Re: PROBLEM: high load average when idle

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



Linus Torvalds wrote:

On Tue, 2 Oct 2007, Andrew Morton wrote:
This is unexpected.  High load average is due to either a task chewing a
lot of CPU time or a task stuck in uninterruptible sleep.

Not necessarily.

We saw high loadaverages with the timer bogosity with "gettimeofday()" and "select()" not agreeing, so they would do things like

	date = time(..)
	select(.. , timeout = <time + 1> )

and when "date" wasn't taking the jiffies offset into account, and thus mixing these kinds of different time sources, the select ended up returning immediately because they effectively used different clocks, and suddenly we had some applications chewing up 30% CPU time, because they were in a loop that *tried* to sleep.

And I wonder if the same kind thing is effectively happening here: the code is written so that it *tries* to sleep, but the rounding of the clock basically means that it's trying to sleep using a different clock than the one we're using to wake things up with, so some percentage of the time it doesn't sleep at all!

I wonder if the whole "round_jiffies()" thing should be written so that it never rounds down, or at least never rounds down to before the current second!
...

On a related note, {set/get}itimer() currently are buggy (since 2.6.11 or so),
also due to this round_jiffies() thing I believe.

If one sets ITIMER_PROF to, say, 5.000000 seconds, and then reads it back
very shortly thereafter, it will give 5.200000 seconds as the value (HZ==1000).

AFAIK, this should *never* be possible --> any read of get_itimer should never
return a value higher than the starting value.  This makes ITIMER_PROF not very
useful for measuring one's own CPU usage, for example.

Cheers
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

[Index of Archives]     [Kernel Newbies]     [Netfilter]     [Bugtraq]     [Photo]     [Stuff]     [Gimp]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux RAID]     [Video 4 Linux]     [Linux for the blind]     [Linux Resources]
  Powered by Linux