CS252/Culler Lec 4.1 1/31/02 CS203A Graduate Computer Architecture Lecture 14 Cache Design Taken from Prof. David Culler’s notes

CS252/CullerLec 4.1

1/31/02

CS203AGraduate Computer Architecture

Lecture 14

Cache Design

Taken from

Prof. David Culler’s notes

CS252/CullerLec 4.2

1/31/02

How to Improve Cache Performance?

1. Reduce the miss rate,

2. Reduce the miss penalty, or

3. Reduce the time to hit in the cache.

yMissPenaltMissRateHitTimeAMAT

CS252/CullerLec 4.3

1/31/02

Where to misses come from?

• Classifying Misses: 3 Cs– Compulsory—The first access to a block is not in the

cache, so the block must be brought into the cache. Also called cold start misses or first reference misses.(Misses in even an Infinite Cache)

– Capacity—If the cache cannot contain all the blocks needed during execution of a program, capacity misses will occur due to blocks being discarded and later retrieved.(Misses in Fully Associative Size X Cache)

– Conflict—If block-placement strategy is set associative or direct mapped, conflict misses (in addition to compulsory & capacity misses) will occur because a block can be discarded and later retrieved if too many blocks map to its set. Also called collision misses or interference misses.(Misses in N-way Associative, Size X Cache)

• 4th “C”:– Coherence - Misses caused by cache coherence.

CS252/CullerLec 4.4

1/31/02

Cache Size (KB)

Mis

s R

ate

per

Typ

e

0

0.02

0.04

0.06

0.08

0.1

0.12

0.141 2 4 8

16

32

64

12

8

1-way

2-way

4-way

8-way

Capacity

Compulsory

3Cs Absolute Miss Rate (SPEC92)

Conflict

CS252/CullerLec 4.5

1/31/02

Cache Size

• Old rule of thumb: 2x size => 25% cut in miss rate• What does it reduce?

Cache Size (KB)

Mis

s R

ate

per

Typ

e

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 2 4 8

16

32

64

12

8

1-way

2-way

4-way

8-way

Capacity

Compulsory

CS252/CullerLec 4.6

1/31/02

Cache Organization?

• Assume total cache size not changed:• What happens if:

1) Change Block Size:

2) Change Associativity:

3) Change Compiler:

Which of 3Cs is obviously affected?

CS252/CullerLec 4.7

1/31/02

Block Size (bytes)

Miss Rate

0%

5%

10%

15%

20%

25%

16

32

64

12

8

25

6

1K

4K

16K

64K

256K

Larger Block Size (fixed size&assoc)

Reduced compulsory

missesIncreasedConflictMisses

What else drives up block size?

CS252/CullerLec 4.8

1/31/02

Cache Size (KB)

Mis

s R

ate

per

Typ

e

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

1 2 4 8

16

32

64

12

8

1-way

2-way

4-way

8-way

Capacity

Compulsory

Associativity

Conflict

CS252/CullerLec 4.9

1/31/02

3Cs Relative Miss Rate

Cache Size (KB)

Mis

s R

ate

per

Typ

e

0%

20%

40%

60%

80%

100%1 2 4 8

16

32

64

12

8

1-way

2-way4-way

8-way

Capacity

Compulsory

Conflict

Flaws: for fixed block sizeGood: insight => invention

CS252/CullerLec 4.10

1/31/02

Fast Hit Time + Low Conflict => Victim Cache

• How to combine fast hit time of direct mapped yet still avoid conflict misses?

• Add buffer to place data discarded from cache

• Jouppi [1990]: 4-entry victim cache removed 20% to 95% of conflicts for a 4 KB direct mapped data cache

• Used in Alpha, HP machines

To Next Lower Level InHierarchy

DATATAGS

One Cache line of DataTag and Comparator





1/31/02

Alpha 21264 Cache Organization


1/31/02

Reducing Misses by Hardware Prefetching of Instructions & Data

• E.g., Instruction Prefetching– Alpha 21064 fetches 2 blocks on a miss– Sequential prefetch– Cache Pollution if unused! – Unnecessarily replaced some

useful data!– Solution: Put incoming block in “stream buffer”– On miss check stream buffer

• Works with data blocks too:– Jouppi [1990] 1 data stream buffer got 25% misses from

4KB cache; 4 streams got 43%– Palacharla & Kessler [1994] for scientific programs for 8

streams got 50% to 70% of misses from 2 64KB, 4-way set associative caches

– Data Prediction is difficult

• Prefetching relies on having extra memory bandwidth that can be used without penalty

• Question: What to prefetch and when to prefetch? Instruction prefetch is fine, but data?


