UIUC - CS 433IBM POWER7

Adam KunkAnil JohnPete Bohman

Quick Facts

Released by IBM in 2010 (~ February) Successor of the POWER6 Shift from high frequency to multi-core Implements IBM PowerPC architecture v2.06

Clock Rate: 2.4 GHz - 4.25 GHz Feature size: 45 nm ISA: Power ISA v 2.06 (RISC) Cores: 4, 6, 8 Cache: L1, L2, L3 – On Chip

References: [1], [5]

Why the POWER7?

PERCS – Productive, Easy-to-use, Reliable Computer System DARPA funded contract that IBM won in order

to develop the Power7 ($244 million contract, 2006)▪ Contract was to develop a petascale supercomputer

architecture before 2011 in the HPCS (High Performance Computing Systems) project.

IBM, Cray, and Sun Microsystems received HPCS grant for Phase II.

IBM was chosen for Phase III in 2006.

References: [1], [2]

Blue Waters

Side note: The Blue Waters system was meant to

be the first supercomputer using PERCS technology.

But, the contract was cancelled (cost and complexity).

History of Power

2004 2001 2007 2010

POWER4/4+

Dual Core Chip Multi Processing Distributed Switch Shared L2 Dynamic LPARs (32)180nm,

POWER5/5+

Dual Core & Quad Core MdEnhanced Scaling2 Thread SMTDistributed Switch +Core Parallelism +FP Performance +Memory bandwidth +130nm, 90nm

POWER6/6+

Dual Core High Frequencies Virtualization + Memory Subsystem + Altivec Instruction Retry Dyn Energy Mgmt 2 Thread SMT + Protection Keys 65nm

POWER7/7+

4,6,8 Core 32MB On-Chip eDRAM Power Optimized Cores Mem Subsystem ++ 4 Thread SMT++ Reliability + VSM & VSX Protection Keys+ 45nm, 32nm

POWER8

Future

First Dual Corein Industry

HardwareVirtualizationfor Unix & Linux

FastestProcessorIn Industry

MostPOWERful &ScalableProcessor inIndustry

References: [3]

POWER7 Layout

Cores: 8 Intelligent Cores / chip (socket) 4 and 6 Intelligent Cores available

on some models 12 execution units per core Out of order execution 4 Way SMT per core 32 threads per chip L1 – 32 KB I Cache / 32 KB D

Cache per core L2 – 256 KB per coreChip: 32MB Intelligent L3 Cache on chip

Core

L2

Core

L2

Memory Interface

Core

L2

Core

L2

Core

L2

Core

L2

Core

L2

Core

L2

GX

SMP

FABRIC

POWER

BUS

Memory++

L3 CacheeDRAM

References: [3]

POWER7 Options (8, 6, 4 cores)

References: [3]

POWER7 TurboCore

TruboCore mode 8 core to 4 Core 7.25% higher core frequency 2X the amount of L3 cache (fluid cache)

Tradeoffs Reduces per core software licenses Increases throughput computing Decreases transactional based

workloads

POWER7 Core

Each core implements “aggressive” out-of-order (OoO) instruction execution

The processor has an Instruction Sequence Unit capable of dispatching up to six instructions per cycle to a set of queues

Up to eight instructions per cycle can be issued to the Instruction Execution units

References: [4]

Pipeline

Instruction Fetch

8 inst. fetched from L2 to L1 I-cache or fetch buffer Balanced instruction rates across active threads Inst. Grouping

Instructions belonging to group issued together Groups contain independent instructions

Branch Prediction

POWER7 uses different mechanisms to predict the branch direction (taken/not taken) and the branch target address.

Instruction Fetch Unit (IFU) supports 3-cycle branch scan loop (to scan instructions for branches taken, compute target addresses, and determine if it is an unconditional branch or taken)

References: [5]

Branch Direction Prediction Tournament Predictor (due to GSEL):

8-K entry local BHT (LBHT)▪ BHT – Branch History Table

16-K entry global BHT (GBHT) 8-K entry global selection array (GSEL)

All arrays above provide branch direction predictions for all instructions in a fetch group (fetch group - up to 8 instructions)

The arrays are shared by all threads

References: [5]

Branch Direction Prediction (cont.)

8-K LBHT directly indexed by 10 bits from instruction fetch address

The GBHT and GSEL arrays are indexed by the instruction fetch address hashed with a 21-bit global history vector (GHV) folded down to 11 bits, one per thread

Value in GSEL chooses between LBHT and GBHT

References: [5]

Branch Direction Prediction (cont.)

Value in GSEL chooses between LBHT and GBHT for the direction of the prediction of each individual branch Hence the tournament predictor!

Each BHT (LBHT and GBHT) entry contains 2 bits Higher order bit determines direction

(taken/not taken) Lower order bit provides hysteresis (history of

the branch)References: [5]

Branch Target Address Prediction

Predicted in two ways: Indirect branches that are not subroutine

returns use a 128-entry count cache (shared by all active threads).

