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Computer Architecture 2008 – Advanced Topics 1 Computer Architecture Advanced Topics.

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Presentation on theme: "Computer Architecture 2008 – Advanced Topics 1 Computer Architecture Advanced Topics."— Presentation transcript:

1 Computer Architecture 2008 – Advanced Topics 1 Computer Architecture Advanced Topics

2 Computer Architecture 2008 – Advanced Topics 2 Pentium ® M Processor

3 Computer Architecture 2008 – Advanced Topics 3 Pentium ® M Processor  Intel’s 1st processor designed for mobility –Achieve best performance at given power and thermal constraints –Achieve longest battery life BaniasDothan transistors77M140M process130nm90nm Die size84 mm 2 85mm 2 Peak power24.5 watts21 watts Freq1.7 GHz2.1GHz L1 cache32KB I$ + 32KB D$ L2 cache1MB2MB

4 Computer Architecture 2008 – Advanced Topics 4 Performance per Watt  Mobile’s smaller form-factor decreases power budget –Power generates heat, which must be dissipated to keep transistors within allowed temperature –Limits the processor’s peak power consumption  Change the target –Old target: get max performance –New target: get max performance at a given power envelope  Performance per Watt  Performance via frequency increase –Power = CV 2 f, but increasing f also requires increasing V –X% performance costs 3X% power  Assume performance linear with frequency  A power efficient feature – better than 1:3 performance : power –Otherwise it is better to just increase frequency –All Banias u-arch features (aimed at performance) are power efficient

5 Computer Architecture 2008 – Advanced Topics 5 Higher Performance vs. Longer Battery Life  Processor average power is <10% of the platform –The processor reduces power in periods of low processor activity –The processor enters lower power states in idle periods  Average power includes low-activity periods and idle-time –Typical: 1W – 3W  Max power limited by heat dissipation –Typical: 20W – 100W  Decision –Optimize for performance when Active –Optimize for battery life when idle Display (panel + inverter) 33% CPU 10% Power Supply 10% Intel® MCH 9% Misc. 8% GFX 8% HDD 8% CLK 5% Intel® ICH 3% DVD 2% LAN 2% Fan 2%

6 Computer Architecture 2008 – Advanced Topics 6 Static Power  The power consumed by a processor consists of –Active power: used to switch transistors –Static power: leakage of transistors under voltage  Static power is a function of –Number of transistors and their type –Operating voltage –Die temperature  Leakage is growing dramatically in new process technologies  Pentium ® M reduces static power consumption –The L2 cache is built with low-leakage transistors (2/3 of the die transistors)  Low-leakage transistors are slower, increasing cache access latency  The significant power saved justifies the small performance loss –Enhanced SpeedStep ® technology  Reduces voltage and temperature on low processor activity

7 Computer Architecture 2008 – Advanced Topics 7 Less is More  Less instructions per task –Advanced branch prediction reduces #wrong instructions executed –SSE instructions reduce the number of instructions architecturally  Less uops per instruction –Uops fusion –Dedicated stack engine  Less transistor switches per micro-op –efficient bus –various lower-level optimizations  Less energy per transistor switch –Enhanced SpeedStep ® technology Power-awareness top to bottom

8 Computer Architecture 2008 – Advanced Topics 8 Improved Branch Predictor  Pentium® M employs best-in-class branch prediction –Bimodal predictor, Global predictor, Loop detector –Indirect branch predictor  Reduces number of wrong instructions executed –Saves energy spent executing wrong instructions  Loop predictor –Analyzes branches for loop behavior  Moving in one direction (taken or NT) a fixed number of times  Ended with a single movement in the opposite direction –Detect exact loop count –Loop predicted accurately

9 Computer Architecture 2008 – Advanced Topics 9 Indirect Branch Predictor  Indirect jumps are widely used in object-oriented code (C++, Java)  Targets are data dependent –Resolved at execution  high misprediction penalty  Initially, allocate indirect branch only in target array (TA) –If TA mispredicts  allocate in iTA according to global history  Multiple targets allocated for a given branch –Indirects with a single target predicted by TA, saving iTA space  Use iTA if TA indicates indirect branch + iTA hits Target Array iTA Branch IP Predicted Target hit indirect branch hit target HIT global history target

10 Computer Architecture 2008 – Advanced Topics 10 Dedicated Stack Engine  PUSH, POP, CALL, RET update ESP (add or sub an offset) –Use a dedicated add uop  Track the ESP offset at the front-end –ID maintains offset in ESP_delta (+/- Osize) –Eliminates need for uops updating ESP –Patch displacements of stack operations  In some cases, ESP actual value is needed –For example: add eax, esp, 3 –A sync uop is inserted before the instruction  if ESP_delta != 0  ESP = ESP + ESP_delta –Reset ESP_delta  ESP_delta recovered on jump misprediction

11 Computer Architecture 2008 – Advanced Topics 11 ESP Tracking Example ESP = SUB ESP, 8 STORE [ESP-8], EBX STORE [ESP-4], EAX PUSH eax PUSH ebx INC eax INC esp ESP = ESP - 4 STORE [ESP], EAX ESP = ESP - 4 STORE [ESP], EBX EAX = ADD EAX, 1 ESP = ADD ESP, 1 Δ = Δ - 4 Δ = 0 Δ = - 4 Δ = - 8Δ = Δ - 4 EAX = ADD EAX, 1 ESP = ADD ESP, 1 Δ = - 8 Δ = 0 Sync ESP !

