CSE 351 Spring 2011 Homework 0
An experiment in C and Java
Assigned: Monday, March 28, 2011
Due: Wednesday, March 30, 2011 at 11:59 PM
- Evaluate a claim made in the lecture slides that two seemingly equivalent
ways of writing a program can have vastly different performance.
- Get a feel for the relative performance of Java and C.
- Get a feel for the effectiveness of turning on C compiler optimizations.
- Verify that you have set up a working environment in which you can edit,
compile, and run C programs.
Let's test the claim that understanding
the memory hierarchy can be useful in writing efficient programs.
An example in the first-day lecture slides said that interchanging two loops has no effect on the
correctness of the results, but can give a 21x difference in performance. Let's see about that.
Here's the important part of the code. It computes exactly the same
thing no matter which of the two loops is outermost.
for ( i = 0; i < 2048; i++ )
for ( j = 0; j < 2048; j++ )
//src[i][j] = i * rep;
dst[i][j] = src[i][j];
You will download a set of four tiny programs—one in C and three in Java—that contain those
loops. You'll compile them and time how long it takes them to run. For the C program, you'll compile
both with and without compiler optimizations enabled, so in total you will have five programs to compare
at a time (3 Java + C compiled two ways).
You will do this several times, making small modifications to see what differences they make—how the
choice of language affects performance and how effective the compiler can be at optimizing your code
- interchange the order of the
- uncomment the commented line
- change the size of the array being copied from 2048 x 2048 to 8192 x 8192
You'll run each version of the code and measure how long it takes to complete. With all the permutations
(5 executables x 2 loop orderings x 2 commented/uncommented line versions x 2 array sizes), that's 40 versions.
(It will be easy—just read all the way through these instructions first.)
You'll then turn in a short document, described below, in which you summarize your test results and
answer a few questions.
The assignment requires use of a Linux system (or a Mac, if the right software is installed by default):
- Your personal machine:
- If you're running Linux, what you need is either already installed or else
- If you're not running Linux, you can run the
CSE home Linux
virtual machine. I highly recommend this option!
- A CSE instructional lab Linux workstation.
- The remote-login CSE Linux instructional machine(s),
Fetch the files, which are provided as a
tar archive: hw0.tar.gz .
Save them to a directory in which you want a new directory (containing the files) created.
Now issue the command '
tar xzf hw0.tar.gz' (without the quotes). That will un-archive the files,
hw0. In that directory you will find these files:
||Rows 'Java' in your tables of test results (see below)
||Rows 'JavaInteger' in your tables
||Rows 'JavaIntegerInteger' in your tables
||Rows 'C' and 'Optimized C' in your tables
||See "Automating" below
To compile the C program without optimizations,
cd to the
hw0 directory and type
gcc -Wall hw0.c'. That results in an executable named
a.out. To compile the program
with optimizations, type '
gcc -Wall -O2 hw0.c', which also produces executable
a.out, you would type '
(Note: You don't actually want to do this. See the next heading about obtaining timings.)
hw0.java type '
javac -Xlint hw0.java', which produces
Do the same thing for the other Java programs.
To run it, type '
java -Xmx640M -cp . hw0'. (Again, this is a command you need
to time, so read on.)
On Linux, you can measure the CPU time consumed by any execution using the
$ /usr/bin/time ./a.out
0.12user 0.03system 0:00.16elapsed 95%CPU (0avgtext+0avgdata 66704maxresident)k
0inputs+0outputs (0major+8287minor)pagefaults 0swaps
This executes the command (
./a.out) and then prints information about the resources it consumed.
man time' to obtain more information about the time program and ways to format its output.)
The only information we'll use is the user time ('0.12user', meaning 0.12 seconds
of CPU time consumed while not in the operating system) and the system time
('0.03system', meaning 0.03 CPU seconds spent by the operating system doing
things for this application). The measured time we want is the sum of those two. For this example,
the measured time would be 0.15 seconds.
Measured times are likely to vary quite a bit from one run to the next, even
without changing anything. (This course will explain some of the reasons why.)
Note that all the programs wrap the two array-copying loops with another loop that
causes the copy to be performed 10 times. One goal of that is to reduce the amount of
variability in the measurements.
The distribution includes an optional script,
run.pl, that automates some of the chore of
running the five executables and gathering measurements.
To run it, type '
./run.pl'. It compiles each of the source files (and
with and without optimizations), runs each with the
time command, and reports the
sum of the user and sytem times.
run.pl works everywhere I've tried it (including the CSE virtual machine). It should work for you,
but it is an optional (and unsupported) tool.
So, to summarize:
- Compile and measure each of the Java implementations as they come in the distribution. Compile and measure
the C program with and without optimizations.
- Edit each source file to uncomment the assignment to array
src. Re-compile and re-measure.
- Edit to switch the order of the
j loops. Recompile and re-measure.
- Edit to re-comment out the statement assigning to array
src (with the
j loops still reversed). Re-compile and re-measure.
- Edit to put the loops back in the original order. (At this point the code is the same as it was when you first fetched it.)
Change the code to copy an array of size 8192 x 8192. Then repeat steps 1–4 above.
Collect your results in a short document (PDF, text, or Word) with the following sections:
- The Test System
- A short string describing the system your ran on (e.g., 'my Mac laptop' or 'the CSE home VM on my
Windows laptop' or 'lab Linux workstation' or ...).
- What the CPU is on that system. You can obtain that on any Linux
system using the command '
cat /proc/cpuinfo'. Give us the model name, as listed.
- Test Results
Four tables of numbers giving the measured
CPU time consumed when executing each of the five executables
under the different configurations. Each table should look like this. (It doesn't have
to be exactly this, to every detail of formatting, but please keep your information in the same order;
it makes reading 50 copies of these tables easier if they're all laid out the same way.)
||i then j
||j then i
Answer these questions:
- What are the source code differences among the three Java implementations?
- Pick a single pair of results that most surprised you. What is it about the results that surprised you?
(That is, from the (40*39)/2 pairs of measurement results, pick one pair whose relationship is least like
what you would have guessed.)
- [Optional extra credit: 0 points] None of these programs appear to actually do anything, so one is tempted
to optimize them by simply eliminating all code (resulting in an empty main()). Is that a correct optimization?
Related to that, try compiling this C program, with and without optimization, and then timing running it:
#define SIZE 1000000
int i, j, k;
int sum = 1;
for (i = 0; i < SIZE; i++)
for (j = 0; j < SIZE; j++)
for (k = 0; k < SIZE; k++)
sum = -sum;
Now replace the
printf line with
and compile/run unoptimized and optimized.
printf("Sum is %d\n", sum);
Please turn in a
.docx file using
the Catalyst turn-in page for
this assignment (you can submit even if you're not yet officially enrolled.) The navigation sidebar
above also has a link to the main Catalyst 351 turnin page.
If you bring your results to section on Thursday, we can discuss them as a group and find out whether the class saw any
Grading is basically binary. If you convince us that you actually did the assignment, on time, then full credit.
If you fail to convince us, then no credit.
Note that there are two kinds of work required to "do the assignment"—the mechnical aspects of modifying and measuring the code, and the intellectual effort of thinking about the results.