Timing

[Dado Sutter]

On Wed, Feb 18, 2009 at 02:19, James Snyder <jbsnyder at fanplastic.org> wrote:

> So, one of the things I've been doing recently is tracking timing data for
> adcscope.lua when running it.  I've found certain things that increase or
> decrease the amount of time spent getting samples, but I hadn't actually
> checked what the bare minimum time would be for a dry loop like the
> following:
>
> term.clrscr()
>
> while 1 do
>  t1 = tmr.start(0)
>  t2 = tmr.read(0)
>  dtime = tmr.diff(0,t2,t1)
>  term.gotoxy(1,1)
>  term.putstr(string.format("Time (us): %06d",dtime))
> end
>
> For LM3S6965, I get 79 us.  Is this correct?

What is the MCLK freq on your board/kit pls ?
I'll check the timming on my EK-LM3S8962 asap.

Thanks for showing me that we not always need a tmr.stop() !! :)


> I get generally the same results if I start the timer outside the loop and
> do two subsequent reads.
> Making t1 and t2 locals shaves off a little but not much.  If I make
> tmr.start and tmr.read locals, things get down to 13 us.


Wow, what a difference ! I knew locals were faster (I think Roberto mentions
this and other gems in http://www.lua.org/gems/sample.pdf) but that seems
quite a difference.

Does this jibe with everyone else's findings?


asap


> If I do a local function between the two timers like math.sqrt, I get a
> stable 48 us/cycle:


Good !!! Determinism is welcome here :)
(pls note this can obviously vary, depending on the function used here, at
least for different arguments, as you've done in your example below)

term.clrscr()

>
> local t1, t2, s2
> local tstart = tmr.start
> local tread = tmr.read
> local speedfun = math.sqrt
>
> while 1 do
>  s2 = 23
>  t1 = tstart(0)
>  s2 = speedfun(s2)
>  t2 = tread(0)
>  dtime = tmr.diff(0,t2,t1)
>  term.gotoxy(1,1)
>  term.putstr(string.format("Delay (us): %06d",dtime))
> en
>
> Making all the timing and sampling functions and variables local, here are
> the timings I'm looking at for adcscope.lua when getting samples from 4
> channels.
>
> Pass table to getsamples, modify specified index (
> adc.getsamples(v,1,adcvals,i ): 293 us (stable)
> Convert returned table to integer, insert into table (adcvals[i] =
> adc.getsamples(v,1)[1]): ~470 us (fluctuates)
> Replace nested table of values within adcvalues table (adcvals[i] =
> adc.getsamples(v,1)): ~450 (fluctuates, sometimes maybe up to 1000-2000 us)

gc seems to be working on 3rd test :(

Very nice !! Thanks for the precise report again !!
Still, it seems a bit too slow to me :( for a Cortex-M3. I hope this is just
a bad impression.

-jsnyder


Dado







>
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