CS 377P: Programming for Performance

Assignment 1: Performance counters

Due date: Feb 6, 2026, 10:00PM

Late submission policy: Submissions can be at most 1 day late. There will be a 10% penalty for late submissions.

Description

1) Write C code for the 6 variants of matrix-matrix multiply (MMM)  you can generate by permuting loops in the standard three-nested loop version of MMM. The data type in the matrix should be complex double. You will need to include the header file <complex.h>.

• Instrument your implementations to use PAPI to measure:
  • Total cycles
  • Total instructions
  • Total Load Store Instructions
  • Total Floating Point Instructions
  • L1 data cache accesses and misses
  • L2 data cache accesses and misses

  Remember that you may not be able to measure all these values at the same time.
  
• Compile your code in gcc with flags  '-lpapi -O3'
• Collect these measurements for the following 8 matrix sizes: 50x50, 100x100, 200x200, 400x400, 800x800, 1200x1200, 1600x1600 and 2000x2000. To ensure that there is no interference, make sure that you are the only one running experiments on the machine, and do one measurement at a time.
• You can collect measurements on the following 5 CS machines (with your CS login): orcrist-20.cs.utexas.edu, orcrist-21, orcrist-24, orcrist-25, orcrist-28.
  
• Create a table in which the rows correspond to the loop-order variant (i-j-k, j-i-k, j-k-i, k-j-i, i-k-j, k-i-j) and the columns correspond to the matrix sizes, and fill in each position in the table with four values: L1 and L2 miss rate, total load and store instructions, and the number of committed floating point instructions. You can create four separate tables if you prefer.
  
• Answer the following questions.
  
• For the smallest matrix size, do the L1 and L2 miss rates vary for the different loop-order variants? Do they vary for the larger matrix sizes? Is there any difference in behavior between the different problem sizes? Can you explain intuitively the reasons for this behavior?
• Re-instrument your code by removing PAPI calls, and using clock_gettime with CLOCK_THREAD_CPUTIME_ID to measure the execution times for the six versions of MMM and the eight matrix sizes specified above. How do your timing measurements compare to the execution times you obtained from using PAPI? Repeat this study using CLOCK_REALTIME. Explain your results briefly.
  

    Hint: To check cache sizes on the machine, run:  lscpu

2) Answer the following questions, using a few sentences for each one.

• What is Moore's Law? What is Amdahl's Law? Which of these is an empirical observation and which of these is a mathematical fact?
  
• In the earliest ISA's, memory could only be accessed using the absolute addressing mode. What problems arise in implementing loops with such an ISA? How we get around these problems in today's ISA's?
  
• What are data and control dependences? Give simple examples to illustrate these concepts.
• Explain out-of-order execution and in-order retirement/commit. Why do high-performance processors execute instructions out of order but retire them in order? What hardware structure(s) are used to implement in-order retirement?
• Consider the invariants for retirement in OOO execution with renaming shown in the lecture slides. Why do we need to check the condition "(R3.PR# = ROB[n].PR#")  before updating R3.v ? Explain what would go wrong if we did not check this condition before updating R3.v.
• What is the typical branch prediction accuracy in modern processors? In lecture, we said "A correctly predicted branch is essentially a NO-OP as far as performance is concerned." Explain this statement in a few sentences.

Deliverables

Submit (in canvas) the following two files:

• A .tar.gz file with your code, a README.txt and a Makefile. 
• The README.txt describes how to run your program and what the output will be. A reasonable output will be pairs of "name of measured event, value".
• With the Makefile, your code should be compiled on the 5 CS machines by running only "make".
• A report (in .pdf) containing the tables, and the answers to the questions in both parts.
  

Grading

Code: 40 points

Measurements (plots): 30 points

Explanation: 10 points

 Answers to short questions in (2): 20 points

PAPI:

To see which papi counters are available on a host, run:

papi_avail

To see which papi counters can be collected at the same time, run:

papi_event_chooser

The PAPI website has a lot of information: https://icl.utk.edu/papi/.
Here's the PAPI users guide:https://icl.utk.edu/projects/papi/files/documentation/PAPI_USER_GUIDE_23.htm

"Warning! num_cntrs is more than num_mpx_cntrs" can be ignored.