CS 61C Great Ideas in Computer Architecture (Machine

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  • 1.CS 61C: Great Ideas in Computer Architecture (Machine Structures)Caches Part 1 Instructors: Nicholas Weaver & Vladimir Stojanovic http://inst.eecs.berkeley.edu/~cs61c/
  • 2.Processor Control Datapath Components of a Computer 2 PC Registers Arithmetic & Logic Unit (ALU) Memory Input Output Bytes Enable? Read/Write Address Write Data ReadData Processor-Memory Interface I/O-Memory Interfaces Program Data
  • 3.Problem: Large memories slow?Library Analogy Finding a book in a large library takes time Takes time to search a large card catalog – (mapping title/author to index number) Round-trip time to walk to the stacks and retrieve the desired book. Larger libraries makes both delays worse Electronic memories have the same issue, plus the technologies that we use to store an individual bit get slower as we increase density (SRAM versus DRAM versus Magnetic Disk) 3 However what we want is a large yet fast memory!
  • 4.Processor-DRAM Gap (latency) 4 Time µProc 60%/year DRAM 7%/year 1 10 100 1000 1980 1981 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 DRAM CPU 1982 Processor-Memory Performance Gap:(growing 50%/yr) Performance 1980 microprocessor executes ~one instruction in same time as DRAM access 2015 microprocessor executes ~1000 instructions in same time as DRAM access Slow DRAM access could have disastrous impact on CPU performance!
  • 5.What to do: Library Analogy Want to write a report using library books E.g., works of J.D. Salinger Go to Doe library, look up relevant books, fetch from stacks, and place on desk in library If need more, check them out and keep on desk But don’t return earlier books since might need them You hope this collection of ~10 books on desk enough to write report, despite 10 being only 0.00001% of books in UC Berkeley libraries 5
  • 6.Second-Level Cache (SRAM) Big Idea: Memory Hierarchy Control Datapath Secondary Memory (Disk Or Flash) On-Chip Components RegFile Main Memory (DRAM) Data Cache Instr Cache Speed (cycles): ½’s 1’s 10’s 100’s 1,000,000’s Size (bytes): 100’s 10K’s M’s G’s T’s 6 Principle of locality + memory hierarchy presents programmer with ≈ as much memory as is available in the cheapest technology at the ≈ speed offered by the fastest technology Cost/bit: highest lowest Third-Level Cache (SRAM)
  • 7.Real Memory Reference Patterns Donald J. Hatfield, Jeanette Gerald: Program Restructuring for Virtual Memory. IBM Systems Journal 10(3): 168-192 (1971) Time Memory Address (one dot per access)
  • 8.Big Idea: Locality Temporal Locality (locality in time) Go back to same book on desktop multiple times If a memory location is referenced, then it will tend to be referenced again soon Spatial Locality (locality in space) When go to book shelf, pick up multiple books on J.D. Salinger since library stores related books together If a memory location is referenced, the locations with nearby addresses will tend to be referenced soon 8
  • 9.Memory Reference Patterns Donald J. Hatfield, Jeanette Gerald: Program Restructuring for Virtual Memory. IBM Systems Journal 10(3): 168-192 (1971) Time Memory Address (one dot per access) Spatial Locality Temporal Locality
  • 10.Principle of Locality Principle of Locality: Programs access small portion of address space at any instant of time (spatial locality) and repeatedly access that portion (temporal locality) What program structures lead to temporal and spatial locality in instruction accesses? In data accesses? 10
  • 11.Memory Reference Patterns Address Time Instruction fetches Stack accesses Data accesses n loop iterations subroutine call subroutine return argument access vector access scalar accesses
  • 12.Cache Philosophy Programmer-invisible hardware mechanism to give illusion of speed of fastest memory with size of largest memory Works fine even if programmer has no idea what a cache is However, performance-oriented programmers today sometimes “reverse engineer” cache design to design data structures to match cache 12
