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

[Daniel Stone]

Hi,

On Thu, Aug 12, 2010 at 09:47:23AM +0800, ykzhao wrote:
>      The issue is that the function of read_hpet will be called more
> than 20000 every second when playing video workload on one machine. And
> the matter is that it will take about 1800 cycles to read the HPET
> counter(About 1us latency). According to the collected info, Xorg is the
> first contributor of read_hpet. And the further analysis shows that one
> contributor of read_hept in Xorg is caused by calling the
> CLOCK_MONOTONIC posix timer. 
> 
>      In 2009 a guy from IBM added a CLOCK_MONOTONIC_COARSE posix timer,
> which will return the time at the last tick. Compared with the
> CLOCK_REALTIME/MONOTONIC, it is much faster as it can avoid the access
> to the hardware. In such case the application can make the trade-off
> between proper speed/granularity.
> http://marc.info/?l=linux-kernel&m=125073483507611&w=2
> 
>    From the xorg server code it seems that the main purpose of
> CLOCK_MONOTONIC posix timer in Xorg is to check the client's
> request periodically. Is is acceptable that we use CLOCK_MONONIC_COARSE
> posix timer with less accuracy instead of CLOCK_MONOTONIC in Xserver?

It's used by anything which calls GetTimeInMillis(), which is often to
timestamp events sent to clients.  We don't really need anything hugely
fine-grained; certainly nothing greater than 1ms granularity.  I'd
happily ack a patch which uses CLOCK_MONOTONIC_COARSE in preference to
CLOCK_MONOTONIC, in preference to gettimeofday().

Cheers,
Daniel
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