PAPI Development Team Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team

PAPI Evaluation

Patricia J. Teller

, Maria G. Aguilera, Thientam Pham, and Roberto Araiza (Leonardo Salayandia, Alonso Bayona, Manuel Nieto, and Michael Maxwell)

University of Texas-El Paso

.

Supported by the Department of Defense PET Program SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Main Objectives

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team Provide DoD users with a set of documentation that enables them to

easily collect, analyze, and interpret hardware performance data

that is highly relevant for analyzing and improving performance of applications on HPC platforms. SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Evaluation: Objectives

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team

Understand and explain

various PAPI metrics counts obtained for Determine reasons why counts may be different from what is expected

Calibrate

counts, excluding PAPI overhead Work with vendors and/or the PAPI team to

fix

errors Provide DoD users with information that will allow them to effectively use collected performance data SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Evaluation: Methodology - 1

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team

1.

2.

3.

4.

Micro-benchmark

: design and implement a micro-benchmark that facilitates event count prediction

P

rediction

: predict event counts using tools and/or mathematical models

D

ata collection -1

: collect hardware-reported event counts using PAPI

Data collection -2

: collect predicted event counts using a simulator (not always necessary or possible) SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Evaluation: Methodology - 2

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team

5.

6.

7.

Comparison

: compare predicted and hardware-reported event counts

Analysis

: analyze results to identify and possibly quantify differences

Alternate approach

: when analysis indicates that prediction is not possible, use an alternate means to either verify reported event count accuracy or demonstrate that the reported event count seems reasonable SC 2003, Phoenix, AZ – November 17-20, 2003

Computer Science Department University of Texas at El Paso PAPI Development Team

Example Findings - 1

PCAT Performance Counter Assessment Team • Some hardware-reported event counts mirror expected behavior , e.g., number of floating-point instructions on the MIPS R10K and R12K.

• Other hardware-reported events can be calibrated , by subtracting that part of the event count associated with the interface (

overhead

or

bias error

), to mirror expected behavior , e.g., number of load instructions on the MIPS and POWER processors and instructions completed on the POWER3. • In some cases, compiler optimizations effect event counts , e.g., the number of floating-point instructions on the IBM POWER platforms. SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Example Findings - 2

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team • Very-long instruction words can affect event counts, e.g., on the Itanium architecture the number of instruction cache misses and instructions retired are dilated by no ops used to compose very long instruction words. • The definition of the event count may be non-standard and, thus, the associated performance data may be misleading , e.g., instruction cache hits on the POWER3.

• The complexity of hardware features and lack of documentation can make it difficult to understand how to tune performance based on information gleaned from event counts —example: data prefetching, page walker.

SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Example Findings - 3

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team • Although we have not been able to determine the algorithms used for prefetching , the ingenuity and performance of these mechanisms is striking. • In some cases, more instructions are completed than issued on the R10K .

• The DTLB miss count on the POWER3 varies upon the method used to allocate memory depending (i.e., static, calloc or malloc).

• Hardware SQRT on POWER3 not counted in total floating-point operations unless combined with another floating-point operation .

SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Publications

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team • • •

Papers

• • • •

DoD Users Group Conference

2003.

(with Shirley Moore), June

LACSI 2002

(with Shirley Moore), October 2002.

DoD Users Group Conference

(with members of PAPI team), June 2002.

“Hardware Performance Metrics and Compiler Switches: What you see is not always what you get,” with Luiz Derose, submitted for publication.

Posters

• “Hardware Performance Counters: Is what you see, what you get?, Poster SC2003.

Presentations

• •

PTools Workshop

, September 2002. Conference Presentations for Papers above.

