02/02/2005 ucy computer architecture group andreas artemiou 1 power awareness through selective...
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02/02/2005 UCY Computer Architecture Group Andreas Artemiou
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Power awareness through selective Dynamically Optimized Traces
Roni Rosner, Yoav Almong, Micha Moffie, Naftali Schwartz and Avi MendelsonMicroprocessor Reseacrch
Intel Labs, Haifa, IsraelISCA 2004
Presented at the
Computer Architecture Group
University of Cyprus
by Andreas Artemiou
02-02-2005
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PARROT Concept
Achieve higher performance with reduced energy consumption through gradual optimization of frequently executed code traces
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Why energy is important?
Current processors have become power limited, that is, they can only operate at limited frequency, preventing them from achieving their full microarchitectural performance potential.
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Modern Processors
Modern Processors Front end: fetch, dispatch, issue Back end: execute, commit
The front end bandwidth is crucial for the overall performance of the system
The power and complexity of dynamic scheduling depends on execution bandwidth as well as on program behavior and the instruction window size
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Amdahl’s Law
This work try to take advantage of the hot/cold (90/10) paradigm following the Amdahl's Law
That is a small portion of the static program code is responsible for the most of its dynamic execution
PARROT applies similar principles like those in profiling compilers, dynamic translators etc.
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Identifying techniques
To identify frequently executed code sections we have: Software based techniques like those proposed by
Mahlke S.A. et al (MICRO 1992) Hardware based techniques like those by Merten M.C.
et al (ISCA 26, 1999) PARROT aims to aggressively exploit the hot/cold
paradigm in hardware for the benefit of both: processor performance and power awareness
PARROT=Power-Aware aRchitecture Running Optimized Traces
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What this work does?
Examine several microarchitectural alternatives based on the concept of PARROT (referred as PARROT Microarchitectures)
Organized around optimized trace cache. What is a trace cache;
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PARROT parts
PARROT (3 parts) Identify the most frequent sequences of
program code with a mechanism that does trace selection and filtering
Aggressively optimize them once with a dynamic optimizer
Efficiently execute them many times (after stored in the trace cache)
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PARROT parts (2)
Key factors for power awareness: Gradual construction of traces Pipeline decoupling Specific Trace optimizations
Cold part: Limiting the hardware for the cold part may exact a
small price in performance Hot part:
More aggressive hardware may be used to improve performance/power tradeoffs for the dominant hot segments of the code
Results show that no additional energy is spent for the optimization of hot traces
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Related work
PARROT's uniqueness is: in the application of decoupling and Dynamic optimization techniques to achieve better
performance with less energy consumption Other similar ideas:
rePLay (Patel J.S. and Lumetta S.S. IEEE Transactions on Computer VOL.50 NO. 6 JUNE 2001) – Hardare based
Turboscalar (B. Black and J. P. Shen ISCA 27, June 2000) – Hardware based
DAISY (K. Ebcioglu and E.R. Altman ISCA 24, 1997) – Software based
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Motivation
Parrot is based on the following observations The working set of a program is relatively small Much of the complexity excesses of modern OOO
processors results from handling rare cases Small segments of code which are repeatedly
executed (“hot traces”) typically cover most of the program´s working set
The hot segments of the code behave differently than the rest of the code, namely they are more regular and predictable, and consequently they exhibit higher potential for ILP extraction than the other, less frequent executed parts of the code
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PARROT Split Microarchitecture
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Microarchitecture Components
Front-end and execution pipelines are tuned for either cold or hot portions of the code
Background components post process the instruction flow out of the foreground pipeline making “off critical path” decisions such as when to move from the cold subsystem to the hot subsystems and when to apply further optimizations
Synchronization elements are required for arbitrating and switching states between pipelines and for preserving global program order
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Cold/Hot Subsystems
A trace cache can be very efficient in handling hot code, provided this code has been sufficiently well identified (Previous studies)
We base the cold subsystem on instructions fetched from an instruction cache whereas the hot subsystem is based on traces fetched from a trace cache
Power awareness and trace cache effectiveness limit trace construction and trace cache insertion to frequently executed code sections
PARROT gradually applies dynamic optimizations – the hotter the trace is, the more aggressive power-aware optimizations are applied
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Decoding/Optimizing
Reusability of hardware work and results is important for both performance and energy savings
In PARROT, the trace-cache stores: decoded traces and is thus a container for
reuse of decoding results optimized traces allowing multiple reuses of
trace optimizations
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Dynamic Optimization
Dynamic Optimizations advantages: Dynamic information available (outcome of trace
internal branches) enables optimizations that are impossible for a static compiler.
