VEAL: Virtualized Execution Accelerator for Loops

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VEAL Virtualized Execution Accelerator for Loops

• Nate Clark1, Amir Hormati2, Scott Mahlke2
• 1 Georgia Tech., 2U. Michigan

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How to get Efficiency?

• Microarchitecture changes
• Multi- / many-core
• Heterogeneity

STI Cell
Core2 Duo
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How is Heterogeneity Used?
Program
Hetero.
GPP
Control Statically Placed in Binary
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Problem With Static Control
CPU
Program

• Not forward/backward compatible

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Solution Virtualization

• Abstract accelerator features
• Reexamine compiler algorithms
• Key do the hard stuff offline

Offline
Online
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This Paper

• Examines loops as heterogeneity target
• ASICs often implement loops
• Design a generalized loop accelerator
• Not covered in this talk
• Explore how to virtualize loop accelerators
• I.e. abstract the accelerator interface

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Loop Accelerator Template
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Why More Efficient Than GPP?

• Simple control flow
• Decoupled memory accesses
• I-Cache unnecessary
• Customize execution resources for loops

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Proposed Loop Accelerator

• 1 CCA
• 2 Int units
• 16 regs
• Memory (4x)
• 16 Input streams
• 8 Output streams
• 0.8 mm2, 90nm

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Modulo Scheduling

• High quality software pipelining technique
• Simple control structure (low HW cost)
• - Can be slow, i.e., hard to do dynamically
• - Loops no side exits, no while, if convertible

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Benchmark Execution Time
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Modulo Scheduling Basics
FU C












Kernel


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Modulo Scheduling Example
1. CCA Mapping 2. II Calculation 3. Priority 4.
Scheduling 5. Reg. assignment/
communication
CCA Int Int



CCA Int Int

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3
4
5
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Time
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Priority 2, 4, 6 3, 5 7
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Measured Scheduling Overhead
70 Priority, 19 CCA
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Supporting Hybrid Compilation
Loop 1 ld 2 add 3 sub 4 brl CCA 5 or 6 or 7
add 8 str CCA and sub xor ret
Data 0 1 4 6 3 Loop 1 ld 2 add 3 sub 4 brl
CCA 5 or
Loop 1 ld 2 add 3 sub and sub xor 5 or 6
or 7 add 8 str
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Speedups
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Summary

• Virtualization key to heterogeneity
• VEAL speedup 2.54
• 2.63 w/o translation (i.e., not binary
  compatible)
• 2.17 fully dynamic
• CCA and priority 89 overhead
• mpeg2dec 2.1 vs. 1.15

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Thank you!

• Questions?
• http//www.cc.gatech.edu/ntclark
• http//cccp.eecs.umich.edu/