What Is Parallel Computing? Parallel Computing is the use of multiple processing units or computers working together to solve a common problem. Work on different tasks or the same task but on different pieces of the problem’s data. 3
Parallel Computing Each Processor works its section of the problem Processors can exchange and share information 4
Why do Parallel Computing? Limits of single CPU computing Performance: Serial is too slow. Available memory: Need for large amount of memory not accessible by a single processor. Parallel computing allows one to: Solve problems that don’t fit on a single CPU Solve problems that can’t be solved in a reasonable time 5
Why do Parallel Computing? We can solve... Larger problems The same problem faster 6
GENERAL CHARACTERISTICS: Shared memory parallel computers vary widely, but generally have in common the ability for all processors to access all memory as global address space. Multiple processors can operate independently but share the same memory resources. 7 Shared Memory
General Characteristics Cont. Changes in a memory location effected by one processor are visible to all other processors. Historically, shared memory machines have been classified as Uniformed Memory Access (UMA) and NUMA (Non-Uniformed Memory Access), based upon memory access times. 8
Uniform Memory Access (UMA) Equal access and access times to memory If one processor updates a location in shared memory, all the other processors know about the update. 9
Non-Uniform Memory Access Often made by physically linking two or more multiprocessors One processor can directly access memory of another processor Not all processors have equal access time to all memories Memory access across link is slower 10
Serial Computing 12 Suppose you want to do a jigsaw puzzle that has, say, a thousand pieces. We can imagine that it’ll take you a certain amount of time. Let’s say that you can put the puzzle together in an hour.
Shared Memory Parallelism 13 If Scott sits across the table from you, then he can work on his half of the puzzle and you can work on yours.
Shared Memory Parallelism 14 Once in a while, you’ll both reach into the pile of pieces at the same time (you’ll contend for the same resource), which will cause a little bit of slowdown. And from time to time you’ll have to work together (communicate) at the interface between his half and yours. The speedup will be nearly 2-to-1: y’all might take 30 minutes instead of hour.
The More the Merrier? 15 Now let’s put Jane and Sam on the other two sides of the table.
The More the Merrier? 16 Each of you can work on a part of the puzzle, but there’ll be a lot more contention for the shared resource (the pile of puzzle pieces) and a lot more communication at the interfaces. So y’all will get noticeably less than a 4-to-1 speedup, but you’ll still have an improvement, maybe something like 3-to-1: the four of you can get it done in 20 minutes instead of an hour.
More People? 17 What Happens – If we now put Sally and Sue and Jane and Bill on the corners of the table?
Diminishing Returns 18 There’s going to be a whole lot of contention for the shared resource, and a lot of communication at the many interfaces. So the speedup you get will be much less than you’d like; you’ll be lucky to get 5-to-1. So we can see that adding more and more workers onto a shared resource is eventually going to have a diminishing return. What activity yesterday demonstrated the diminishing returns?
Advantages of Shared Memory Global address space provides a user-friendly programming perspective to memory Data sharing between tasks is both fast and uniform due to the proximity of memory to CPUs 19
Disadvantage of Shared Memory Primary disadvantage is the lack of scalability between memory and CPUs. Adding more CPUs can geometrically increases traffic on the shared memory-CPU path Programmer responsibility for synchronization constructs that ensure "correct" access of global memory 20
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