Description
1. Consider the problem of computing the sum of n numbers, x = a1 + a2 + · · · + an. For the
case n = 8, show the Dependence Analysis Graph (DAG) of the sum operation when it is
performed as
x = a1 + (a2 + (a3 + (a4 + (a5 + (a6 + (a7 + a8)))))).
Recall that the DAG is a binary tree with data at the leaves and operations (in this case ’+’)
at all other nodes. Still for the case n = 8, show the same DAG for the case when the sum is
done by the cascade algorithm.
What is the shortest height that a binary tree can have, among all possible binary trees with
n leaves? [No proofs required]. Assuming you have enough adders, what is the smallest
number of time units needed to add n numbers, if a time unit corresponds to the time an
adder completes one operation (and if communication times are neglected)? What algorithm
achieves this mininum?
2. Given a vector x with the n components x0, x1, · · · , xn−1 the prefix sum (or scan-sum, or
cumulative sum) of x is the vector consisting of the n scalars yi
, for i = 0, 1, · · · , n − 1 where
yi = x0 + x1 + · · · + xi
.
(a) Design an algorithm for computing prefix sums in which the following property is exploited:
[ym, ym+1, · · · , yn−1] = ym−1 + PrefSum(xm, xm+1, · · · , xn−1),
where m < n − 1 and PrefSum(xm, xm+1, · · · , xn−1) is the prefix sum of xm, xm+1, · · · , xn−1.
For example if you have 4 numbers you can calculate the half prefix sums y0 = x0, y1 = x0+x1
and z0 = x2, z1 = x2 + x3 and then get the whole prefix sum of x by calculating y2 = y1 + z0,
y3 = y1+z3. Write down the algorithm in pseudo-code [or matlab code] – Indicate which loop
(or loops) is (are) parallel. Show the results of the outer loop steps (sequential loop) for the
case when x = [1, 2, −1, 4, −1, 6, −1, 8]. What will be the execution time of your algorithm in
a PRAM model assuming an addition takes τ seconds? What memory access protocol will
be needed (EREW? CREW? ….). How many processors will be needed? (you may assume
that n is a power of 2 for simplicity)
(b) A related problem is that of determining the ‘nearest one from the left’: in an array of
zeros and ones, determine for any j the nearest index i of an x(i) that is equal to one, where
0 ≤ i ≤ j. [If all elements x(i), for i ≤ j are zero then set y(j) = −1.] For example if
x = [0, 0, 1, 1, 0] then the answer is y = [−1, −1, 2, 3, 3]. How can you adapt the prefix sum
algorithm to solve this problem? Show the result of the sequential loop of your algorithm
for the case when x = [0, 1, 0, 1, 0, 1, 1, 0]. Answer the same questions as in (a): Number of
processors needed and parallel time complexity with the PRAM model.
3. The time it takes to perform a given operation (e.g., an addition) of two vectors of length n
on a pipelined vector computer is usually cast in the form
t(n) = σ + n/ρ
where σ is the start-up time and ρ is the peak speed (number of FLoating point Operations
Per Second or FLOPS).
Copyright
c 2019, the University of Minnesota. 2
(a) What is the peak MFLOPS (Mega FLOPS) rate of a vector computer when σ = 10−6
sec,
ρ = 108
? What is the MFLOPS rate that is actually achieved for vectors of length 1000?
100? 10?
(b) What is the number n1/2
for which the MFLOPS rate achieved is half the peak rate
(obtain a general formula for n1/2 which depends on the parameters σ and ρ)? What is
n1/2
for the example in (a)? Show that t(n) can be written as
t(n) =
n1/2 + n
ρ
The number n1/2
is termed the half-performance length or half vector length.
(c) Determine the peak performance rate and the half vector length for the CRAY X-MP
(a machine from the early 1990s) which had a clock cycle is 8.5 ns, knowing that it took
10 cycles to get one result out (“start-up of 9 cycles”).
4. A ‘master’ computer A is connected to a number p of identical processors Pi
, i = 1, …, p, via
a high-speed bus. Consider the problem of adding N numbers by having processor A split
the computatoin on p equal pieces and have the ’workers’ do the the p subsums. Then A
will obtain the subsums and add them (see notes). It is assumed that the high-speed bus is
infinitely fast but that moving data with the local bus of processor A to any processor takes
a time of the form
β + N/τ
where N is the size of the data, β is the communication start-up time, and τ is the memory
bandwidth of A’s bus in words/sec. All processors (A and the Pi
’s) can perform ρ floating
point operations per second, and the computations are synchronus (the adds start at the same
time and end at the same time).
P4P3P2 P5P1
A
High Speed Bus
(a) Estimate the execution time for adding N numbers with when p processors are used. Is
there a speed-ups when τ < ρ?
(b) Show a typical curve of execution time versus p. Is there an optimal number of processors
p∗ to use? If so what is it?
(c) Assume that β = 50/ρ and τ = ρ/10. What is the optimal number of processors
when N = 10, 000? What is the corresponding optimal time (expressed in terms of 1/ρ
seconds)?
5. Consider a memory system with a level 1 cache of 32KB, and DRAM of 512MB with the
processor operating at 1 GHz. The latency to L1 cache is one cycle and the latency to
DRAM is 100 cycles. In each memory cycle, the processor fetches 4 words (cache line size
is 4 words). What is peak achievable performance of a dot-product of 2 vectors? [Where
necessary assume an optimal cache replacement policy]
