CFD Online Logo CFD Online URL
www.cfd-online.com
[Sponsors]
Home > Forums > General Forums > Main CFD Forum

CPU efficiency over 100% !!

Register Blogs Community New Posts Updated Threads Search

Reply
 
LinkBack Thread Tools Search this Thread Display Modes
Old   September 1, 1999, 09:03
Default CPU efficiency over 100% !!
  #1
X. Ye
Guest
 
Posts: n/a
Running our parallel CFD code, I have obtained the experience that the CPU efficiency exceeds sometimes 100%, e.g., with single processor I need 10 hours for a calculation, but with 8 processors I need only 1 hour, that means the speedup factor is 10 and the CPU efficiency is 125%. This fact suprised me. Now I suppose that the cache plays a very important role: With single processor its cache load is too high and the data must be transfered to RAM, then the calculation speed will be reduced through this data transfer; with 8 processors the cache load is distributed to the sevral processors and the calculation speed is as high as there is no data transfer to RAM. Has anybody the same experience?

X. Ye
  Reply With Quote

Old   September 1, 1999, 10:01
Default Re: CPU efficiency over 100% !!
  #2
Dr. Hrvoje Jasak
Guest
 
Posts: n/a
Yeah, superlinear behaviour of parallel CFD codes just means that you picked (or wrote) a good one! The thing is that the performance of a CFD code is critically dependent of memory fetch - sometimes you get unbelievable scalability results. If you look into the commercial parallel CFD code brochures, you'll find that lots of people report this sort of thing (it looks cool!). Nothing to worry about. My guess is that: a) you've got a really decent parallel platform with big bus-bandwidth (SGI?) b) you might be a bit low on the number of cells per processor (I would say the optimum is between 30 and 50k, depending on the communication width vs. type of solver you're using). If, on top of everything, you have the option of running partial (processor-by-processor) IC preconditioning (I assume you're using conjugate gradient solvers) rather than diagonal preconditioning, you'll get a tiny difference in results (round-off errors are different) but also a LOT of music (performance) for your money!
  Reply With Quote

Old   September 3, 1999, 13:17
Default Re: CPU efficiency over 100% !!
  #3
clifford bradford
Guest
 
Posts: n/a
i wouldn't say cpu efficiency is >100% since cpu efficiency is measured flop rate/peak speed (flop rate) *100. however you are getting a superlinear speeedup which is good. some parallel codes have a "sweet spot" where the cache load is just right and little or no data has to be fetched from RAM. what code are you using
  Reply With Quote

Old   September 6, 1999, 10:00
Default Re: CPU efficiency over 100% !!
  #4
X. Ye
Guest
 
Posts: n/a
I am using finite volume, multiblock and Runge-Kutta explicite time marching code. The machine is HP Convex 220 with native parallel compiler.

X. Ye
  Reply With Quote

Old   September 6, 1999, 15:39
Default Re: CPU efficiency over 100% !!
  #5
John C. Chien
Guest
 
Posts: n/a
(1). Are you getting the same trend at 2-CPU, 4-CPU, 6-CPU ? (2). If you can establish the trend, they should extend the machine to more CPUs to take advantage of this over 100% speed gain.
  Reply With Quote

Old   September 7, 1999, 04:34
Default Re: CPU efficiency over 100% !!
  #6
X. Ye
Guest
 
Posts: n/a
Dear John,

Thanks for your message. I am getting the same trend with lower number of CPU's. We'll scale the number of CPU's in the near future.

X. Ye
  Reply With Quote

Old   September 7, 1999, 11:07
Default Re: CPU efficiency over 100% !!
  #7
frank tse
Guest
 
Posts: n/a
This superlinear speedup, of course, do not last for ever. As more processors are used, the time needed for loading data into cache reduces to a minimum and remain stable while the time for message passing increases. The speedup will quickly becomes sublinear.

Regards Frank
  Reply With Quote

Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On


Similar Threads
Thread Thread Starter Forum Replies Last Post
stop when I run in parallel Nolwenn OpenFOAM 36 March 21, 2021 05:56
Superlinear speedup in OpenFOAM 13 msrinath80 OpenFOAM Running, Solving & CFD 18 March 3, 2015 06:36
OpenFOAM 13 Intel quadcore parallel results msrinath80 OpenFOAM Running, Solving & CFD 13 February 5, 2008 06:26
OpenFOAM 13 AMD quadcore parallel results msrinath80 OpenFOAM Running, Solving & CFD 1 November 11, 2007 00:23
Dual Core CPU hjasak OpenFOAM Running, Solving & CFD 5 July 22, 2006 04:57


All times are GMT -4. The time now is 10:01.