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ECE 565 Assignment #3

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ECE 565

Assignment #3
Submit instructions: Submit a file named hw3_writeup.pdf to your Sakai drop box.
1. Programming Models. For the following code, identify variables that are read-only,
read/write non-conflicting, and read-write conflicting, if we consider parallelizing the “for i”
loop only or if we consider parallelizing the “for j” loop only. In the code, the function
‘ceil(arg)’ returns the integer ceiling of the passed argument.
float product;
int i, j, N, sum;
int data_array[N];
int data_gridX[N][N], data_gridY[N][N];
float measurement[N];
for (i=1; i<N; i++) {
product = 1;
for (j=1; j<N; j++) {
product = product * sum * measurement[j]
measurement[ceil(product)]++;
if ((i-j) >= 0)
data_gridX[i-j][j] = data_gridX[i-j][j] * 2;
if ((j-i) >= 0)
data_gridY[i][j-i] = data_gridY[i][j-i] / 4;
}
sum = sum + data_array[i] + product;
}
2. Code Analysis for Parallel Task Identification. For the code shown below, analyze the
loop-carried data dependencies in order to answer each question about the presence of
independent, parallel tasks. To help answer the questions, you may want to list out the data
dependencies and/or draw the LDG. If you show your work (on the front or back of this
page), and how you arrived at each answer, you may receive some partial credit if the final
answer is not correct.

for (i=1; i<N; i++) {
for (j=N-3; j>0; j–) {
S1: b[i][j] = b[i][j+2] + a[i][j];
S2: a[i][j] = a[i][j] + a[i-1][j] + a[i+1][j];
}
}

(a) Is each “for i” loop iteration an independent parallel task (i.e. DOALL parallelism)?
(b) Is each “for j” loop iteration an independent parallel task?
(c) Assume that we change the loop structure to traverse across the diagonals or antidiagonals (i.e. as in chart 9 of the shared memory programming slides), and then traverse
each node within the diagonal or anti-diagonal in the inner loop. Is the update of each
node along a diagonal an independent parallel task? Is the update of each node along an
anti-diagonal an independent parallel task?
(d) Other than any identified in parts (a) – (c) are there any other parallel tasks that you can
identify? If so, describe this available parallelism.
3. Code Profiling & Performance Counters. In this problem, you will work with a (most
likely new to you) existing code base to identify the functions which dominate execution time,
calculate theoretical possible speedups, and extract various metrics using performance counters.
To complete this problem, you may use your own personal machine of your choosing, or you
may compile, build, run, and measure your code on one of two available server machines:
kraken.egr.duke.edu or leviathan.egr.duke.edu. An individual account will be created for you,
and your username and initial password will be emailed. The code that you will work with is a
Finite Element mini-application, which assembles and solves a linear system of equations. You
may download the code, and view additional documentation and information at these links:
https://asc.llnl.gov/sites/asc/files/2020-06/MiniFE_Summary_v2.0.pdf
https://asc.llnl.gov/sites/asc/files/2020-09/miniFE_openmp-2.0-rc3.zip
(a) Performance profiling: After downloading and unpackaging the code, prepare the
makefile for compilation. We will use this makefile: makefile.gnu.serial. The “serial”
indicates that we will only use a single-threaded run of the program. To ‘CFLAGS’ and
‘CXXFLAGS’ add the flags needed to enable use of ‘gprof’ (refer to the class notes and
don’t forget optimization!). Also add the following flag ‘-fno-inline’ which prevents
function inlining and will make this exercise more interesting. Compile the program with:
‘make –f makefile.gnu.serial’.
Next, execute the program with the following input: ./miniFE.x -nx 40 -ny 80 -nz 160
Finally, collect & examine the flat profile using gprof. For each of the top 8 functions,
report: 1) the function name, 2) the number of calls, and 3) the percentage of execution
time for all calls to the function.
(b) Amdahl’s Law: Based on the percentage of execution time identified for the top function
from part (a), calculate the overall application speedup that would be possible if this
function were optimized to produce a 5x speedup for that function alone.
(c) Performance Counters: Use the ‘perf’ tool to collect performance counter data over the
execution of the miniFE application (remember to run with the program arguments as
specified above). Recall from the lecture notes that you may use ‘perf list’ to show a list
of events to collect.
Collect and report counts for the following events:
Instructions
CPU cycles (and also show IPC, instructions per cycle)
Branch instructions
Branches misses (mispredictions)
Cache references
L1 data cache load misses
L1 instruction cache load misses
LLC (last level cache) loads
LLC (last level cache) load misses
Data TLB load misses
4. Performance Counters. For this question, you will revisit your matrix multiplication code
from Assignment #2 (question 4, part a). In Assignment #2, you measured the performance of
your code across three different orderings of the loop nests: I-J-K, J-K-I, and I-K-J. The
assumption was that performance differed due to different memory access patterns with different
cache miss rates. In this problem, you will use ‘perf’ as in question 3 to study the performance
across these 3 loop nest orderings in more detail.
First, re-run your program across different loop nest orderings on the machine where you are
using ‘perf’, and record you new performance results. Then, run your program across different
loop nest orderings and use ‘perf’ to see if the miss rates of various levels of cache explain your
performance results. Show your results and analysis in your writeup.

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