Decoupling these optimizations allows more aggressive optimizations than on-the-fly optimizations that can be performed within a standard execution of the pipeline
To take full advantage of these optimizations atomicity of a trace is assumed. That permits very aggressive optimizations across basic-block boundaries
Architectural transparency (the hardware is able to optimize legacy code without the need of recompilation)
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Traces
An execution trace is a sequence of operations representing a continuous segment of the dynamic flow of a program. Trace may contain execution beyond CTI and so a trace may extend over several basic blocks
In the current study they consider decoded atomic traces Decoded = contains decoded micro-operations (uops)
and enable reuse of decode activity, thus saving energy
Atomic traces are single-entry and single-exit
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Trace Selection
Trace selection is the process of constructing particular traces out of a dynamic sequence of instructions. It may be: Deterministic: if applied to the fully
predictable sequence of in-order committed instructions
Speculative: if applied to any previous stage in pipeline which instructions are potentially mispredicted
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Trace Construction Criteria Deterministic selection criteria:
Capacity limitation: traces are constructed into frames of at most 64 uops
Complete basic blocks: with the exceptions of large basic blocks, traces always terminate on CTIs
Terminating CTIs: all indirect jumps and software exceptions terminate basic blocks, except RETURN instructions. In addition, backward taken branches terminate a trace
RETURN instructions terminate traces only if they exit the outermost procedure context already encountered in the current trace
If two or more consecutive traces are identical, they are joined into a single trace, until capacity limit is reached (achieves the effects of explicit loop unrolling)
Unique trace identifiers (TIDs) can be compared into a single address and a sequence of branch directions (taken/not taken)
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Split Execution Microarchitecture
A split-execution implementation, consists of two disjoint sub-systems for the cold and for the hot paths.
Different execution engines can be employed by each subsystem (wider execution engine, higher bandwidth etc)
There is an optimized unified-execution engine that shares the execution resources between the hot and cold subsystems
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Unified Core Microarchitecture
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Pipeline Phases
The description following applies to both split-execution and unified execution implementations.
Both cold and hot pipelines operate in two phases: foreground phase which is responsible for
the fetch to execution pipeline Background phase selects for selecting
frequent parts of the just executed code, optimizing them and potentially promoting them to a “hotter” level
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Background phase
The background phase of the cold subsystem identifies frequent IA32 instruction sequences and captures them as traces in the trace cache.
Composed of TID selection, TID hot-filtering, trace-construction and insertion into trace cache
Continuous training of both trace predictor and hot filter is assured.
Only those TIDs that pass the hot-filter continue to the trace construction stage.
The background phase of the hot subsystem identifies the most frequent traces, optimizes them and inserts them back into trace cache.
Post-processing is used gradually, so the longer a trace is used the more aggressive optimizations are applied to it
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Detailed Schema
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Predictors
Two predictors: Branch predictor predicts the next cache line to be
fetched from the I$ for execution on the cold pipeline. A trace predictor predicts the TID of the next trace to
be fetched from the trace cache and executed on the hot pipeline
Both based on a global history register (GHR) GHR is updated for each CTI being executed
Both support speculative update upon fetch and real upon commit
NOTE!!! Is important that a trace predictor may predict a TID that reflect a trace not present in the trace cache.
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Fetch Selector
Fetch selector chooses between the execution pipelines by consulting both the lower priority branch predictor and the higher priority trace predictor
When the trace predictor is successful in making a next TID prediction the hot pipeline is selected and if a trace is successfully fetched it is executed on the hot pipeline
All other cases result in cold pipeline directed by branch prediction
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Foreground phase
In the foreground phase the pipelines are executing sequences of uops originating from either cold instructions or hot traces.
A split core enables core specialization: The cold core may focus on the execution of rare but
complex operations or be less performance aggressive while the hot core may excel at aggressive execution of atomic traces, employ simplified renaming schemes or rely on dynamic scheduling performed by the optimizer
On the other hand split core increases die size and introduces complexities with cold/hot stage switches
A unified core reduces both die size and idle power This study considers standard superscalar out-of-order cores
only, in both split and unified configurations
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Switch Mechanism in Split Core
For the split core: The state switch mechanism ensures that
values computed and stored in the register file of one core are used at the appropriate time and place in the second
By tracking for each register the last writer uop preceding the switch and the first reader uop of the code following the switch, and assuring that the reader is not executed until writer completes writeback and the value has communicated to the second core
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Commit Stage
The commit stage is responsible for committing IA32 instructions to the architectural state.
Two synchronization issues: to commit instruction in program order (in a split
microarchitecture instructions must contain markers to reconstruct global order)
The atomic hot traces should be committed at once as a single entity, requiring a mechanism for state accumulation
Note that for such a gradual scheme only moderate enlargement of non critical machine resources is necessary.
If any intermediate event prevents full completion of the trace, the remaining uops are flushed and the architectural stage returns before the fetch of the trace. This may happen from exceptions, failed assert uops or from external interrupts!