12 Computer Architecture 2008 – Advanced Topics 12 Uop Fusion  The Instruction Decoder breaks an instruction into uops –A conventional uop consists of a single operation operating on two sources  An instruction requires multiple uops when –the instruction operates on more than two sources, or –the nature of the operation requires a sequence of operations  Uop fusion: in some cases the decoder fuses 2 uops into one uop –A short field added to the uop to support fusing of specific uop pairs  Uop fusion reduces the number of uops by 10% –Increases performance by effectively widening rename, and retire bandwidth –More instructions can be decode by all decoders  The same task is accomplished by processing fewer uops –Decreases the energy required to complete a given task

13 Computer Architecture 2008 – Advanced Topics 13 A 2-uop Load-Op Decoder add eax,[ebp+4*esi+8] Scheduler LD MEUALU OP LD OP LD OP tmp=load[ebp+4*esi+8] eax = eax + tmp Load-op with 3 reg. operands Decoded into 2 uops LD: read data from mem OP: reg ← reg op data The LD and OP are inherently serial OP dispatched only when LD completes

14 Computer Architecture 2008 – Advanced Topics 14 A 1-uop Load-Op Decoder add eax,[ebp+4*esi+8] Scheduler Cache LD + OP LD ALU OP eax = eax + load[ebp+4*esi+8] Decoded into 1 uop Fused uops has a 3 rd source – new field in uop holds index register Increase decode BW Increase alloc BW and ROB/RS effective size Dispatched twice OP dispatched after LD fused uop retires after both LD&OP complete Increase retire BW

15 Computer Architecture 2008 – Advanced Topics 15 Enhanced SpeedStep™ Technology  The “Basic” SpeedStep™ Technology had –2 operating points –Non-transparent switch  The “Enhanced” version provides –Multi voltage/frequency operating points. The Pentium M processor 1.6GHz operation ranges:  From 600MHz @ 0.956V  To 1.6GHz @ 1.484V –Transparent switch –Frequent switches  Benefits –Higher power efficiency 2.7X lower frequency  2X performance loss  >2X energy gain –Outstanding battery life –Excellent thermal mgmt. 2.7X6.1X Efficiency ratio = 2.3

16 Computer Architecture 2008 – Advanced Topics 16 Trace Cache (Pentium® 4 Processor)

17 Computer Architecture 2008 – Advanced Topics 17 Trace Cache  Decoding several IA-32 inst/clock at high frequency is difficult –Instructions have a variable length and have many different options –Takes several pipe-stages  Adds to the branch mis-prediction penalty  Trace-cache: cache uops of previously decoded instructions –Decoding is only needed for instructions that miss the TC  The TC is the primary (L1) instruction cache –Holds 12K uops –8-way set associative with LRU replacement  The TC has its own branch predictor (Trace BTB) –Predicts branches that hit in the TC –Directs where instruction fetching needs to go next in the TC

18 Computer Architecture 2008 – Advanced Topics 18 Traces  Instruction caches fetch bandwidth is limited to a basic blocks –Cannot provide instructions across a taken branch in the same cycle  The TC builds traces: program-ordered sequences of uops –Allows the target of a branch to be included in the same TC line as the branch itself  Traces have variable length –Broken into trace lines, six uops per trace line –There can be many trace lines in a single trace Jump into the line Jump out of the linejmp jmp jmpjmpjmp

19 Computer Architecture 2008 – Advanced Topics 19 Hyper Threading Technology (Pentium® 4 Processor ) Based on Hyper-Threading Technology Architecture and Micro-architecture Intel Technology Journal

20 Computer Architecture 2008 – Advanced Topics 20 Thread-Level Parallelism  Multiprocessor systems have been used for many years –There are known techniques to exploit multiprocessors  Software trends –Applications consist of multiple threads or processes that can be executed in parallel on multiple processors  Thread-level parallelism (TLP) – threads can be from –the same application –different applications running simultaneously –operating system services  Increasing single thread performance becomes harder –and is less and less power efficient  Chip Multi-Processing (CMP) –Two (or more) processors are put on a single die

21 Computer Architecture 2008 – Advanced Topics 21 Multi-Threading  Multi-threading: a single processor executes multiple threads  Time-slice multithreading –The processor switches between software threads after a fixed period –Can effectively minimize the effects of long latencies to memory  Switch-on-event multithreading –Switch threads on long latency events such as cache misses –Works well for server applications that have many cache misses  A deficiency of both time-slice MT and switch-on-event MT –They do not cover for branch mis-predictions and long dependencies  Simultaneous multi-threading (SMT) –Multiple threads execute on a single processor simultaneously w/o switching –Makes the most effective use of processor resources  Maximizes performance vs. transistor count and power