  • 13.Memory Access without Cache Load word instruction: lw $t0,0($t1) $t1 contains 0x12F0, Memory[0x12F0] = 99 Processor issues address 0x12F0 to Memory Memory reads word at address 0x12F0 (99) Memory sends 99 to Processor Processor loads 99 into register $t0 13
  • 14.Processor Control Datapath Adding Cache to Computer 14 PC Registers Arithmetic & Logic Unit (ALU) Memory Input Output Bytes Enable? Read/Write Address Write Data ReadData Processor-Memory Interface I/O-Memory Interfaces Program Data Cache
  • 15.Memory Access with Cache Load word instruction: lw $t0,0($t1) $t1 contains 0x12F0, Memory[0x12F0] = 99 With cache: Processor issues address 0x12F0 to Cache Cache checks to see if has copy of data at address 0x12F0 2a. If finds a match (Hit): cache reads 99, sends to processor 2b. No match (Miss): cache sends address 0x12F0 to Memory Memory reads 99 at address 0x12F0 Memory sends 99 to Cache Cache replaces word which can store 0x12F0 with new 99 Cache sends 99 to processor Processor loads 99 into register $t0 15
  • 16.Administrivia Fill In 16
  • 17.Clicker Question… Consider the following statements 1: The J instructions in MIPS have a delay slot 2: JAL records PC + 4 into $ra on MIPS with a delay slot 3: The location where to jump to on a JR is known in the ID stage Which are true? A) None B) 1, 3 C) 1, 2 D) 2, 3 E) 1, 3 17
  • 18.Cache “Tags” Need way to tell if have copy of location in memory so that can decide on hit or miss On cache miss, put memory address of block as “tag” of cache block 1022 placed in tag next to data from memory (99) 18 From earlier instructions
  • 19.Anatomy of a 16 Byte Cache, 4 Byte Block Operations: Cache Hit Cache Miss Refill cache from memory Cache needs Address Tags to decide if Processor Address is a Cache Hit or Cache Miss Compares all 4 tags "Fully Associative cache"Any tag can be in any location so you have to check them all 19 Processor 32-bit Address 32-bit Data Cache 32-bit Address 32-bit Data Memory 0x12F0 99 0x1F00 7 20 12 0xF214 0x001C
  • 20.Cache Replacement Suppose processor now requests location 0x050C, which contains 11? Doesn’t match any cache block, so must “evict” one resident block to make room Which block to evict? Replace “victim” with new memory block at address 0x050C 20
  • 21.Block Must be Aligned in Memory Word blocks are aligned, so binary address of all words in cache always ends in 00two How to take advantage of this to save hardware and energy? Don’t need to compare last 2 bits of 32-bit byte address (comparator can be narrower) => Don’t need to store last 2 bits of 32-bit byte address in Cache Tag (Tag can be narrower) 21
  • 22.Anatomy of a 32B Cache, 8B Block 22 Blocks must be aligned in pairs, otherwise could get same word twice in cache Tags only have even-numbered words Last 3 bits of address always 000two Tags, comparators can be narrower Can get hit for either word in block Processor 32-bit Address 32-bit Data Cache 32-bit Address 32-bit Data Memory 0x1028 99 0xF258 42 1947 12 0xA130 0x2040 1000 7 20 -10
  • 23.Hardware Cost of Cache Need to compare every tag to the Processor address Comparators are expensive Optimization: use 2 “sets” of data with a total of only 2 comparators 1 Address bit selects which set (ex: even and odd set) Compare only tags from selected set Generalize to more sets 23 23 Processor 32-bit Address Tag Data 32-bit Data Cache 32-bit Address 32-bit Data Memory Tag Data Set 0 Set 1 Tag Data Tag Data
  • 24.Processor Address Fields used by Cache Controller Block Offset: Byte address within block Set Index: Selects which set Tag: Remaining portion of processor address Size of Index = log2 (number of sets) Size of Tag = Address size – Size of Index – log2 (number of bytes/block) Block offset Set Index Tag 24 Processor Address (32-bits total)
  • 25.What is limit to number of sets? For a given total number of blocks, we can save more comparators if have more than 2 sets Limit: As Many Sets as Cache Blocks => only one block per set – only needs one comparator! Called “Direct-Mapped” Design 25 Block offset Index Tag
