Multicore: Panic or Panacea

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Multicore Panic or Panacea?

• Mikko H. Lipasti
• Associate Professor
• Electrical and Computer Engineering
• University of Wisconsin Madison

http//www.ece.wisc.edu/pharm
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Multicore Mania

• First, servers
• IBM Power4, 2001
• Then desktops
• AMD Athlon X2, 2005
• Then laptops
• Intel Core Duo, 2006
• Soon, your cellphone
• ARM MPCore, prototypes for a while now

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What is behind this trend?

• Moores Law
• Chip power consumption
• Single-thread performance trend
• source Intel

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Dynamic Power

• Static CMOS current flows when active
• Combinational logic evaluates new inputs
• Flip-flop, latch captures new value (clock edge)?
• Terms
• C capacitance of circuit
• wire length, number and size of transistors
• V supply voltage
• A activity factor
• f frequency
• Future Fundamentally power-constrained

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Easy answer Multicore
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Amdahls Law
n
f
CPUs
1
f
1-f
Time

• f fraction that can run in parallel
• 1-f fraction that must run serially

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Fixed Chip Power Budget
n
CPUs

• Amdahls Law
• Ignores (power) cost of n cores
• Revised Amdahls Law
• More cores ? each core is slower
• Parallel speedup lt n
• Serial portion (1-f) takes longer
• Also, interconnect and scaling overhead

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Fixed Power Scaling

• Fixed power budget forces slow cores
• Serial code quickly dominates

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Predictions and Challenges

• Parallel scaling limits many-core
• gt4 cores only for well-behaved programs
• Optimistic about new applications
• Interconnect overhead
• Single-thread performance
• Will degrade unless we innovate
• Parallel programming
• Express/extract parallelism in new ways
• Retrain programming workforce

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Research Agenda

• Programming for parallelism
• Sources of parallelism
• New applications, tools, and approaches
• Single-thread performance and power
• Most attractive to programmer/user
• Chip multiprocessor overheads
• Interconnect, caches, coherence, fairness

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Finding Parallelism

• Functional parallelism
• Car engine, brakes, entertain, nav,
• Game physics, logic, UI, render,
• Automatic extraction UW Multiscalar
• Decompose serial programs
• Data parallelism
• Vector, matrix, db table, pixels,
• Request parallelism
• Web, shared database, telephony,

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Balancing Work

• Amdahls parallel phase f all cores busy
• If not perfectly balanced
• (1-f) term grows (f not fully parallel)
• Performance scaling suffers
• Manageable for data request parallel apps
• Very difficult problem for other two
• Functional parallelism
• Automatically extracted
• Scale power to mismatch Multiscalar

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Coordinating Work

• Synchronization
• Some data somewhere is shared
• Coordinate/order updates and reads
• Otherwise ? chaos
• Traditionally locks and mutual exclusion
• Hard to get right, even harder to tune for perf.
• Research Transactional Memory UW Multifacet
• Programmer Declare potential conflict
• Hardware and/or software speculate check
• Commit or roll back and retry

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Single-thread Performance

• Still most attractive source of performance
• Speeds up parallel and serial phases
• Can use it to buy back power
• Must focus on power consumption
• Performance benefit Power cost

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Single-thread Performance

• Hardware accelerators and circuits
• Domain-specific UW MESA
• Reconfigurable UW Compton
• VLSI and design automation UW WISCAD, Kursun
• Increasing frequency
• Seems prohibitive clock power
• Clever clocking schemes can help UW Pharm
• Increasing instruction-level parallelism
• UW Multiscalar, UW Pharm, UW Smith
• Without blowing power budget
• Alternatively, reduce power for same performance

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Chip Multiprocessor Overheads

• Core Interconnect UW Pharm
• 80 of chip power Borkar, ISLPED 07 panel
• Need fundamentally different approach
• Revisit circuit switching
• Cache coherence UW Multifacet, Pharm
• Match workload behavior
• Optimize for on-chip communication

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Chip Multiprocessor Overheads

• Shared caches UW Multifacet, Multiscalar, Smith
• On-chip memory can be shared
• Optimize replacement, replication
• Fairness UW Smith
• Maintain Performance isolation
• Share resources fairly (memory, caches)

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Research Groups _at_ UW
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Conclusion

• Forecast
• Limited multicore (4) is here to stay
• Manycore (gt4) will find its place
• Hardware Challenges
• Single-thread performance and power
• Multicore overhead
• Software Challenges
• Finding application parallelism
• Creating correct parallel programs
• Creating scalable parallel programs

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Questions?

• http//www.ece.wisc.edu/pharm