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Amdahl equation and scalability

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Hello,

I have come to another interresting subject, the Amdahl equation
that is equal to 1/(S+P/N)
(S:the percentage of the serial part,
P: percentage of the parallel part
N: the number of cores)

So we have to be smart, so follow with me,  i have read on some 
documents something that look like this: if the Serial part is 0.1% and 
the parallel part is 0.9% and we have 4 cores, so the Amdahl equatoin
will equal  to 1/ 0.1 + 0.225 = 3X , so this will scale to 3X, but i 
don't agree with this , cause i think
this Amdahl equation do not give a correct picture,so imagine that the 
serial part take 1 second and
the parallel part 9 seconds, that means S= 0.1% and P=0.9%,so
with 4 cores you will say that this will run four P parts in 9 seconds
and four S parts in 4 seconds this will equal 13 seconds, but the serial 
part will run in 4*10 , so the scalability will equal 40 seconds divide
by 13 seconds so this will scale to 3X, this is exactly what i have found
with the Amdahl equation. But the Amdhal equation is not
so precise and it doesn't give a correct picture, cause i think
the Amdhal equation is for only the ideal contention scenario as
i have just explain to you , but in a none contention scenario
you will for example run four S parts in 1 seconds and four P parts in 9 
seconds this will give a scalability equal to 40 seconds divide by 10
so this will scale to 4X in a none contention scenario, hence if you 
have less contention it will scale better than 3X , so this is why i say 
that
the Amdahl equation doesn't give you a correct picture, and more than
that if in pratice the serial part is small and there is more randomness 
in the parallel part, you will have less contention i think, so the 
example that i just gave you will scale to much better than 3X , so hope 
you have undertood this important ideas that i am giving you.



Thank you,
Amine Moulay Ramdane.

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Amdahl equation and scalability aminer <aminer@toto.net> - 2013-10-09 16:25 -0700
  Re: Amdahl equation and scalability Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-10-10 14:47 +0100

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