  • 26.Cache Names for Each Organization “Fully Associative”: Block can go anywhere First design in lecture Note: No Index field, but 1 comparator/block “Direct Mapped”: Block goes one place Note: Only 1 comparator Number of sets = number blocks “N-way Set Associative”: N places for a block Number of sets = number of blocks / N N comparators Fully Associative: N = number of blocks Direct Mapped: N = 1 26
  • 27.9/27/2021 27 Memory Block-addressing example
  • 28.Block number aliasing example 9/27/2021 28 Block # Block # mod 8 Block # mod 2 12-bit memory addresses, 16 Byte blocks
  • 29.Direct Mapped Cache Ex: Mapping a 6-bit Memory Address In example, block size is 4 bytes (1 word) Memory and cache blocks always the same size, unit of transfer between memory and cache # Memory blocks >> # Cache blocks 16 Memory blocks = 16 words = 64 bytes => 6 bits to address all bytes 4 Cache blocks, 4 bytes (1 word) per block 4 Memory blocks map to each cache block Memory block to cache block, aka index: middle two bits Which memory block is in a given cache block, aka tag: top two bits 29 0 5 1 Byte Within Block Byte Offset 2 3 Block Within $ 4 Mem Block Within$ Block Tag Index
  • 30.Caching: A Simple First Example 00 01 10 11 Cache Main Memory Q: Where in the cache is the mem block? Use next 2 low-order memory address bits – the index – to determine which cache block (i.e., modulo the number of blocks in the cache) Tag Data Q: Is the memory block in cache? Compare the cache tag to the high-order 2 memory address bits to tell if the memory block is in the cache (provided valid bit is set) Valid 0000xx 0001xx 0010xx 0011xx 0100xx 0101xx 0110xx 0111xx 1000xx 1001xx 1010xx 1011xx 1100xx 1101xx 1110xx 1111xx One word blocks Two low order bits (xx) define the byte in the block (32b words) Index 30
  • 31.One More Detail: Valid Bit When start a new program, cache does not have valid information for this program Need an indicator whether this tag entry is valid for this program Add a “valid bit” to the cache tag entry 0 => cache miss, even if by chance, address = tag 1 => cache hit, if processor address = tag 31
  • 32.One word blocks, cache size = 1K words (or 4KB) Direct-Mapped Cache Example 20 Tag 10 Index Data Index Tag Valid 0 1 2 . . . 1021 1022 1023 31 30 . . . 13 12 11 . . . 2 1 0 Byte offset What kind of locality are we taking advantage of? 20 Data 32 Hit 32 Valid bit ensures something useful in cache for this index Compare Tag with upper part of Address to see if a Hit Readdata from cache instead of memory if a Hit Comparator
  • 33.Four words/block, cache size = 1K words Multiword-Block Direct-Mapped Cache 8 Index 2 Data Index Tag Valid 0 1 2 . . . 253 254 255 31 30 . . . 13 12 11 . . . 4 3 2 1 0 Byte offset 20 20 Tag Hit Data 32 Word offset What kind of locality are we taking advantage of? 33
  • 34.Processor Address Fields used by Cache Controller Block Offset: Byte address within block Set Index: Selects which set Tag: Remaining portion of processor address Size of Index = log2 (number of sets) Size of Tag = Address size – Size of Index – log2 (number of bytes/block) Block offset Set Index Tag 34 Processor Address (32-bits total)
  • 35.One word blocks, cache size = 1K words (or 4KB) Direct-Mapped Cache Review 20 Tag 10 Index Data Index Tag Valid 0 1 2 . . . 1021 1022 1023 31 30 . . . 13 12 11 . . . 2 1 0 Byte offset 20 Data 32 Hit 35 Valid bit ensures something useful in cache for this index Compare Tag with upper part of Address to see if a Hit Readdata from cache instead of memory if a Hit Comparator
  • 36.Four-Way Set-Associative Cache 28 = 256 sets each with four ways (each with one block) 31 30 . . . 13 12 11 . . . 2 1 0 Byte offset Data Tag V 0 1 2 . . . 253 254 255 Data Tag V 0 1 2 . . . 253 254 255 Data Tag V 0 1 2 . . . 253 254 255 Set Index Data Tag V 0 1 2 . . . 253 254 255 8 Index 22 Tag Hit Data 32 4x1 select Way 0 Way 1 Way 2 Way 3 36
