Lecture 9: Improving Cache Performance: Reduce miss rate Reduce miss penalty Reduce hit time
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1 Lecture 9: Improving Cache Performance: Reduce miss rate Reduce miss penalty Reduce hit time
2 Review ABC of Cache: Associativity Block size Capacity Cache organization Direct-mapped cache : A =, S = C/B N-way set-associative: A = N, S =C /(BA) Fully assicaitivity S = 3 kinds of cache misses (3Cs) Compulsory Conflict Capacity How to improve cache performance? Average memory access time = hit time + miss-rate X miss-penalty 2
3 . Reduce Misses via Larger Block Size Block size Compulsory misses Spatial locality Example: access patter x,x4,x8,x2,.. Block size = 2 Word x (miss) x4 (hit) x8 (miss) x2 (hit) Block size = 4 Word x (miss) x4 (hit) x8 (hit) x2 (hit) 3
4 . Reduce Misses via Larger Block Size Block size Miss penalty Larger block size may increase capacity misses & conflict misses Example (conflict misses):» access pattern,2,4,6,9,,3,5,,2,4,6, Block size = B, direct-mapped cache Block size = 2B, direct--mapped cache
5 Reduce Misses via Larger Block Size Example2 (capacity misses) Access pattern,2,4,6,8,2,4,6,,2,4, Block size = B, fully-associative cache Block size = 2B, fully-associative cache
6 . Reduce Misses via Larger Block Size 25% Miss Rate 2% 5% % 5% % K 4K 6K 64K 256K Block Size (bytes) 6
7 2. Reduce Misses via Higher Associativity 2: Cache Rule: Miss Rate DM cache size N - Miss Rate 2-way cache size N/2 Beware: Execution time is only final measure! Will Clock Cycle time increase? Hill [988] suggested hit time external cache +%, internal + 2% for 2-way vs. -way 7
8 2: Cache Rule way 2-way 4-way Conflict x x 8-way Capacity Cache Size (KB) Compulsory 8
9 Example: Avg. Memory Access Time vs. Miss Rate Example: assume CCT =. for 2-way,.2 for 4-way,.4 for 8-way vs. CCT direct mapped Cache Size Associativity (KB) -way 2-way 4-way 8-way (Red means A.M.A.T. not improved by more associativity) 9
10 3. Reducing Misses via Victim Cache How to combine fast hit time of Direct Mapped yet still avoid conflict misses? victim cache Add buffer to place data discarded from cache Jouppi [99]: 4-entry victim cache removed 2% to 95% of conflicts for a 4 KB direct mapped data cache
11 4. Reducing Misses via Pseudo- Associativity How to combine fast hit time of Direct Mapped and have the lower conflict misses of 2-way SA cache? Divide cache: on a miss, check other half of cache to see if there, if so have a pseudo-hit (slow hit) Hit Time Pseudo Hit Time Miss Penalty Time Drawback: CPU pipeline is hard if hit takes or 2 cycles Better for caches not tied directly to processor
12 5. Reducing Misses by HW Prefetching of Instruction & Data Bring a cache block up the memory hierarchy before it is requested by the processor Example: Stream Buffer for instruction prefetching (alpha 264) Alpha 264 fetches 2 blocks on a miss Extra block placed in stream buffer On miss check stream buffer stream buffer 4 5 CPU Issue prefetch request 6 memory 2
13 5. Reducing Misses by HW Prefetching of Instruction & Data Works with data blocks too: Jouppi [99] data stream buffer got 25% misses from 4KB cache; 4 streams got 43% Palacharla & Kessler [994] for scientific programs for 8 streams got 5% to 7% of misses from 2 64KB, 4-way set associative caches Prefetching relies on extra memory bandwidth that can be used without penalty 3
14 6. Reducing Misses by SW Prefetching Data Data Prefetch Compiler insert prefetch instructions to the request the data before they are needed 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 Need a non-blocking cache Issuing Prefetch Instructions takes time Is cost of prefetch issues < savings in reduced misses? 4
15 7. Reducing Misses by Compiler Optimizations Instructions Data Reorder procedures in memory so as to reduce misses Profiling to look at conflicts McFarling [989] reduced caches misses by 75% on 8KB direct mapped cache with 4 byte blocks Merging Arrays: improve spatial locality by single array of compound elements vs. 2 arrays Loop Interchange: change nesting of loops to access data in order stored in memory Loop Fusion: Combine 2 independent loops that have same looping and some variables overlap Blocking: Improve temporal locality by accessing blocks of data repeatedly vs. going down whole columns or rows 5
