Re: [RFC][PATCH] Introduce CLOCK_REALTIME_COARSE

[Peter Zijlstra <peterz@infradead.org>]

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.