1. Constructing a system with multiple computers or processors
ITCS 4/5145 Parallel Programming, UNC-Charlotte, B. Wilkinson, slides1-b.ppt Aug 28, 2012

2. Conventional Computer
Consists of a processor executing a program stored in a (main) memory:
Each main memory location located by its address.
Addresses start at 0 and extend to 2b - 1 when there are b bits (binary digits) in address.
Main memory
Instr
uctions (to processor)
Data (to or from processor)
Processor

3. Types of Parallel Computers
Two principal approaches:
Shared memory multiprocessor
Distributed memory multicomputer

4. 1. Shared Memory Multiprocessor System
Natural way to extend single processor model - have multiple processors connected to multiple memory modules, such that each processor can access any memory module:
Memory module
One
address
space
Processor-memory Interconnections
Processors

5. Using a processor-memory bus as the interconnection network Example – Dual and Quad Processor Shared Memory Multiprocessors
Processor
Processor
Processor
Processor
L1 cache
L1 cache
L1 cache
L1 cache
L2 Cache
L2 Cache
L2 Cache
L2 Cache
Bus interface
Bus interface
Bus interface
Bus interface
Processor/
memory b us
coit-grid01 – coit-grid04 are of this form (dual processor servers with 8GB shared memory)
Set of lines
100+
Memory controller
Memory
Shared memory

6. “Recent” innovation(since 2005)
Dual-core and multi-core processors
Two or more independent processors in one package
Actually an old idea but not put into wide practice until recently because limits of making single processors faster principally caused by:
Power dissipation (power wall) and clock frequency limitations
Limits in parallelism within a single instruction stream
Memory speed limitations (memory wall)

7. Single quad core shared memory multiprocessor
Chip
Processor
L1 cache
Processor “core”
L2 Cache
Memory controller
Memory
Shared memory

8. Multiple quad-core multiprocessors
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L1 cache
L2 Cache
L2 Cache
possible L3 cache
Memory controller
Memory
Shared memory
Example coit-grid05.uncc.edu -- four processors each quad core. All 16 cores have access to 64 GB shared main memory (thro multilevel caches)

9. Programming Shared Memory Multiprocessors
Several possible ways – Usual approach is to use threads
Threads - individual parallel sequences (threads), each thread having their own local variables but being able to access shared variables declared outside threads.
1. Low–level thread libraries - programmer calls thread routines to create and control the threads. Example Pthreads, Java threads.
2. Higher level library functions and preprocessor compiler directives.
Example OpenMP - industry standard. Consists of library functions, compiler directives, and environment variables

10. Tasks
Rather than program with threads, which are closely linked to the physical hardware, can program with parallel “tasks.”
Promoted by Intel with their TBB (Thread Building Blocks) tools.
Other programming alternatives:
Parallelizing compilers compiling regular sequential programs and making them parallel programs
Special parallel languages
(both not now common).

11. 2. Distributed Memory Multicomputer
Complete computers connected through an interconnection network:
Many interconnection networks explored in 1970s and 1980s
including 2- and 3-dimensional meshes, hypercubes, and multistage interconnection networks
Interconnection
network
Messages
Processor
Local
memory
Computers

12. Networked Computers as a Computing Platform
A network of computers became a very attractive alternative to expensive supercomputers and parallel computer systems for high-performance computing in early 1990s.
Several early projects. Notable:
Berkeley NOW (network of workstations) project.
NASA Beowulf project.

13. Key advantages of using commodity networked computers:
Very high performance workstations and PCs readily available at low cost.
The latest processors can easily be incorporated into the system as they become available.
Existing software can be used or modified.

14. Beowulf Clusters*
A group of interconnected “commodity” computers achieving high performance with low cost.
Typically using commodity interconnects - high speed Ethernet, and Linux OS.
* Beowulf comes from name given by NASA Goddard Space Flight Center cluster project.

15. Cluster Interconnects
Originally fast Ethernet on low cost clusters
Gigabit Ethernet - easy upgrade path
More specialized/higher performance interconnects available including Myrinet and Infiniband.

16. Dedicated cluster with a master node and compute nodes
User
Master node
Compute nodes
Dedicated Cluster
Ethernet interface
Switch
External network
Computers
Local network

17. Software Tools for Clusters
Based upon message passing programming model
User-level libraries provided for explicitly specifying messages to be sent between executing processes on each computer .
Use with regular programming languages (C, C++, ...).
Can be quite difficult to program correctly as we shall see.

18. Using GPUs for high performance computing
GPUs (graphics processing units) originally designed to speed and support graphics operations now used for high performance computing.
GPUs have 100’s of processing cores and can provide orders of magnitude increase in execution speed.
We will look at GPU devices and how to program them in the last few weeks of the course

19. GPU clusters
Recent trend for clusters – incorporating GPUs for high performance.
At least three of the five fastest computers in the world are GPU clusters (as of late 2011)
UNC-C cluster used in the course has two GPU servers, coit-grid06.uncc.edu and coit-grid07.uncc.edu

20. Next step Learn how to program multiprocessor systems
New for Fall 2012, we will start with a new pattern programming approach and later consider lower level tools.