16 Merging Arrays Example /* Before */ int val[size]; int key[size]; /* After */ struct merge { int val; int key; }; struct merge merged_array[size]; Reducing conflicts between val & key Val Val2 Val3 Val4 Key Key2 Key3 Key4 Val Key Val2 Key2 V al3 Key3 Val4 Key4 conflict 6
17 Loop Interchange Example /* Before */ for (k = ; k < ; k = k+) for (j = ; j < ; j = j+) for (i = ; i < 5; i = i+) x[i][j] = 2 * x[i][j]; /* After */ for (k = ; k < ; k = k+) for (i = ; i < 5; i = i+) for (j = ; j < ; j = j+) x[i][j] = 2 * x[i][j]; Sequential accesses Instead of striding through memory every words (improve spatial locality) 7
18 Loop Fusion Example /* Before */ for (i = ; i < N; i = i+) for (j = ; j < N; j = j+) a[i][j] = /b[i][j] * c[i][j]; for (i = ; i < N; i = i+) for (j = ; j < N; j = j+) d[i][j] = a[i][j] + c[i][j]; /* After */ for (i = ; i < N; i = i+) for (j = ; j < N; j = j+) { a[i][j] = /b[i][j] * c[i][j]; d[i][j] = a[i][j] + c[i][j];} 2 misses per access to a & c vs. one miss per access (improve temporal locality) 8
19 Blocking Example for (i = ; i < N; i = i+) for (j = ; j < N; j = j+) {r = ; for (k = ; k < N; k = k+){ r = r + y[i][k]*z[k][j];}; x[i][j] = r; }; j k j i k x y z 9
20 Blocking Example /* Before */ for (i = ; i < N; i = i+) for (j = ; j < N; j = j+) {r = ; for (k = ; k < N; k = k+){ r = r + y[i][k]*z[k][j];}; x[i][j] = r; }; Capacity Misses a function of N & Cache Size: 3 NxN => no capacity misses; otherwise... Idea: compute on BxB submatrix that fits 2
21 Blocking Example /* After */ for (jj = ; jj < N; jj = jj+b) for (kk = ; kk < N; kk = kk+b) for (i = ; i < N; i = i+) for (j = jj; j < min(jj+b-,n); j = j+) {r = ; for (k = kk; k < min(kk+b-,n); k = k+) { r = r + y[i][k]*z[k][j];}; x[i][j] = x[i][j] + r; }; 2
22 Blocking Example /* After */ for (jj = ; jj < N; jj = jj+b) for (kk = ; kk < N; kk = kk+b) for (i = ; i < N; i = i+) for (j = jj; j < min(jj+b-,n); j = j+) {r = ; for (k = kk; k < min(kk+b-,n); k = k+) { r = r + y[i][k]*z[k][j];}; x[i][j] = x[i][j] + r; }; Capacity Misses from 2N 3 + N 2 to 2N 3 /B +N 2 B called Blocking Factor Conflict Misses Too? 22
23 Reducing Conflict Misses by Blocking..5 Direct Mapped Cache Fully Associative Cache 5 5 Conflict misses in caches not FA vs. Blocking size Lam et al [99] a blocking factor of 24 had a fifth the misses vs. 48 despite both fit in cache Blocking Factor 23
24 Summary of Compiler Optimizations to Reduce Cache Misses vpenta (nasa7) gmty (nasa7) tomcatv btrix (nasa7) mxm (nasa7) spice cholesky (nasa7) compress Performance Improvement merged arrays loop interchange loop fusion blocking 24
25 Reducing Miss Rate Summary Reducing Miss Rate. Reduce Misses via Larger Block Size 2. Reduce Misses via Higher Associativity 3. Reducing Misses via Victim Cache 4. Reducing Misses via Pseudo-Associativity 5. Reducing Misses by HW Prefetching Instr, Data 6. Reducing Misses by SW Prefetching Data 7. Reducing Misses by Compiler Optimizations Remember danger of concentrating on just one parameter when evaluating performance reducing Miss penalty 25
26 . Reducing Miss Penalty: Read Priority over Write on Miss Write buffer dilemma: Decrease write-miss penalty ; increase read-miss penalty SW 52(R), R3 ; M[52]<- R3 (cache index ) LW R, 24(R) ; R <- M[24] (cache index ) LW R2, 52(R) ; R2 <- M[52] (cache index ) RAW Hazard (assume direct-mapped, write-through cache) cache Write buffer write New value (r+52) Read old value (r+52) memory 26
27 . Reducing Miss Penalty: Read Priority over Write on Miss If simply wait for write buffer to empty might increase read miss penalty by 5% (old MIPS ) Check write buffer contents before read; if no conflicts, let the memory access continue Write Back? 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 27
28 2. Subblock Placement to Reduce Miss Penalty Don t have to load full block on a miss Have bits per subblock to indicate valid (Originally invented to reduce tag storage) Sub-blocks Valid Bits 28
29 3. 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 a problem; tend to want next sequential word, so not clear if benefit by early restart 29
