Transcript Milestone Slides

Migration Cost Aware Task Scheduling
18-743 Milestone
Shraddha Joshi, Brian Osbun
10/24/2013
Outline
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Problem description
Approach
Milestone I
Methodology
Results
Observations
Future Work
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Problem Description
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Dynamic schedulers allow task migration during program
execution
Migrating threads to new cores can have hidden costs
 Cold cache misses
 Overhead of moving architectural state
 Congestion on interconnect during transfer
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Most task schedulers ignore migration overhead
Problem statement: quantify and consider the task migration
cost when evaluating scheduling possibilities
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Approach
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Architectural transfer is essentially fixed cost
 Can be modeled independently
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Cache effects have a larger and variable cost
Published theory requires static analysis*
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Determination of “useful” blocks in cache
Reaching memory blocks (RMB): all blocks that may be in cache at a point
Live memory blocks (LMB): all blocks that may be referenced before eviction
Usefulness is intersection of RMB and LMB
Dynamic analysis
 Need prediction!
 For now: everything’s useful!
*Source-Hardy et. al. Estimation of Cache Related Migration Delays for Multi-Core Processors with
Shared Instruction Caches
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Milestone I
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To be able to quantify migration cost in terms of a metric
 Our predictor: past cache accesses
 Our prediction: future cache misses
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Methodology
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Sniper simulation framework
Clustered architecture
 Homogeneous cores
 Clustered L1 caches (shared by 4 cores)
 Shared L2 cache
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Two benchmarks
 Radix (SPLASH-2) (compute intensive)
 x264 (PARSEC) (memory intensive)
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Variables
 Time interval before and after migration
 Cache size and parameters
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Results
Benchmark Comparison
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Additional Misses
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0
0
10000
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40000
-50
50000
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Previous Accesses
Radix
x264
Log. (Radix)
Log. (x264)
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Results
Cache Comparison
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350
Additional Misses
300
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200
150
100
50
0
0
10000
20000
30000
-50
40000
50000
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70000
80000
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Previous Accesses
32k 4way
32k 8way
16k 4way
16k 8way
Log. (32k 4way)
Log. (32k 8way)
Log. (16k 4way)
Log. (16k 8way)
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Observations
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Large cost variance between applications
 Compute-intensive v. memory-intensive
 Radix: 0.115 accesses per instruction
 x264: 0.368 accesses per instruction
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Some sensitivity to cache parameters
 Smaller cache may evict contents anyway
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Future Work
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Refine accuracy of cost mechanism
Correlate future cost with past predictors
Incorporate migration cost metric into scheduling algorithm
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Q&A
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