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

[ykzhao]

On Fri, 2010-08-13 at 23:30 +0800, Adam Jackson wrote:
> 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?

I have a question about the granularity. Do we have to assure CONFIG_HZ
>= 1000 for the granularity? 
   For one system with 250HZ, what will be affected when using the
CLOCK_MONOTONIC_COARSE timer in X? 

Thanks.

> 
> - ajax