30 4. Non-blocking Caches to reduce stalls on misses Non-blocking cache or lockup-free cache allowing the data cache to continue to supply cache hits during a miss hit under miss reduces the effective miss penalty by being helpful during a miss instead of ignoring the requests of the CPU 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 3
31 5th Miss Penalty Reduction: Second Level Cache L2 Equations AMAT = Hit Time L + Miss Rate L x Miss Penalty L Miss Penalty L = Hit Time L2 + Miss Rate L2 x Miss Penalty L2 AMAT = Hit Time L + Miss Rate L x (Hit Time L2 + Miss Rate L2 + Miss Penalty L2 ) Definitions: Local miss rate misses in this cache divided by the total number of memory accesses to this cache (Miss rate L2 ) Global miss rate misses in this cache divided by the total number of memory accesses generated by the CPU (Miss Rate L x Miss Rate L2 ) 3
32 Comparing Local and Global Miss Rates Figure KByte st level cache; Increasing 2nd level cache Global miss rate close to single level cache rate provided L2 >> L Don t use local miss rate 32
33 L2 Cache Design Principle L2 not tied to CPU clock cycle Consider Cost & A.M.A.T. Generally Fast Hit Times and fewer misses Since hits are few, target miss reduction Larger cache, higher associativity and larger blocks 33
34 L2 cache block size & A.M.A.T. Relative CPU Time Block Size 32KB L, 8 byte path to memory 34
35 Reducing Miss Penalty Summary Five techniques Read priority over write on miss Subblock placement Early Restart and Critical Word First on miss Non-blocking Caches (Hit 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 35
36 Review: Improving Cache Performance. Reduce the miss rate, 2. Reduce the miss penalty, or 3. Reduce the time to hit in the cache. 36
37 . Fast Hit times via Small and Simple Caches Why Alpha 264 has 8KB Instruction and 8KB data cache + 96KB second level cache Direct Mapped, on chip 37
38 2. Fast hits by Avoiding Address Translation Address translation from virtual to physical addresses Physical cache:. Virtual -> physical 2. Use physical address to index cache => longer hit time Virtual cache: Use virtual cache to index cache => shorter hit time problem: aliasing More on this after covering virtual memory issues! 38
39 3. Fast Hit Times Via Pipelined Writes Pipeline Tag Check and Update Cache as separate stages; current write tag check & previous write cache update Only Write in the pipeline; empty during a miss write x write x2 tag check x write data tag check x2 write data In color is Delayed Write Buffer; must be checked on reads; either complete write or read from buffer 39
40 3. Fast Hit Times Via Pipelined Writes In color is Delayed Write Buffer; must be checked on reads; either complete write or read from buffer =? address CPU Data in data out tag Delayed write buffer =? mux Data Write buffer Lower level memory 4
41 4. Fast Writes on Misses Via Small Subblocks If most writes are word, subblock size is word, & write through then always write subblock & valid bit immediately Tag match and valid bit already set: Writing the block was proper, & nothing lost by setting valid bit on again. Tag match and valid bit not set: The tag match means that this is the proper block; writing the data into the subblock makes it appropriate to turn the valid bit on. Tag mismatch: This is a miss and will modify the data portion of the block. As this is a write-through cache, however, no harm was done; memory still has an up-to-date copy of the old value. Only the tag to the address of the write and the valid bits of the other subblock need be changed because the valid bit for this subblock has already been set Doesn t work with write back due to last case 4
42 Cache Optimization Summary Technique MR MP HT Complexity Larger Block Size + Higher Associativity + Victim Caches + 2 Pseudo-Associative Caches + 2 HW Prefetching of Instr/Data + 2 Compiler Controlled Prefetching + 3 Compiler Reduce Misses + Priority to Read Misses + Subblock Placement + Early Restart & Critical Word st + 2 Non-Blocking Caches + 3 Second Level Caches + 2 Small & Simple Caches + Avoiding Address Translation + 2 Pipelining Writes + 42
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