Talianpaip

• Talianpaip ialah satu teknik utk meningkatkan prestasi mesin dgn melakukan arahan bertindih utk mengurangkan masa perlaksanaan. Ini merupakan kunci kepantasan CPU pd hari ini.

1

Pipelining Is Natural!

• Laundry Example

• Aini, Boon, Chong, David each have one load of clothes to wash, dry, and fold

• Washer takes 30 minutes

• Dryer takes 30 minutes

• “Folder” takes 30 minutes

• “Stasher” takes 30 minutesto put clothes into drawers

A B C D

Sequential Laundry

• Sequential laundry takes 8 hours for 4 loads

• If they learned pipelining, how long would laundry take?

30Task

Order

B

C

D

ATime

3030 3030 30 3030 3030 3030 3030 3030

6 PM 7 8 9 10 11 12 1 2 AM

Pipelined Laundry: Start Work ASAP

• Pipelined laundry takes 3.5 hours for 4 loads!

Task

Order

12 2 AM6 PM 7 8 9 10 11 1

Time

B

C

D

A

303030 3030 3030

Pipelining Lessons

• Talian tidak mempercepatkan pendaman satu tugas, ia mempercepatkan truput utk keseluruhan beban kerja.

• Multi tugas dilaksanakan serentak dgn menggunakan sumber yg berlainan.

• Keupayaan speedup = Bil. segmen talianpaip

• Kadar talianpaip dihadkan oleh segmen talianpaip yg terlembap

• Ketidakseimbangan pjg segmen talianpaip mengurangkan speedup

• Masa utk penuhkan talianpaip & pengosongan talianpaip mengurangkan speedup.

6 PM 7 8 9

Time

B

C

D

A

303030 3030 3030Task

Order

Pengenalan

• Dlm sistem tipikal kepantasan dicapai melalui teknik keselarian dlm semua peringkat: Multi-user, multi-tasking, multi-processing, multi-programming, multi-threading, compiler optimizations.

• Talianpaip : adl teknik pertindihan operasi semasa perlaksanaan.

• Different types of pipeline: instruction pipeline, operation pipeline, multi-issue pipelines.

Apakah talianpaip?

2

• Ia spt automobile assembly line.• Ia m’punyai bbrp langkah atau langkah atau segmen. • Setiap segmen melakukan arahan atau operasi yg

berbeza.• Segmen2 ini disambung menjadikan paip.• An inst or operation enters through one end and

progresses through the stages and exit through the other end.

• Talianpaip ialah teknik perlaksanaan yg mengeksplotasikan keselarian sesama arahan dlm aliran arahan berjujukan.

Krateria2 Talianpaip

• Truput: Bil. item (cars, instructions, operations) yg keluar drpd talianpaip per unit masa. – Ex: 1 inst / clock cycle, 10 cars/ hour, 10 fp operations /cycle.

• Masa segmen: Matlamat pereka talianpaip ialah keseimbangan masa setiap segmen talianpaip. (Balanced pipeline) – masa segmen = Masa arahan dlm mesin yg bukan talianpaip /

bil.segmen.

– Dlm kebanyakan kes, masa segmen = mak (masa utk semua segmen).

• CPI : Pipeline yields a reduction in cycles per instruction. CPI approx = stage time.

A Machine Without Pipelines

I1 I2 I3

Below is a diagram which shows the execution of three instructions in sequence.

Computer Instructions

ExecutionTime

Time

A Machine With Pipelines

I1

I2

I3

Computer Instructions

}Instructions

are overlapped

Time

Below is a diagram to show the execution of three instructions which are overlapped.

ExecutionTime

Instruction Composition

A single instruction executing within a machine normally consist of a few segments which performs specific tasks.

An example of an instruction which has 6 segments:-

1) Fetch instruction from memory. (IFetch)

2) Decode the instruction. (Dcd)

3) Calculate the effective address. (Eadd)

4) Fetch operand from memory. (Mem)

5) Execute the instruction. (Exec)

6) Store results in the proper place (or location). (WB)

Space Time Diagram

A space-time diagram is normally used to represent the execution of instructions in a pipeline.Below is an example of a space time diagram.

Whereby there are six instructions executing (I1 to I6) and each instruction has four segments.

IFetch Dcd Exec Mem WB

IFetch Dcd Exec Mem WB

IFetch Dcd Exec Mem WB

IFetch Dcd Exec Mem WB

IFetch Dcd Exec Mem WB

IFetch Dcd Exec Mem WBProgram Flow

Time

Pipeline Performance - Example2

• Assume times for five functional units of a pipeline to be: 10ns, 8ns, 10ns, 10ns and 7ns. Overhead 1ns per stage. Compute the speed of the data path.

• Pipelined: Stage time = MAX(10,8,10,10,10,7) + overhead

= 10 + 1 = 11ns.This is the average instruction execution time at steady state.

