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Chapter 3. The Cray T3E system 25
two words in each cp. An SCACHE line is 64 bytes. Therefore, data is
moved in consecutive blocks of 64 bytes from the main memory.
When you are optimizing your code, the most important thing is to
optimize the usage of the DCACHE. Almost as important is to optimize
the usage of the SCACHE.
Because of the reasons mentioned above, try to avoid step sizes of 8 kB
or 32 kB when you are referencing memory. The most optimal way is
to use stride one, which in the case of Fortran means changing the first
index of arrays with a step size of one.
Here is a simple example of memory references:
REAL, DIMENSION(n) :: a, b
REAL, DIMENSION(n,n) :: c
INTEGER :: i, j
DOi=1,n
DOj=1,n
c(i,j) = c(i,j) + a(i)*b(j)
END DO
END DO
If the constant n is of size 1024, the code runs very slowly due to the
memory references c(1,1), c(1,2), c(1,3) etc., which are 8 kB apart
in memory. You should rearrange the loops as follows to get better
performance:
DOj=1,n
DOi=1,n
c(i,j) = c(i,j) + a(i)*b(j)
END DO
END DO
3.6 Interprocessor communication
The system PEs of the Cray T3E are connected through a high-speed, low-
latency interconnection network. The peak data-transfer speed between
processors is 480 MB/s in every direction through the bi-directional 3D
torus network. The hardware latency is less than 1 µs.
The T3E system interconnection network operates asynchronously and
independently from the PEs to access and redistribute global data. The
3D torus topology ensures short connection paths. The bisectional band-
width is also high (measured by splitting the machine in half and finding
out the maximum transfer rate between these parts). The topology has
also the ability to avoid failed communication pathways.
An example of routing through the interconnection network is presented
in Figure 3.4.
- Cray T3E 1
- User’s Guide 1
- Cray T3E User’s Guide 3 3
- Contents 4
- Contents 5 5
- B Glossary 113 6
- Bibliography 118 6
- Index 120 6
- Introduction 7
- 1.2 Usage policy 8
- 1.3 Overview of the system 9
- 1.4 Programming environment 9
- 10 Cray T3E User’s Guide 10
- Chapter 1. Introduction 11 11
- 12 Cray T3E User’s Guide 12
- Using the Cray T3E at CSC 13
- 2.2 Files 14
- 2.3 Editing files 16
- 2.5 Executing in batch mode 17
- 2.6 More information 18
- The Cray T3E system 20
- 3.2 Distributed memory 21
- 3.3 Processing elements 21
- 3.4 Processor architecture 23
- 3.5 Local memory hierarchy 24
- 26 Cray T3E User’s Guide 26
- Processing Element 27
- I/O Controller 27
- Interconnection 27
- Network 27
- 3.9 File systems 28
- 3.10 Resource monitoring 29
- 30 Cray T3E User’s Guide 30
- 3.11 More information 31
- Program development 32
- 4.3.1 BLAS 34
- 4.3.2 LAPACK 34
- 4.3.3 BLACS 35
- 4.3.4 PBLAS and ScaLAPACK 35
- 4.3.5 Details 35
- 36 Cray T3E User’s Guide 36
- 4.6 More information 38
- Fortran programming 40
- 5.2 Basic usage 41
- 5.4 Compiler options 42
- 5.5 Optimization options 42
- 5.6 Optimizing for cache 44
- 5.7 Compiler directives 45
- 5.8 Fortran 90 modules 48
- 5.9 Source code preprocessing 49
- 5.10 More information 51
- C and C++ programming 52
- 6.2 The C compiler 53
- 6.3 Calling Fortran from C 54
- 6.4 C compiler options 55
- #pragma _CRI symmetric var 59
- #pragma _CRI unroll [n] 59
- 6.6 The C++ compiler 61
- 6.7 More information 61
- Interprocess communication 62
- 7.2.1 Format of the MPI calls 63
- 7.2.2 Some MPI routines 64
- 7.2.3 An example of using MPI 64
- 7.2.5 MPI data types 67
- 7.3.1 Using PVM on the T3E 68
- #include <mpp/shmem.h> 71
- INCLUDE ’mpp/shmem.fh’ 71
- 7.4.2 Data addresses 72
- 7.4.4 Reduction routines 74
- 7.4.6 Example of using SHMEM 76
- 78 Cray T3E User’s Guide 78
- 0:c= 10121416 78
- 1:c= 2468 78
- 80 Cray T3E User’s Guide 80
- Batch queuing system 81
- 82 Cray T3E User’s Guide 82
- # QSUB -l p_mpp_t=7000 82
- 8.3 Status of the NQE job 83
- 84 Cray T3E User’s Guide 84
- 8.4 Deleting an NQE batch job 86
- 8.5 Queues 86
- 8.6 More information 87
- Programming tools 88
- 9.2 Program Browser 89
- 9.3 Debugging programs 92
- 9.3.2 Examining core files 93
- 94 Cray T3E User’s Guide 94
- 9.4.1 MPP Apprentice 95
- 9.4.2 The appview command 98
- 9.4.3 PAT 98
- 9.5.1 The VAMPIRtrace library 100
- 102
- 9.5.3 More information 103
- Miscellaneous notes 104
- 10.1.3 The CPU clock timer 105
- 10.1.4 Example of timing 106
- 108 Cray T3E User’s Guide 108
- 10.4 More information 110
- About CSC 111
- 112 Cray T3E User’s Guide 112
- Glossary 113
- 114 Cray T3E User’s Guide 114
- Appendix B. Glossary 115 115
- Metacomputer Environment 116
- Bibliography 118
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