Re: CONFIG_PRINTK_TIME woes

Andrew Morton
2005-08-22T20:14:03Z

tony.luck@intel.com wrote:
>
> At the other extreme ... the current use of sched_clock() with
> potentially nano-second resolution is way over the top. Logging
> to a serial console at 115200 a typical line from printk will take
> 2-4 milli-seconds to print ... so there would seem to be little
> benefit from a sub-millisecond resolution (in fact at 250HZ jiffies
> are on the ragged edge of being good enough).
>
> If that isn't sufficient ... it should be possible to make a cut-down,
> lockless version of do_gettimeofday that meets Andrew's suggestion
> of good resolution with occasional theoretical weirdness. But before
> we go there ... I'd like to hear whether there are usage models that
> really need better resolution than jiffies can provide?

I think so. Say you're debugging or performance tuning filesystem requests
and I/O completions, etc. You disable the console with `dmesg -n', run the
test then do `dmesg -s 1000000 > foo'. Having somewhat-sub-millisecond
timestamping in the resulting trace is required.