On Fri, 2007-06-01 at 20:30 +0200, Ingo Molnar wrote:
> * Daniel Walker <[email protected]> wrote:
>
> > > So, having two interfaces, one fast and one accurate is the right
> > > answer IMHO.
> >
> > In the case of lockstat you have two cases fast and functional, and
> > non-functional .. Right now your patch has no slow and functional
> > state.
>
> let me explain it to you:
>
> 1) there is absolutely no problem here to begin with. If a rare
> architecture is lazy enough to not bother implementing a finegrained
> sched_clock() then it certainly does not care about the granularity of
> lockstat fields either. If it does, it can improve scheduling and get
> more finegrained lockstat by implementing a proper sched_clock()
> function - all for the same price! ;-)
There is a problem, which we are discussing ... sched_clock() can be
lowres in lots of different situations, and lockstat fails to account
for that .. That in turn makes it's timing non-functional.
> 2) the 'solution' you suggested for this non-problem is _far worse_ than
> the granularity non-problem, on the _majority_ of server systems today!
> Think about it! Your suggestion would make lockstat _totally unusable_.
> Not "slow and functional" like you claim but "dead-slow and unusable".
I'm not sure how to respond to this.. You taking a big ball of
assumptions, and molding it into what ever you want ..
> in light of all this it is puzzling to me how you can still call Peter's
> code "non-functional" with a straight face. I have just tried lockstat
> with jiffies granular sched_clock() and it was still fully functional.
> So if you want to report some bug then please do it in a proper form.
Clearly you can't have sane microsecond level timestamps with a clock
that doesn't support microsecond resolution.. This is even something
Peter acknowledged in his first email to me.
> > As I said before there is no reason why and architectures should be
> > forced to implement sched_clock() .. Is there some specific reason why
> > you think it should be mandatory?
>
> Easy: it's not mandatory, but it's certainly "nice" even today, even
> without lockstat. It will get you:
>
> - better scheduling
> - better printk timestamps
> - higher-quality blktrace timestamps
>
> With lockstat, append "more finegrained lockstat output" to that list of
> benefits too. That's why every sane server architecture has a
> sched_clock() implementation - go check the kernel source. Now i wouldnt
> mind to clean the API up and call it get_stat_clock() or whatever - but
> that was not your suggestion at all - your suggestion was flawed: to
> implement sched_clock() via the GTOD clocksource.
At this point it's not clear to me you know what my suggestion was ..
Your saying you want a better API for sched_clock(), and yes I agree
with that 100% sched_clock() needs a better API .. The paragraph above
it looks like your on the verge of agreeing with me ..
You think my words are puzzling, try it from this end..
Daniel
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