Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!ncar!ncar.ucar.EDU!ftower
From: ftower@ncar.ucar.EDU (Francis Tower)
Newsgroups: comp.lang.fortran
Subject: Re: vectorization question reposed
Message-ID: <10851@ncar.ucar.edu>
Date: 31 Mar 91 14:49:44 GMT
References: <1991Mar30.142903.5225@ariel.unm.edu>
Sender: news@ncar.ucar.edu
Reply-To: ftower@ncar.ucar.EDU (Francis Tower)
Organization: Climate and Global Dynamics Division, NCAR
Lines: 58

Dear John,

   Most of the following I've taken from the Cray manual TR-OPT,
CF77 & Standard C Features and Optimization. The rest, I just made
up. 


1.  Each CPU on the Y-MP has 4 each buffers.  Each can hold 128
instruction 'parcels'.  However there is no data or instruction
cache as others systems might use to speed up serial processing.

2.  I don't believe 'fpp' will attempt to reorder your indices.
Even if it did try, it wouldn't be able to do much because of
the use of indirect indices.  For irregular grids (many Finite 
Element problems)  the indirect way is frequently used.  The
indicies are fixed, we know that, but the pre-compiler doesn't.
'fpp' or 'cft77' must assume the worst case.  If there are 
bank conflicts, you eat them.

3.  Each Y-MP CPU has 4 ports to memory so it's possible for each
CPU to be accessing  each of the Y-MP's 4 memory sections.
Each memory section has 8 subsections.  Depending on the amount
of memory, each section is broken into banks.

The minimum configuration (Y-MP 2/116 has 64 banks. The 4/132 has
128 banks, and the 8/432 upwards have 256 banks.  Each bank has a
5 clock period cycle time, and all the banks are interleaved.  The
worst case is when the stride =0 or the number of banks you have.

On a 256 bank machine:

     STRIDE                    Relative Performance


      0 (or 256)                    1 / 5

      192                           4 / 5

      128                           2 / 5

      64                            4 / 5

   every-thing else                 1 / 1


Sorry, I gotta run and play B-Ball.  I'll Be back


Francis G Tower
Software QA
NCAR/CGD/ICS

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