[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [f-cpu] Prefetch and Preload



Ben Franchuk wrote:
> Thomas Lavergne wrote:
> 
>>>1) What distance should be used for prefectch (how many cycle beteween a
>>>prefetch and the effective load)
>>>
>>>It depend, if you're in cache no need for it (you lose a cycle for
>>>nothing). If it's not in cache you could take 150-200 cycle. But if the
>>>distance is too small, depending of the implementation, you could also
>>>lose one cycle (because the underlying system didn't find the adress in
>>>cache and relaunch a read to the DRAM). If it's too far, the data should
>>>have been trash by another data.
>>>
>>>Then trash or not and cache hit or not depend on the cache type (size
>>>and associativity) so it depend on the effective memory adress of the
>>>manipulated data. That's why i didn't like prefetch...but preload.
>>
>>Ok so Prefetch is very hardware specific. Very bad :-( but I seem we
>>don't have any other solutions.
> 
> 
> So why can't one make them pesudo-variables in special registers?
> It seems a lot of timing information is needed for tomorrow's designs
> where real memory and internal processing speeds differ by so much.
> If we have this information on timing the software could better make use
> of free time. A test for 25% full or empty on buffered information could
> be very useful for dynamic optimzation.
> 

OK but you can't realy use this information at runtime to place your 
prefetch at a good place because you can't reorder your code durrinf 
execution (too complicated).
The only thing you can do is to produce different code and execute the good.


-- 
Thomas Lavergne                       "Le vrai rÍveur est celui qui rÍve
                                        de l'impossible."  (Elsa Triolet)
thomas.lavergne@laposte.net
d-12@laposte.net    ICQ:#137121910     http://assoc.wanadoo.fr/thallium/

*************************************************************
To unsubscribe, send an e-mail to majordomo@seul.org with
unsubscribe f-cpu       in the body. http://f-cpu.seul.org/