Re: [PATCH] - sched_clock() broken for ia64 SN platform

[Grant Grundler <iod00d@hp.com>]

On Wed, Nov 19, 2003 at 08:09:15PM -0800, John Hawkes wrote:
> I'd like to hear an argument about why sched_clock() needs sub-microsecond
> accuracy, instead of just using jiffies, when one use of sched_clock() is to
> compare a delta time against cache_decay_ticks, which is a
> "jiffies"-granularity value, and the other use is to determine the relative
> computebound-vs-interactive characteristics of the process.

In general, it seems like bouncing the jiffies cacheline around is more of
a problem than the need for accuracy. This sounds like a similar problem
Jack Steiner wrote about before (updating interrupt counts):

| Updating the counter causes a cache line to be bounced between
| cpus at a rate of at least HZ*active_cpus. (The number of bus transactions
| is at least 2X higher because the line is first obtained for
| shared usage, then upgraded to modified. In addition, multiple references
| are made to the line for each interrupt. On a big system, it is unlikely that
| a cpu can hold the line for entire time that the interrupt is being
| serviced).

hth,
grant