[PATCH] use CLOCK_MONOTONIC_COARSE posix timer instead of CLOCK_MONOTONIC in Xorg

[Daniel Stone]

Hi,

On Tue, Aug 24, 2010 at 11:05:23AM +0800, ykzhao wrote:
> Maybe there is no definition of "CLOCK_MONOTONIC_COARSE"
> in /usr/include/bits/time.h when compiling the xorg. But the xorg is
> executed on the linux kernel that supports the CLOCK_MONOTONIC_COARSE
> posix timer.(In fact for most previous Linux distribution there is no
> definition of CLOCK_MONOTONIC_COARSE in /usr/include/bits/time.h). 
> If it is executed on the kernel that doesn't support the
> CLOCK_MONOTONIC_COARSE timer, it will fallback to the CLOCK_MONOTOIC
> posix timer(the function of clock_getres will return the invalid value).
> 
> Do we need to consider the above scenario? If the above scenario doesn't
> need to be cared, I will update the patch to assure that
> the CLOCK_MONOTONIC_COARSE posix timer will be tried only when there
> exists the corresponding definition. 

Why not just fix glibc to include the definition?

> > > If so, is there an approach that helps us to detect whether the
> > > CLOCK_MONOTONIC_COARSE posix timer is supported on one OS?
> > 
> > Try the following completely untested patch (hey, it compiles).  It's
> > not perfect though: if CLOCK_MONOTONIC or CLOCK_MONOTONIC_COARSE were
> > ever 0, we'd make two or three syscalls for GetTimeInMillis() instead of
> > one, and if either of them were ~0L, we'd never use them.
> 
> the corresponding code is put under the condition definition of
> MONOTONIC_CLOCK. This is already checked by using configure script.
> 
>     In theory the CLOCK_MONOTONIC exists if the MONOTONIC_CLOCK is
> defined.  Not sure whether it is still necessary to check the
> CLOCK_MONOTONIC again?

It was just an overly paranoid check to make sure that CLOCK_MONOTONIC
actually works on the target system, as well as exists on the build
system.  I'm not sure if there's a Linux kernel we really support that
doesn't have a working CLOCK_MONOTONIC, but I wasn't sure, so.

I'd say CLOCK_MONOTONIC_COARSE -> CLOCK_MONOTONIC -> gettimeofday is the
best order, given that it's in descending order of usefulness, but
ascending order of likelihood of working.

Cheers,
Daniel
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