larrabee eric jogerst cortlandt schoonover francis tan
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Larrabee
Eric JogerstCortlandt Schoonover
Francis Tan
Larrabee
• Intel’s new approach to a GPU
• Considered to be a hybrid between a multi-core CPU and a GPU
• Combines functions of a multi-core CPU with the functions of a GPU
Larrabee
FETCHLarrabee
Fetch
• Utilizes a hardware prefecther• Supports four threads of execution– Separate register files for each thread– Switches threads in order to cover cases where
the compiler is unable to schedule code without stalls or if the prefetcher has not received new instructions
– Inactive thread data is written to the core’s local L2 cache
PIPELINE ORGANIZATIONLarrabee
Pipeline
• Pipeline derived from the dual-issue Pentium processor, which is 5-stages– Short, inexpensive execution pipeline
• Pairing rules for primary and secondary instruction pipes are deterministic– Allows compilers to perform offline analysis with a
wide scope
Pipeline
• Pairing rules for primary and secondary instruction pipes are deterministic– Allows compilers to perform offline analysis with a wide
scope • All instructions can be issued on the primary pipeline– Minimizes the combinational problems for a compiler
• Secondary pipeline can execute a large x86 instruction set– Small and cheap– Power wasted by failing to dual-issue on every cycle is
minimal
Pipeline
• Each core has own pipeline– Based upon the 5 stage Pentium– Dual issues instructions– In order execution
• Pipeline is shared between threads– Hardware can switch between threads that have
instructions that have instructions ready to execute
Pipeline
• Designed software-rendering pipeline to minimize the number of locks and other synchronization events
• Graphics-rendering pipeline written with high-level languages and tools– Enables developers to add innovative rendering
capabilities
SIMD ORGANIZATIONLarrabee
Vector Processor Unit
• 16-wide vector processor unit (VPU) – executes integer, single-precision float, and
double-precision float instructions– VPU and register are approximately one-third the
area of the processor core
• Tradeoff– Increased computational density– Wider VPU’s have higher utilization
Vector Processor Unit
• VPU instructions can be predicated by a mask register
• Mask controls which parts of a vector register or memory location are written and which are left untouched
• Advantages– Reduces branch misprediction penalties – Gives instruction scheduler greater freedom
Number of Cores
• Many-core processor– Planned to have 24 to 48
cores
Number of Cores
Number of Cores
SYSTEM ON-CHIP COMPONENTSLarrabee
System On-Chip Components
• x86 computer cores - Dual issue, in order processors that support the x86 protocol with Larrabee extensions. Connected to ring network and high bandwidth connection to adjacent L2 Cache subset.
System On-Chip Components
• L2 Cache subsets– High bandwidth access to adjacent CPU– Connected directly to the ring network– Coherent cache, uses the ring network to check
coherency when allocating new cache lines
System On-Chip Components
• Ring Network Nodes– Simple bi-directional routers with a 512 bit data
path in each direction (1024 bit total bandwidth)– Organized in rings of 8-16 cores and other devices– Interconnected with other rings– All data moved between cores and fixed
functional units passes through the ring network
System On-Chip Components
• Fixed function logic components– Provides rasterization, interpolation and other
commonly needed functions– Directly connected to the ring network– Will be spread among the cores to provide lower
latency and load balancing on the ring network
System On-Chip Components
• Memory & I/O interface– Provides and manages communication between
the Ring Network and off chip devices.– Manages initial routing and tasking of cores
MEMORY HIERARCHYLarrabee
Memory Interface
ON-CHIP INTERCONNECTLarrabee
On-Chip Interconnect
• Ring interconnect bus• Similar to the Sony Cell processor.
Ring Bus
Ring Bus Features
• Bi-directional• 512 Bits in each direction• Presumably running at core speed.• Each element can take from one direction on
odd CC and other direction on even CC.
Ring Bus Comparisons
• Compared to AMD’s R600/RV670 bus, it is half the bit-width.
• The higher clock speed of Larrabee’s bus should make up for the difference in bandwidth.
Ring Bus Tradeoff Analysis
Ring Bus Tradeoff Analysis
Pros:•Straightforward, not complex•Able to deliver high bandwidth•Great performance if memory clients need high bandwidth.
Cons:•Waste of chip area if most applications don’t need high memory bandwidth•That area could be spent elsewhere to increase performance in a different way.
MULTITHREADING ORGANIZATION Larrabee
Multithreading Organization
• Superscalar• In-Order• Four Threads of execution• Dual issue (with a vector processing unit)
Comparison to OO Execution# CPU cores: 2 out-of-order 10 in-order
Instruction issue: 4 per clock 2 per clock
VPU per core: 4-wide SSE 16-wide
L2 cache size: 4 MB 4 MB
Single-stream: 4 per clock 2 per clock
Vector throughput: 8 per clock 160 per clock
8 per clock 8 per clock
Larrabee Vector Processor
Scheduling Policy
• Software Controlled• More flexible due to the software controlled
scheduling than a typical GPU.
Software Controlled Scheduling
Pros• Flexible: can choose the
scheduler to suit the application.
• Worst case won’t be so bad. (As compared to a hardware encoded scheduling policy)
Cons• Overhead of scheduler
takes a bite out of performance
• Programmer overhead of selecting the correct scheduler.
Criticism
• NVIDIA– “like a GPU from 2006”– Unrealistic performance projections– Motivated by interest to retain market share
Possible Market
• Dreamworks Animation• Xbox / Playstation• Scientific research
Questions?
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