Kristof Beyls,Erik D’Hollander,

Frederik Vandeputte

ICCS 2005 – May 23

RDVIS: A Tool That Visualizes theCauses of Low Locality and Hints

Program Optimizations

Overview

1. Motivation: cache bottleneck

2. Some theoretical background: reuses

3. View 1: cache-missing reuses

4. View 2: reuse pair clusters corresponding to program optimizations

5. Experimental results

6. Implementation details

7. Conclusion

1. Motivation

• Many programs incur large cache bottlenecks.

• Mainly caused by poor locality (temporal or spatial)

• Temporal locality is hard to optimize automatically in a compiler

• Therefore: need to help programmer to pin-point sources of low temporal locality.

2. Theoretical background

• Stream of memory accesses:accesses:a b c a a breferences: r1 r1 r2 r1 r1 r1basic block: bb1 bb1 bb2 bb1 bb1 bb1

• Reuses / Reuse Distance• Reference pair / Reference pair histogram• Basic Block Vector of Intermediately

executed code.

20

2

Cache miss

reuse distance ≥ cache size

2. Theoretical background

• Stream of memory accesses:accesses:a b c a a breferences: r1 r1 r2 r1 r1 r1basic block: bb1 bb1 bb2 bb1 bb1 bb1

• Reuses / Reuse Distance• Reference pair / Reference pair histogram• Basic Block Vector of Intermediately

executed code.

Reference pair r1-r1

Reuse distance

1

2

10 2

20

2

3. RDVIS by example:matrix multiplication

Reuses between a[i*N+k] at distance 2^9 Reuses between b[k*N+j] at distance 2^17

How to bring reuses of b[k*N+j] closer together?

What separates reuses?

What code is executed between reuses?

3. RDVIS by example:matrix multiplication

Reuses occur betweeniterations of i-loop

Solution: bring iterationsof i-loop inwards

3. RDVIS by example:matrix multiplication

Next to optimize: reuses of A[i*N+k]

3. Matrix multiplication:final resultL1 cache

L2 cacheMain memory

Exec. Time on P4:

Orig: 0.740s

Opt.: 0.223s

Speedup: 3.3

4. Cluster Analysis

• In more complex programs, there can be many arrows.

• Many arrows can often by optimized by the same program transformation.

• Key idea: “When the same code is executed between use and reuse, probably the same program transformation is needed.”

4. Cluster Analysisby example: equake

Many different arrows

contribute to long-distance

reuse

2(bis). Theoretical background

• Stream of memory accesses:accesses:a b c a a breferences: r1 r1 r2 r1 r1 r1basic block: bb1 bb1 bb2 bb1 bb1 bb1

• Reuses / Reuse Distance• Reference pair / Reference pair histogram• Basic Block Vector of Intermediately

executed code of a reference pair.

BBV(Reference pair r1-r1)

.66.66% exec. betw. reuses

bb2bb1Basic block

4. Cluster Analysisby example: equake

LOOP FUSION!

5. Experimental Results

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101112

athlonXP Alpha Itanium average

Speedup

mcf

art

equake

6. Some Implementation Details

• Instrumentation added to GCC 4:– Exact source location info added to all abstract syntax

tree nodes.– Source location info is added in language-specific

front-end (currently only C, Fortran is being added).– Instrumentation occurs in language-independent

middle-end.• Inserts function call for each memory reference• Inserts function call at begin of each basic block• Writes out source location info for memory references and

basic blocks

7. Conclusion

• Visualization indicates reuses at a long distance, and the code that is executed between those reuses.

• Clustering of intermediately executed code leads to reference pairs that are optimizable with the same program transformation.

• Give RDVIS a try: http://www.elis.UGent.be/~kbeyls/rdvis

QUESTIONS?

MCF

AMMP

AMMP