22 Computer Architecture 2008 – Advanced Topics 22 Hyper-threading (HT) Technology  HT is SMT –Makes a single processor appear as 2 logical processors = threads  Each thread keeps a its own architectural state –General-purpose registers –Control and machine state registers  Each thread has its own interrupt controller –Interrupts sent to a specific logical processor are handled only by it  OS views logical processors (threads) as physical processors –Schedule threads to logical processors as in a multiprocessor system  From a micro-architecture perspective –Thread share a single set of physical resources  caches, execution units, branch predictors, control logic, and buses

23 Computer Architecture 2008 – Advanced Topics 23 Two Important Goals  When one thread is stalled the other thread can continue to make progress –Independent progress ensured by either  Partitioning buffering queues and limiting the number of entries each thread can use  Duplicating buffering queues  A single active thread running on a processor with HT runs at the same speed as without HT –Partitioned resources are recombined when only one thread is active

24 Computer Architecture 2008 – Advanced Topics 24 Front End  Each thread manages its own next-instruction-pointer  Threads arbitrate TC access every cycle (Ping-Pong) –If both want to access the TC – access granted in alternating cycles –If one thread is stalled, the other thread gets the full TC bandwidth  TC entries are tagged with thread-ID –Dynamically allocated as needed –Allows one logical processor to have more entries than the other TC Hit TC Miss

25 Computer Architecture 2008 – Advanced Topics 25 Front End (cont.)  Branch prediction structures are either duplicated or shared –The return stack buffer is duplicated –Global history is tracked for each thread –The large global history array is a shared  Entries are tagged with a logical processor ID  Each thread has its own ITLB  Both threads share the same decoder logic –if only one needs the decode logic, it gets the full decode bandwidth –The state needed by the decodes is duplicated  Uop queue is hard partitioned –Allows both logical processors to make independent forward progress regardless of FE stalls (e.g., TC miss) or EXE stalls

26 Computer Architecture 2008 – Advanced Topics 26 Out-of-order Execution  ROB and MOB are hard partitioned –Enforce fairness and prevent deadlocks  Allocator ping-pongs between the thread –A thread is selected for allocation if  Its uop-queue is not empty  its buffers (ROB, RS) are not full  It is the thread’s turn, or the other thread cannot be selected

27 Computer Architecture 2008 – Advanced Topics 27 Out-of-order Execution (cont)  Registers renamed to a shared physical register pool –Store results until retirement  After allocation and renaming uops are placed in one of 2 Qs –Memory instruction queue and general instruction queue  The two queues are hard partitioned –Uops are read from the Q’s and sent to the scheduler using ping-pong  The schedulers are oblivious to threads –Schedule uops based on dependencies and exe. resources availability  Regardless of their thread –Uops from the two threads can be dispatched in the same cycle –To avoid deadlock and ensure fairness  Limit the number of active entries a thread can have in each scheduler’s queue  Forwarding logic compares physical register numbers –Forward results to other uops without thread knowledge

28 Computer Architecture 2008 – Advanced Topics 28 Out-of-order Execution (cont)  Memory is largely oblivious –L1 Data Cache, L2 Cache, L3 Cache are thread oblivious  All use physical addresses –DTLB is shared  Each DTLB entry includes a thread ID as part of the tag  Retirement ping-pongs between threads –If one thread is not ready to retire uops all retirement bandwidth is dedicated to the other thread

29 Computer Architecture 2008 – Advanced Topics 29 Single-task And Multi-task Modes  MT-mode (Multi-task mode) –Two active threads, with some resources partitioned as described earlier  ST-mode (Single-task mode) –There are two flavors of ST-mode  single-task thread 0 (ST0) – only thread 0 is active  single-task thread 1 (ST1) – only thread 1 is active –Resources that were partitioned in MT-mode are re-combined to give the single active logical processor use of all of the resources  Moving the processor from between modes ST0 ST1 MT Thread 1 executes HALT Low Power Thread 1 executes HALT Thread 0 executes HALT Interrupt

30 Computer Architecture 2008 – Advanced Topics 30 Operating System And Applications  An HT processor appears to the OS and application SW as 2 processors –The OS manages logical processors as it does physical processors The OS should implement two optimizations:  Use HALT if only one logical processor is active –Allows the processor to transition to either the ST0 or ST1 mode –Otherwise the OS would execute on the idle logical processor a sequence of instructions that repeatedly checks for work to do –This so-called “idle loop” can consume significant execution resources that could otherwise be used by the other active logical processor  On a multi-processor system, –Schedule threads to logical processors on different physical processors before scheduling multiple threads to the same physical processor –Allows SW threads to use different physical resources when possible


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