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Post Processing
For post-processing cold code, PARROT employs a deterministic TID/trace build scheme.
Uops from cold committed instructions are collected until a termination condition of a trace is reached. Then a new TID is generated from the entry address and the CTI’s, and this TID is used to train the trace predictor
If this TID is identified as frequent, the collected uops are used to construct an executable trace that can be inserted into the trace cache.
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Filtering Mechanisms
PARROT employs two filtering mechanisms: Hot filter to select frequent TID’s from among those
constructed on cold pipeline Blazing filter which is used for selecting the most frequent
TID’s from among those executed on the hot pipeline Both are small caches that maintain counters for each TID Each trace execution increment the counter of the executed TID Once the hot filter threshold is reached, the trace is constructed
and inserted into the trace cache. When the blazing filter threshold is reached, the execution trace
is optimized and written back to the trace cache, replacing the original.
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Optimizations PARROT employs dynamic optimizations on blazing
traces. The optimizations can be classified as:
general purpose which are independent of the underlying execution core (logic simplifications, constant propagation and dead code elimination)
or core-specific which include functional transformations as (micro-operation fusion and SIMDification and global transformations)
Optimization results in: Uop reduction Dependency elimination Simplified renaming Improved scheduling
Virtual renaming results to power/energy savings
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Simulation Framework
Performance simulation Energy simulation
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Performance Simulation (1)
Incorporates full memory hierarchy Newly designed components for the post
processing phases Software architecture includes a generic
highly configurable object oriented execution core class which can be instantiated with a variable number of execution cores of widely differing characteristics
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Performance Simulation (2)
One feature of the simulation framework is the abstract instruction which can be defined as a “commitable work unit” and so has a different interpretation within the cold and hot pipelines i.e. in cold pipeline is an instruction, in hot pipeline is the trace
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Energy Simulation (1)
Use tools which are based on a combination of the WATTCH-like and TEM2P2EST like approaches.
Assume uniform leakage In space In time
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Energy Simulation (2)
Total leakage: LE=PMAX*(0.05*M+0.4*K)* CYC PMAX is tha average dynamic-power of the base OOO
model. 0.05 is the technology constant for each MB of level 2
cache 0.4 is the technology constant for the standard core K is the factor by which the current core is larger than
the standard OOO core CYC is the number of cycles the application is running
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Models (1)
Model a variety of configurations Reference model (N)
Standard 4-wide OOO machine Narrow is the different variants of this
standard 4-wide Wide is the different variants of a more
generous 8-wide machine Created a theoretical configuration where all
stages are wide (W)
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Models (2)
PARROT configurations are denoted TON for narrow TOW for wide TOS for split
T stands for selective trace cache O stands for dynamic optimizations Another two models
TN (without trace optimization) TW (without trace optimization)
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Models (3)
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Benchmarks
SpecInt2000: 30M instructions SPECFP 2000: 30M instructions OFFICE/Windows applications: 100M
instructions Multimedia: from 30M to 100M instructions DotNet: 100M instructions
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Metrics
For processor performance: IPC Total energy Cubic-MIPS-per-WATT(CMPW)
Parameters characterizing PARROT: Coverage Uop reduction Energy breakdown
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Results
Presents the results of alternative enhancements applied to the reference machines N and W
The TOS conceptual microarchitecture statistics are presented only as a reference for alternative future development
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Performance and Power Awareness (1)
TW/W raises 7% TN/N negligible 2% increase TON/N 17% performance improvement TOW/W 25% improvement
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Performance and Power Awareness (2)
All extensions of the wide machine actually save energy
Extensions of the narrow even with PARROT style optimizations increase energy
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Performance and Power Awareness (3)
CMPW criterion weights both performance gain and energy loss here: TON/N about 32% TOW/W about 92%
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Front-End Capabilities (1)
N model with a 4K-entries branch predictor TON with branch predictor (cold) and trace
predictor (hot) 2k entries each
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Front-End Capabilities (2)
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Optimizer Capabilities (1)
Average uop reduction with PARROT 19% Average dependency reduction 8%
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Optimizer Capabilities (2)
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Energy Breakdown (1)
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Conclusions (1)
Presents PARROT Improving processor performance and
power awareness Consists of asymmetric decoupling of the
processor into subsystems responsible for handling the cold-infrequent and hot-frequent portions of code
Designs each part according to different power and performance considerations
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Conclusions (2)
The presented simulation results demonstrate that applying the PARROT concept to a standard 4-wide, OOO processor yields comparable performance to an 8-wide processor, however, consuming significantly less energy
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Future Work
One major topic for future research is related to split-core micro architectures
Investigate potential advantage of such design for establishing even better performance/energy tradeoffs by considering different alternatives
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Thank you!!!!