• Non-pipelined: 10+8+10+10+7 = 45ns• Speedup = 45/11= 4.1 times

Masalah2 Talianpaip•Kebergantungan Data

Masalah yg wujud pabila dua arahan bersebelahan dalam talian paip ingin mencapai secebis data yg berada dlm lokasi memori yg sama.

•Percabangan Arahan

Masalah yg berlaku pabila branch perlu dilaksanakan dan arahan2 pre-fetched yg lain dlm talian paip akan/mesti dibuang.

Kebergantungan Data

Dlm cth ini, andaikan arahan terdiri drpd 4 segment, dan arahan satu (I1) dan arahan dua (I2) mencapai lokasi memori yg sama.

FI DA FO EX

FI DA FO EX

Instruction Execution

I1

I2

Both instructions are trying to access the same variable at the same time.

Selesaian Masalah Kebergantungan Data

• Hardware Interlocks (membabitkan perkakasan)

• Operand Forwarding (membabitkan perkakasan)

• Delayed Load (membabitkan perisian)

Hardware Interlocks

FI DA FO EX

FI DA FO EX

Instruction Execution

I1

I2

Ia mengesan masalah kebergantungan data dan lengahkan arahan kedua.

Operand Forwarding

Ia mengesan masalah kebergantungan data dan hantar (forwards) keputusan arahan satu ke dlm arahan berikutnya.

FI DA FO EX

FI DA FO EX

Instruction Execution

I1

I2

The offending instructions will have the output of one, sent directly into the next instruction.

Delayed LoadIa mengesan masalah kebergantungan data ketika aturcara tersebut dikompilasikan. Pengkompilasi kemudiannya akan memasukkan arahan no-operation di antara arahan2 yg terbabit.

FI DA FO EX

FI DA FO EX

Instruction Execution

I1

I2

FI DA FO EXNOP

Percabangan ArahanWhat if instruction I1 is a branch instruction to I3?

.......... what happens to I2?

FI DA FO EX

FI DA FO EX

Instruction Execution

I1

I2

FI DA FO EXI3

Selesaian Arahan Bercabang

• Pre-fetch Target Instruction

• Branch Target Buffer, A form of cache implementation (but for instructions)

• Loop Buffer Find out more details for

these • Branch Prediction solutions ..

• Delayed Branch

Operasi Talianpaip• Apakah yg memudahkan?

–Semua arahan adl sama panjang

–sedikit format arahan

–memori operan hanya wujud ketika muat & simpan

• Apakah yg menyusahkan?–Katakan hanya ada satu memori (structural hazard)

–masalah arahan2 bercabang (control hazard)

–Arahan yg bergantung kpd arahan sebelumnya (data hazards)

Bencana Talianpaip

• Bencana akan mengurangkan prestasi talian drpd ideal speedupnya:

• Structural hazard: Konflik sumber. Perkakasan tidak boleh menyokong kesemua arahan gabungan yg wujud dlm simultaneous overlapped execution.

• Data hazard: Pabila arahan memerlukan keputusan drpd arahan sebelumnya.

• Control hazard: Akibat drpd cabangan2 & arahan lain yg memberi kesan kpd PC.

Structural Hazards

• Pabila lebih drpd satu arahan dlm talianpaip yg ingin mencapai sumber di mana ini menyebabkan laluan data mengalami structural hazard.

• Cth sumber spt: register file, memori, ALU.• Selesaian: Melengahkan talianpaip utk satu kitaran

jam apabila konflik tersebut dikesan. Hasilnya ialah pipeline bubble.

• Komflik capaian memori dan bgmn ia diatasi dengan melengahkan arahan. Problem: one memory port.

Data Hazard

• Consider the inst sequence:

ADD R1,R2,R3 ; result is in R1

SUB R4,R5,R1

AND R6,R1,R7

OR R8,R1,R9

XOR R10,R1,R11

All instructions use R1 after the first inst.

Selesaian- Data Hazard• Selalunya diselesaikan oleh data forwarding atau

register forwarding (bypassing or short-circuiting).• Bgmn? The data selected is not really used in

identification flag (ID) but in the next stage: ALU.• Forwarding works as follows:• Keputusan ALU drpd buffer EX/MEM selalunya

disuapbalik ke selak input ALU.• If the forwarding hardware detects that its source

operand has a new value, the logic selects the newer result than the value read from the register file.

Perlengahan Talianpaip

• Stall adl perlengahan dlm kitaran yg disbbkan oleh sebarang bencana yg tlh dinyatakan.

• Speedup :

1/(1+pipeline stall per instruction)* Number of stages

• Jadi apakah speedup utk talianpaip unggul tanpa stall?

• Bil. Kitaran yg diperlukan utk pemenuhan talian paip mungkin boleh dimasukkan dlm penghitungan kepurataan stall per arahan.