Messages in this thread | | | Date | Sat, 19 Apr 2008 09:13:40 +0200 (CEST) | From | Thomas Gleixner <> | Subject | Re: [linux-pm] Higer latency with dynamic tick (need for an io-ondemand govenor?) |
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On Fri, 18 Apr 2008, David Brownell wrote: > On Friday 18 April 2008, Woodruff, Richard wrote: > > When capturing some traces with dynamic tick we were noticing the > > interrupt latency seems to go up a good amount. If you look at the trace > > the gpio IRQ is now offset a good amount. Good news I guess is its > > pretty predictable. > > That is, about 24 usec on this CPU ... an ARM v7, which I'm guessing > is an OMAP34xx running fairly fast (order of 4x faster than most ARMs). > > Similar issues were noted, also using ETM trace, on an ARM920 core [1] > from Atmel. There, the overhead of NO_HZ was observed to be more like > 150 usec of per-IRQ overhead, which is enough to make NO_HZ non-viable > in some configurations. > > > > I was wondering what thoughts of optimizing this might be. > > Cutting down the math implied by jiffies updates might help. > The 64 bit math for ktime structs isn't cheap; purely by eyeball, > that was almost 1/3 the cost of that 24 usec (mostly __do_div64).
Hmm, I have no real good idea to avoid the div64 in the case of a long idle sleep. Any brilliant patches are welcome :)
Thanks, tglx | |