[RFC] use CLOCK_MONOTONIC_COARSE posix timer instead of CLOCK_MONOTONIC when calling the function of clock_gettime in xserver/os/util.c

[Adam Jackson]

On Fri, 2010-08-13 at 13:08 +0800, ykzhao wrote:
> On Fri, 2010-08-13 at 11:32 +0800, Julien Cristau wrote:
> > On Thu, Aug 12, 2010 at 09:47:23 +0800, ykzhao wrote:
> > 
> > > when playing flash h.264 cif video workload, the cpu C0 usage is reduced
> > > from 43% to 42% while the video quality is not affected.
> > > 
> > Is 43% vs 42% a really significant win?  How close is it to measurement
> > noise?
> 
> The purpose of using CLOCK_MONOTONIC_COARSE posix timer is to reduce
> the overhead of read_hpet. 
> 
> Yes. The 1% ratio seems tiny. But the tiny ratio is related with that
> the total overhead of read_hpet is not very high on the CPU usage(about
> 3%) when playing the video workload. In such case it is not easy to get
> one higher improvement.

On second thought, this seems like it's pretty awkward to use if we
don't know the granularity.  And given that we're assuming milliseconds
everywhere, you'd need CONFIG_HZ >= 1000 to get that.  Do we have a way
of getting HZ from userspace?

- ajax
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