Messages in this thread |  | | | Subject | Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE | | From | Peter Zijlstra <> | | Date | Mon, 20 Jul 2009 13:17:02 +0200 |
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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.
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.
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.
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