 500MHz.

remember that switching C states isn't free (in either energy or time), so it may not be a win if you don't stay there very long.

We obviously have very different expectations in how the hardware is going to behave at the different states. But keep in mind that I'm not saying that reducing the clock speed is always the right thing to do, I am just unconvinced that it's never the right thing to do the way that you seem to be.

Trading off safety and performance in the kernel

Posted May 14, 2015 23:56 UTC (Thu) by mjg59 (subscriber, #23239) [Link]

Shallow C states are basically free on modern CPUs. Deeper ones will drop cache, but that's basically irrelevant in the case we're discussing.

Trading off safety and performance in the kernel

Posted May 15, 2015 4:46 UTC (Fri) by marcH (subscriber, #57642) [Link] (1 responses)

> But keep in mind that I'm not saying that [X] is always the right thing to do, I am just unconvinced that it's never the right thing to do the way that you seem to be.

I had to waste 5 minutes reading the entire thread again to make sure I did not dream and that the exact opposite happened.

Trading off safety and performance in the kernel

Posted May 15, 2015 19:55 UTC (Fri) by bronson (subscriber, #4806) [Link]

Me too. That was surreal.

Trading off safety and performance in the kernel

Posted May 15, 2015 4:51 UTC (Fri) by mjg59 (subscriber, #23239) [Link]

Oh, right. Yes.

> But keep in mind that I'm not saying that reducing the clock speed is always the right thing to do, I am just unconvinced that it's never the right thing to do the way that you seem to be.

From https://lwn.net/Articles/644541/ (written by you)

> If you are saving the contents of RAM to disk, then you aren't going to finish any sooner at 5GHz clock than at 500MHz, the limiting factor is going to be your disk I/O performance. So if you can do this at 500MHz rather than 5GHz, you generate significantly less heat.

From https://lwn.net/Articles/644549/ (written by me)

> I'm not saying that it's impossible, but it's certainly not obvious.

…