  • 37.Handling Stores with Write-Through Store instructions write to memory, changing values Need to make sure cache and memory have same values on writes: 2 policies 1) Write-Through Policy: write cache and write through the cache to memory Every write eventually gets to memory Too slow, so include Write Buffer to allow processor to continue once data in Buffer Buffer updates memory in parallel to processor 37
  • 38.Write-Through Cache Write both values in cache and in memory Write buffer stops CPU from stalling if memory cannot keep up Write buffer may have multiple entries to absorb bursts of writes What if store misses in cache? 38 Processor 32-bit Address 32-bit Data Cache 32-bit Address 32-bit Data Memory 1022 99 F252 7 20 12 131C 2041 Addr Data Write Buffer
  • 39.Handling Stores with Write-Back 2) Write-Back Policy: write only to cache and then write cache block back to memory when evict block from cache Writes collected in cache, only single write to memory per block Include bit to see if wrote to block or not, and then only write back if bit is set Called “Dirty” bit (writing makes it “dirty”) 39
  • 40.Write-Back Cache Store/cache hit, write data in cache only & set dirty bit Memory has stale value Store/cache miss, read data from memory, then update and set dirty bit “Write-allocate” policy On any miss, write back evicted block, only if dirty. Update cache with new block and clear dirty bit. 40 Processor 32-bit Address 32-bit Data Cache 32-bit Address 32-bit Data Memory 1022 99 F252 7 20 12 131C 2041 D D D D Dirty Bits
  • 41.Write-Through vs. Write-Back Write-Through: Simpler control logic More predictable timing simplifies processor control logic Easier to make reliable, since memory always has copy of data (big idea: Redundancy!) Write-Back More complex control logic More variable timing (0,1,2 memory accesses per cache access) Usually reduces write traffic Harder to make reliable, sometimes cache has only copy of data 41
  • 42.Write Policy Choices Cache hit: write through: writes both cache & memory on every access Generally higher memory traffic but simpler pipeline & cache design write back: writes cache only, memory `written only when dirty entry evicted A dirty bit per line reduces write-back traffic Must handle 0, 1, or 2 accesses to memory for each load/store Cache miss: no write allocate: only write to main memory write allocate (aka fetch on write): fetch into cache Common combinations: write through and no write allocate write back with write allocate 42
  • 43.Cache (Performance) Terms Hit rate: fraction of accesses that hit in the cache Miss rate: 1 – Hit rate Miss penalty: time to replace a block from lower level in memory hierarchy to cache Hit time: time to access cache memory (including tag comparison) Abbreviation: “$” = cache (A Berkeley innovation!) 43
  • 44.Average Memory Access Time (AMAT) Average Memory Access Time (AMAT) is the average time to access memory considering both hits and misses in the cache AMAT = Time for a hit + Miss rate × Miss penalty 44
  • 45.B: 400 psec C: 600 psec D: ≥ 800 psec A: ≤200 psec ☐ ☐ ☐ ☐ 45 Clickers/Peer instruction AMAT = Time for a hit + Miss rate x Miss penalty Given a 200 psec clock, a miss penalty of 50 clock cycles, a miss rate of 0.02 misses per instruction and a cache hit time of 1 clock cycle, what is AMAT?
  • 46.Example: Direct-Mapped Cachewith 4 Single-Word Blocks, Worst-Case Reference String 0 4 0 4 0 4 0 4 Consider the main memory address reference string of word numbers: 0 4 0 4 0 4 0 4 Start with an empty cache - all blocks initially marked as not valid 46
  • 47.Example: Direct-Mapped Cachewith 4 Single-Word Blocks, Worst-Case Reference String 0 4 0 4 0 4 0 4 miss miss miss miss miss miss miss miss 00 Mem(0) 00 Mem(0) 01 4 01 Mem(4) 0 00 00 Mem(0) 01 4 00 Mem(0) 01 4 00 Mem(0) 01 4 01 Mem(4) 0 00 01 Mem(4) 0 00 Start with an empty cache - all blocks initially marked as not valid Ping-pong effect due to conflict misses - two memory locations that map into the same cache block 8 requests, 8 misses 47 Consider the main memory address reference string of word numbers: 0 4 0 4 0 4 0 4