1/31/02

Leave it to the Programmer? Software Prefetching Data

• Data Prefetch – Explicit prefetch instructions– Load data into register (HP PA-RISC loads)– Cache Prefetch: load into cache (MIPS IV, PowerPC, SPARC

v. 9)– Special prefetching instructions cannot cause faults; a

form of speculative execution

• Prefetching comes in two flavors:– Binding prefetch: Requests load directly into register.

» Must be correct address and register!– Non-Binding prefetch: Load into cache.

» Can be incorrect. Faults?

• Issuing Prefetch Instructions takes time– Is cost of prefetch issues < savings in reduced misses?– Higher superscalar reduces difficulty of issue bandwidth


1/31/02

Summary: Miss Rate Reduction

• 3 Cs: Compulsory, Capacity, Conflict0. Larger cache1. Reduce Misses via Larger Block Size2. Reduce Misses via Higher Associativity3. Reducing Misses via Victim Cache4. Reducing Misses via Pseudo-Associativity5. Reducing Misses by HW Prefetching Instr, Data6. Reducing Misses by SW Prefetching Data7. Reducing Misses by Compiler Optimizations

• Prefetching comes in two flavors:– Binding prefetch: Requests load directly into register.

» Must be correct address and register!– Non-Binding prefetch: Load into cache.

» Can be incorrect. Frees HW/SW to guess!

CPUtimeIC CPIExecution

Memory accesses

InstructionMiss rateMiss penalty

Clock cycle time


1/31/02

Review: Improving Cache Performance





1/31/02

1. Reducing Miss Penalty: Read Priority over Write on

Miss• Write-through w/ write buffers => RAW

conflicts with main memory reads on cache misses– If simply wait for write buffer to empty, might increase read

miss penalty (old MIPS 1000 by 50% )– Check write buffer contents before read;

if no conflicts, let the memory access continue

• Write-back want buffer to hold displaced blocks– Read miss replacing dirty block– Normal: Write dirty block to memory, and then do the read– Instead copy the dirty block to a write buffer, then do the

read, and then do the write– CPU stall less since restarts as soon as do read


1/31/02

2. Reduce Miss Penalty: Early Restart and Critical

Word First• Don’t wait for full block to be loaded before

restarting CPU– Early restart—As soon as the requested word of the block

arrives, send it to the CPU and let the CPU continue execution

– Critical Word First—Request the missed word first from memory and send it to the CPU as soon as it arrives; let the CPU continue execution while filling the rest of the words in the block. Also called wrapped fetch and requested word first

• Generally useful only in large blocks, • Spatial locality => tend to want next

sequential word, so not clear if benefit by early restart

block


1/31/02

3. Reduce Miss Penalty: Non-blocking Caches to reduce stalls

on misses• Non-blocking cache or lockup-free cache allow data

cache to continue to supply cache hits during a miss– requires F/E bits on registers or out-of-order execution– requires multi-bank memories

• “hit under miss” reduces the effective miss penalty by working during miss vs. ignoring CPU requests

• “hit under multiple miss” or “miss under miss” may further lower the effective miss penalty by overlapping multiple misses– Significantly increases the complexity of the cache controller as

there can be multiple outstanding memory accesses– Requires muliple memory banks (otherwise cannot support)– Penium Pro allows 4 outstanding memory misses


1/31/02

4: Add a second-level cache

• L2 EquationsAMAT = Hit TimeL1 + Miss RateL1 x Miss PenaltyL1

Miss PenaltyL1 = Hit TimeL2 + Miss RateL2 x Miss PenaltyL2

AMAT = Hit TimeL1 +

Miss RateL1 x (Hit TimeL2 + Miss RateL2 + Miss PenaltyL2)

• Definitions:– Local miss rate— misses in this cache divided by the total

number of memory accesses to this cache (Miss rateL2)– Global miss rate—misses in this cache divided by the total

number of memory accesses generated by the CPU

– Global Miss Rate is what matters


1/31/02

Comparing Local and Global Miss Rates

• 32 KByte 1st level cache;Increasing 2nd level cache

• Global miss rate close to single level cache rate provided L2 >> L1

• Don’t use local miss rate• L2 not tied to CPU clock

cycle!• Cost & A.M.A.T.• Generally Fast Hit Times

and fewer misses• Since hits are few, target

miss reduction

Linear

Log

Cache Size

Cache Size


1/31/02

Example• For every 1000 instructions, 40 misses in L1

and 20 misses in L2;

