Parallel Computing - PRAM -- LECTURE - 1

1. PRAM

2. 9A

3. Concurrent Write (CW)
 Priority CW
 Higher priority processor (normally lower
index) wins
 Common CW
 Succeeds only if all writes have the same
value
 Arbitrary/Random CW
 One of the values is randomly chosen
EREW CREW Common Arbitrary
≤ ≤ ≤ ≤
Priority
Simplest
to
Implemen
Hardest
to
Implemen

4. Example CRCW-PRAM
 Initially
 Table A contains 0 and 1
 Output contains 0
for i = 1 to 5
if A[i] == 1
output = 1
 The program computes the “Boolean
OR” of A[1], A[2], A[3], A[4], A[5]

5. Parallel Addition
 log(n) time
 n/2 processors
 Speed-up: n/log(n)
 Efficiency: 1/log(n)
 Work: n log n *
P0 + P1 + P2 + P3 +
step 1
P0 + P2 +
step 2
P0 + step
3