Messages in this thread | | | Date | Mon, 20 Jul 2009 06:33:05 -0700 | From | Arjan van de Ven <> | Subject | Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE |
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On Mon, 20 Jul 2009 13:17:02 +0200 Peter Zijlstra <peterz@infradead.org> wrote:
> On Sat, 2009-07-18 at 15:30 -0700, Arjan van de Ven wrote: > > On Sat, 18 Jul 2009 15:09:38 -0700 > > john stultz <johnstul@us.ibm.com> wrote: > > > > > After talking with some application writers who want very fast, > > > but not fine-grained timestamps, I decided to try to implement a > > > new clock_ids to clock_gettime(): CLOCK_REALTIME_COARSE and > > > CLOCK_MONOTONIC_COARSE which returns the time at the last tick. > > > This is very fast as we don't have to access any hardware (which > > > can be very painful if you're using something like the acpi_pm > > > clocksource), and we can even use the vdso clock_gettime() method > > > to avoid the syscall. The only trade off is you only get low-res > > > tick grained time resolution. > > > > Does this tie us to having a tick? I still have hope that we can get > > rid of the tick even when apps are running .... since with CFS we > > don't really need the tick for the scheduler anymore for example.... > > On the hardware side to make this happen we'd need a platform that > has: > > - cheap, high-res, cross-cpu synced, clocksource > - cheap, high-res, clockevents > > Maybe power64, sparc64 and s390x qualify, but certainly nothing on x86 > does.
the x86 on my desk disagrees.
> Furthermore, on the software side we'd need a few modifications, such > as doing lazy accounting for things like u/s-time which currently > rely on the tick and moving the load-balancing into a hrtimer.
I thought the load balancer no longer runs as a timer.. but I could well be wrong.
> Also, even with the above done, we'd probably want to tinker with the > clockevent/hrtimer code and possibly use a second per-cpu hardware > timer for the scheduler, since doing the whole hrtimer rb-tree dance > for every context switch is simply way too expensive. > > But even with all that manged, there's still other bits that rely on > the tick -- RCU being one of the more interesting ones.
we need to at least keep our options open to go there, since even the early measurements (iirc from Andrea 5 years ago) of the 1 KHz time show that it has a real performance impact, as much as 1%. While we may not need to switch over RIGHT NOW, adding more dependencies on this timer is just not a good idea...
-- Arjan van de Ven Intel Open Source Technology Centre For development, discussion and tips for power savings, visit http://www.lesswatts.org
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