Re: API explosion (Re: [RFC/RFT] calloutng)

On Wed, 19 Dec 2012, Poul-Henning Kamp wrote:

> --------
> In message <CACYV=-Eg542iHm9KfujPvCzZrA4TqepEBVA8RzT1YOHnCgfJnA_at_mail.gmail.com>
> , Davide Italiano writes:
>
>> Right now -- the precision is specified in 'bintime', which is a binary number.
>> It's not 32.32, it's 32.64 or 64.64 depending on the size of time_t in
>> the specific platform.
>
> And that is way overkill for specifying a callout, at best your clock
> has short term stabilities approaching 1e-8, but likely as bad as 1e-6.

So you always agreed with me that bintimes are unsuitable for almost
everything, and especially unsuitable for timeouts? :-)

> (The reason why bintime is important for timekeeping is that we
> accumulate timeintervals approx 1e3 times a second, so the rounding
> error has to be much smaller than the short term stability in order
> to not dominate)

bintimes are not unsuitable for timekeeping, but they a painful to use
for other APIs.  You have to either put bintimes in layers in the other
APIs, or convert them to a more suitable format, and there is a problem
placing the conversion at points where it is efficient.  This thread
seems to be mostly about putting the conversion in wrong places.  My
original objection was about using bintimes for almost everything at
the implementation level.

>> I do not really think it worth to create another structure for
>> handling time (e.g. struct bintime32), as it will lead to code
>
> No, that was exactly my point:  It should be an integer so that
> comparisons and arithmetic is trivial.   A 32.32 format fits
> nicely into a int64_t which is readily available in the language.

I would have tried a 32 bit format with a variable named 'ticks'.
Something like:
- ticks >= 0.  Same meaning as now.  No changes in ABIs or APIs to use
   this.  The tick period would be constant but for virtual ticks and
   not too small.  hz = 1000 now makes the period too small, and not a
   power of 2.  So make the period 1/128 second.  This gives a 1.24.7
   binary format.  2**24 seconds is 194 days.
- ticks < 0.  The 31 value bits are now a cookie (descriptor) referring
   to a bintime or whatever.  This case should rarely be used.  I don't
   like it that a tickless kernel, which is needed mainly for power
   saving, has expanded into complications to support short timeouts
   which should rarely be used.

> As I said in my previous email:
>
>        typedef dur_t   int64_t;        /* signed for bug catching */
>        #define DURSEC  ((dur_t)1 << 32)
>        #define DURMIN  (DURSEC * 60)
>        #define DURMSEC (DURSEC / 1000)
>        #define DURUSEC (DURSEC / 10000000)
>        #define DURNSEC (DURSEC / 10000000000)
>
> (Bikeshed the names at your convenience)
>
> Then you can say
>
> 	callout_foo(34 * DURSEC)
> 	callout_foo(2400 * DURMSEC)
> or
> 	callout_foo(500 * DURNSEC)

Constructing the cookie for my special case would not be so easy.

> With this format you can specify callouts 68 years into the future
> with quarter nanosecond resolution, and you can trivially and
> efficiently compare dur_t's with
> 	if (d1 < d2)

This would make a better general format than timevals, timespecs and
of course bintimes :-).  It is a bit wasteful for timeouts since
its extremes are rarely used.  Malicious and broken callers can
still cause overflow at 68 years, so you have to check for it and
handle it.  The limit of 194 days is just as good for timeouts.

Bruce