(Nearly) full tickless operation in 3.10

Posted May 8, 2013 22:56 UTC (Wed) by bgmarete (guest, #47484)
Parent article: (Nearly) full tickless operation in 3.10

With regard to the necessity of setting CPU affinities so that processors marked for tickless operation do not acquire more than one process, wouldn't something like systemd ease the related administrative work? After all, I don't want to be messing around with taskset(1) on my laptop.

Also, how does this relate to CPU hyperthreads? Can a hyperthread be in tickless mode while a sibling hyperthread is not? Or must the entire core be woken up (or not) by the timer ticks?

Hyperthreading

Posted May 8, 2013 23:05 UTC (Wed) by corbet (editor, #1) [Link] (8 responses)

If you want tickless for latency reasons, the last thing you're going to do is turn on hyperthreading.

Hyperthreading

Posted May 9, 2013 1:15 UTC (Thu) by bgmarete (guest, #47484) [Link] (7 responses)

Thanks for the reply Corbet. I have just finished reading a long series of articles, including some at Intel, which make your point clear.

I would still like to know if each hyperthread can be independently put into full tickless mode (independent, that is, from its sibling), with the attendant power savings (if any). (Assume that I am interested only in saving battery power).

Hyperthreading

Posted May 9, 2013 13:14 UTC (Thu) by sheepdestroyer (guest, #54968) [Link] (6 responses)

Also interested by that particular issue. Got a SandyBridge ultra-portable so 2 "real" cores + 2 HT and would like to know how much if any power save should i expect?

Hyperthreading

Posted May 9, 2013 15:59 UTC (Thu) by drago01 (subscriber, #50715) [Link] (5 responses)

I doubt this will get you any noticeable power savings if at all. The most interesting part for power saving is the idle part which is already handled by CONFIG_NO_HZ

Hyperthreading

Posted May 10, 2013 8:43 UTC (Fri) by akeane (guest, #85436) [Link] (4 responses)

The config option should be called: CONFIG_WINDOWS_311

Really, "cooperative" multi-tasking?

Really?

Really...

Hyperthreading

Posted May 10, 2013 18:59 UTC (Fri) by PaulMcKenney (✭ supporter ✭, #9624) [Link]

Cute, but inaccurate. ;-)

If there is only one CPU-bound task runnable on a given CPU, there is no point in any scheduling decisions.

If there are multiple tasks runnable on a given CPU, and if the currently running task is CPU-bound, then there is no point in any scheduling decisions until the next timeslice.

Of course, things might change in the meantime, but in that case, this CPU will receive an interrupt and can therefore adjust as appropriate at that point in time.

Hyperthreading

Posted May 13, 2013 21:46 UTC (Mon) by chloe_zen (guest, #8258) [Link] (2 responses)

I think you haven't been keeping up with current events (no pun intended); event-driven loops in a single OS thread are the New Way of getting the most I/O through a CPU. This is a realistically useful optimization.

Hyperthreading

Posted May 20, 2013 23:13 UTC (Mon) by marcH (subscriber, #57642) [Link] (1 responses)

So, ticks to look some day as outdated as polling?

ticks vs. polling

Posted May 21, 2013 7:49 UTC (Tue) by cladisch (✭ supporter ✭, #50193) [Link]

> ticks to look some day as outdated as polling?

Polling is regularly checking the status, just because something that needs handling might have happened.

Ticks are polling.