Don Capps,
On Thu, May 25, 2006 at 09:51:32PM -0500, Iozone wrote:
> >On Thu, May 25, 2006 at 08:53:09AM -0500, Iozone wrote:
> >>Wu Fengguang,
> >>
> >> I noticed someone asking for real-world examples
> >> where a novel read-ahead would be helpful.
> >>
> >> Ok...
> >>
> >> Examples:
> >>
> >> Structural analysis codes:
> >> Writes large files, forward sequential.
> >> then reads them backwards.
> >>
> >> Weather codes:
> >> Input from vertical scanning radar is written
> >> to a file, then the file is processed by strata.
> >> (Written in column order, read in row order..
> >> thus a strided reader)
> >
> >Precious data! Thanks very much.
>
> The next time you get in your car, or on an airplane,
> remember that the structural design was done using
> the structures codes. Without these codes, people
> , to put it simply, would die. Also these codes permit
> molecular modeling that increases material strength of
> the design while reducing weight and cost. GM, Ford,
> AirBus, Boing, and many others run these codes every hour
> of every day. Yes, they are important, really :-)
>
> Without the weather codes, folks would not have
> received early notification of the tornados, again
> it's pretty important to those programs :-)
>
> Humorus note: Golf club manufacturers are also using
> structure codes to design better clubs that help bad
> golfers hit the ball further... Not exactly a noble goal,
> but yet another example of the critical competitive
> nature of the business world and their depencency
> on structural analysis codes :-)
Wow...
> >> It was the study of these codes that lead to the inclusion
> >> of Iozone's backwards reader, and strided reader
> >> test cases :-)
> >>
> >> If your system does not have a novel read-ahead
> >> algorithm then these types of applications will blind
> >> side the traditional read-ahead algorithm and the
> >> performance will be unacceptable.
> >
> >Till now I have not implemented the strided readahead feature.
> >Maybe we should tamp the existing features first :)
>
> Or, you could implement the strided feature, as it's
> pretty easy to detect. Just need to trigger on two
> equa-distance accesses: A -> B and B -> C where
> C-B = B-A then the prediction D is C + distance between
> A->B or B->C.
> The math turns out to be easy to optimize:
> D= C+(B-A)
> for both forward and backward strided readers.
>
> Example:
> A = 1, B = 3 C=5
>
> Forward strided reader reading 1, 3, 5 -> predict 7
> 3-1=2 and 5-3 = 2 and therefore predict 5+2 = 7
> or going backwards, reading 7,5, 3 -> predict 1
> 5-7 = -2 and 3-5 = -2 and 3 + (-2) = 1 ... Cool huh :-)
Maybe it's useful for HPC applications, too. I'll consider it on
time of convenient. Though it can still be tricky for implementing.
So I'd concentrate on the top-priority thing first: to stabilize the
existing features, so that the majority can enjoy them earlier.
> Hmmm, I still remember what I wrote 8 years ago... Now that's
> amazing. I can't remember what I had for breakfast today,
> but this stuff seems fresh in memory banks :-) (weird)
It's a lucky thing for you to do it 8 years ago ;-)
Thanks,
Wu