Hit cycle in L1 is 1, L2 is 10; Miss penalty from L2 to memory is 100 cycles; there are 1.5 memory references per instruction. What is AMAT and average stall cycles per instruction?– AMAT = [1 + 40/1000 * (10 + 20/40 * 100) ] *cc = 3.4

cycles– AMAT without L2 = 1 + 40/1000 * 100 = 5 cycles =>

An improvement of 1.6 cycles due to L2– Average stall cycles per instruction = 1.5 * 40/1000 *

10 + 1.5 * 20/1000 * 100 = 3.6 cycles

• Note: We have not distinguished reads and writes. Access L2 only on L1 miss, i.e. write back cache


1/31/02

Reducing Misses: Which apply to L2 Cache?

• Reducing Miss Rate1. Reduce Misses via Larger Block Size2. Reduce Conflict Misses via Higher Associativity3. Reducing Conflict Misses via Victim Cache4. Reducing Conflict Misses via Pseudo-Associativity5. Reducing Misses by HW Prefetching Instr, Data6. Reducing Misses by SW Prefetching Data7. Reducing Capacity/Conf. Misses by Compiler

Optimizations


1/31/02

Relative CPU Time

Block Size

11.11.21.31.41.51.61.71.81.9

2

16 32 64 128 256 512

1.361.28 1.27

1.34

1.54

1.95

L2 cache block size & A.M.A.T.

• 32KB L1, 8 byte path to memory


1/31/02

Reducing Miss Penalty Summary

• Four techniques– Read priority over write on miss– Early Restart and Critical Word First on miss– Non-blocking Caches (Hit under Miss, Miss under

Miss)– Second Level Cache

• Can be applied recursively to Multilevel Caches– Danger is that time to DRAM will grow with multiple

levels in between– First attempts at L2 caches can make things worse,

since increased worst case is worse

CPUtimeIC CPIExecution

Memory accesses

InstructionMiss rateMiss penalty

Clock cycle time


1/31/02

What is the Impact of What You’ve Learned

About Caches?• 1960-1985: Speed

= ƒ(no. operations)• 1990

– Pipelined Execution & Fast Clock Rate

– Out-of-Order execution

– Superscalar Instruction Issue

• 1998: Speed = ƒ(non-cached memory accesses)

• Superscalar, Out-of-Order machines hide L1 data cache miss ( 5 clocks) but not L2 cache miss ( 50 clocks)?

1

10

100

1000

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

DRAM

CPU


1/31/02

Cache Optimization Summary

Technique MR MP HT ComplexityLarger Block Size + – 0Higher Associativity + – 1Victim Caches + 2Pseudo-Associative Caches + 2HW Prefetching of Instr/Data + 2Compiler Controlled Prefetching + 3Compiler Reduce Misses + 0Priority to Read Misses + 1Early Restart & Critical Word 1st + 2Non-Blocking Caches + 3Second Level Caches + 2

mis

s r

ate

mis

s p

en

alty


1/31/02

How to Improve Cache Performance?




yMissPenaltMissRateHitTimeAMAT


1/31/02

1. Small and simple caches

• Small on-chip L1 caches – less access time

• Direct mapped cache faster because no hardware comparison between blocks, but higher miss ratio

• Compromise – Direct L1 cache and Set-associative L2 cache

How to predict cache access time at the design stage? – Use CACTI program


1/31/02

2. Virtual Caches

• No address translation from virtual to physical address – no TLB

Problems (Read section 5.7):• How to ensure protection? – Add a bit

to distinguish between processes? • Add Process-identifier tag (PID) and

flush the cache for another process – too much overhead!! Increases miss rate.

• O/S can not use same physical address for two or more virtual processes – called aliasing problem!


1/31/02

3. Pipeline the cache access – Does it

not increase hit latency?

4. Trace Caches – What are they? Read pp. 447 - Homework

Documents

CS252/Culler Lec 4.1 1/31/02 CS203A Graduate Computer Architecture Lecture 14 Cache Design Taken from Prof. David Culler’s notes