▪ Count cache is indexed by doing an XOR of 7 bits from the instruction fetch address and the GHV (global history vector)

▪ Each entry in the count cache contains a 62-bit predicted address with 2 confidence bits

Subroutine returns are predicted using a link stack (one per thread).

▪ This is like the “Return Address Stack” discussed in lecture

References: [5]

Execution Units

Each POWER7 core has 12 execution units: 2 fixed point units 2 load store units 4 double precision floating point units (2x

power6) 1 vector unit 1 branch unit 1 condition register unit 1 decimal floating point unit

References: [4]

ILP

Exceptions

POWER7 Demo

IBM POWER7 Demo

Visual representation of the SMT capabilities of the POWER7

Brief introduction to the on-chip L3 cache

SMT

Simultaneous Multithreading Separate instruction streams running

concurrently on the same physical processor

POWER7 supports: 2 pipes for storage instructions (load/stores) 2 pipes for executing arithmetic instructions

(add, subtract, etc.) 1 pipe for branch instructions (control flow) Parallel support for floating-point and vector

operationsReferences: [7], [8]

SMT (cont.)

Simultaneous Multithreading Explanation: SMT1: Single instruction execution thread per

core SMT2: Two instruction execution threads per core SMT4: Four instruction execution threads per

core

This means that an 8-core Power7 can execute 32 threads simultaneously

POWER7 supports SMT1, SMT2, SMT4References: [5], [8]

Multithreading History

Thread 1 Executing

Thread 0 Executing

No Thread Executing

FX0FX1FP0FP1LS0LS1BRXCRL

Single thread Out of Order

FX0FX1FP0FP1LS0LS1BRXCRL

S80 HW Multi-thread

FX0FX1FP0FP1LS0LS1BRXCRL

POWER5 2 Way SMT

FX0FX1FP0FP1LS0LS1BRXCRL

POWER7 4 Way SMT

Thread 3 Executing

Thread 2 ExecutingReferences: [3]

TLP

Memory

Memory Access

(Look at section 2.1.4 in http://www.redbooks.ibm.com/redpapers/pdfs/redp4639.pdf)

Cache Overview

Parameter L1 L2 L3 (Local) L3 (Global)

Size 64 KB (32K I, 32K D)

256 KB 4 MB 32 MB

Location Core Core On-Chip On-Chip

Access Time

.5 ns 2 ns 6 ns 30 ns

Associativity

4-way I-cache8-way D-cache

8-way 8-way 8-way

Write Policy

Write Through

Write Back Partial Victim

Adaptive

Line size 128 B 128 B 128 B 128 B

Cache Design Considerations

On-Chip cache required for sufficient bandwidth to 8 cores. Previous off-chip socket interface unable

to scale Support dynamic cores Utilize ILP and increased SMT latency

overlap

L1 Cache

I and D cache split to reduce latency Way prediction bits reduce hit

latency Write-Through

No L1 write-backs required on line eviction

High speed L2 able to handle bandwidth B-Tree LRU replacement Prefetching

On each L1 I-Cache miss, prefetch next 2 blocks

L2 Cache

Superset of L1 (inclusive) Reduced latency by decreasing capacity L2 utilizes L3-Local cache as victim cache

Increased associativity

L3 Cache

32 MB Fluid L3 cache 4 MB of local L3 cache per 8 cores▪ Local cache closer to respective core, reduced latency

L3 cache access routed to the local L3 cache first Cache lines cloned when used by multiple cores

Cache Coherency

eDRAM

Embedded Dynamic Random-Access memory Less area (1 transistor vs. 6 transistor SRAM) Enables on-chip L3 cache ▪ Reduces L3 latency▪ Larger internal bus size which increases bandwidth

Compared to off chip SRAM cache▪ 1/6 latency▪ 1/5 standby power

Utilized in game consoles (PS2, Wii, Etc.)References: [5], [6]

Performance

Closing/Wrap-up

References

1. http://en.wikipedia.org/wiki/POWER7 2. http://en.wikipedia.org/wiki/PERCS 3. Central PA PUG POWER7 review.ppt

http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=1&ved=0CCEQFjAA&url=http%3A%2F%2Fwww.ibm.com%2Fdeveloperworks%2Fwikis%2Fdownload%2Fattachments%2F135430247%2FCentral%2BPA%2BPUG%2BPOWER7%2Breview.ppt&ei=3El3T6ejOI-40QGil-GnDQ&usg=AFQjCNFESXDZMpcC2z8y8NkjE-v3S_5t3A

References (cont.)

4. http://www.redbooks.ibm.com/redpapers/pdfs/redp4639.pdf

5. http://www.serc.iisc.ernet.in/~govind/243/Power7.pdf

6. http://en.wikipedia.org/wiki/EDRAM 7.

http://www.spscicomp.org/ScicomP16/presentations/Power7_Performance_Overview.pdf

8. http://www-03.ibm.com/systems/resources/pwrsysperf_SMT4OnP7.pdf