SC 2003, Phoenix, AZ – November 17-20, 2003

Computer Science Department University of Texas at El Paso PAPI Development Team

Calibration Example - 1

PCAT Performance Counter Assessment Team Instructions completed  PAPI overhead: 139 on POWER3-II

Number of C-level instructions Predicted Count Mean Reported Count Standard Deviation Reported - Predicted

0 (base)

0 139 0 139

10

34 173 0 139

100

340 479 0 139

1000

3400 3539 0 139

10000

34000 34139 0 139

100000

340000 340139 0 139 SC 2003, Phoenix, AZ – November 17-20, 2003

Computer Science Department University of Texas at El Paso PAPI Development Team

Calibration Example - 2

PCAT Performance Counter Assessment Team Instructions completed  PAPI overhead: 141 for small micro benchmarks

Number of C-level instructions Predicted Count Mean Reported Count Standard Deviation Reported – Predicted

0 (base)

0 141 0 141

10

34 175 0 141

100

340 481 0 141

1000

3400 3541.04

1.14e-5 141.04

10000

34000 34152.57

0.000339

152.57

100000

340000 340267.9

0.000373

267.90

SC 2003, Phoenix, AZ – November 17-20, 2003

Computer Science Department University of Texas at El Paso

RIB/OKC for

PCAT PAPI Development Team

Evaluation Resources

• Object-oriented data model to store benchmarks, results and analyses Performance Counter Assessment Team • Information organized for ease of use by colleagues external to PCAT • To be web-accessible to members • Objects linked between them as appropriate Benchmark General description of a benchmark Case Specific implementation and results Machine Description of platform Organization Contact information SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

PCAT RIB/OKC Data Repository Example

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team Benchmark name: DTLB misses Development date: 12/2002 Benchmark type: Array Abstract: Code traverses though an array of integers once at regular strides of PAGESIZE. The intention is to create compulsory misses on each array access. Input parameters are: Page size (bytes) and array size (bytes). The number of misses normally expected should be: Array Size / Page Size.

Links to files Files included: dtlbmiss.c, dtlbmiss.pl

About included files: dtlbmiss.c, benchmark source code in C, requires pagesize and arraysize parameters for input and outputs PAPI event count.

dtlbmiss.pl, perl script that executes the benchmark 100 times for increasing arraysize parameters and saves benchmark output to text file. Script should be customized for pagesize parameter and arraysize range.

SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

PCAT RIB/OKC Example Case Object

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team Name: Date: DTLB misses on Itanium 12/2002 Compiler and options: gcc ver 2.96 20000731 (Red Hat Linux 7.1 2.96-101) –O0 PAPI Event: Native Event: PAPI_TLB_DM, Data TLB misses DTLB_MISSES Experimental methodology: Ran benchmark 100 times with perl script, averages and standard deviations reported Input parameters used: Page size = 16K, Array size = 16K – 160M (increments by multiples of 10) Links to other objects Platform used: HP01.cs.utk.edu (Itanium) Developed by: PCAT Benchmark used: DTLB misses SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

PCAT RIB/OKC Example Case Object

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team Results summary: Reported counts closely match the predicted counts, showing differences close to 0% even in the cases with a small number of data references, which may be more susceptible to external perturbation. The counts indicate that prefetching is not performed at the DTLB level. Included files and description: dtlbmiss.itanium.c: Source code of benchmark, instrumented with PAPI to count PAPI_TLB_DM dtlbmiss.itanium.pl: dtlbmiss.itanium.txt: Perl script used to run the benchmark Raw data obtained, each column contains results for a particular array size, each case is run 100 times (i.e., 100 rows included) dtlbmiss.itanium.xls: Includes raw data, averages of runs, standard deviations and graph of % difference between reported and predicted counts dtlbmiss.itanium.pdf: Same as dtlbmiss.itanium.xls

SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

Contributions

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team Infrastructure that facilitates user access of hardware performance data that is highly relevant for analyzing and improving the performance of their applications on HPC platforms. Information that allows users to effectively use the data with confidence.

SC 2003, Phoenix, AZ – November 17-20, 2003

PAPI Development Team

QUESTIONS?

Computer Science Department University of Texas at El Paso PCAT Performance Counter Assessment Team SC 2003, Phoenix, AZ – November 17-20, 2003