CPU Memory Hierarchy and Caching Techniques
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COU Memory Hierarchy. Widening processor-memory performance Gap. Cache replacement options. Basic Cache Optimization Techniques
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CPU Memory Hierarchy and Caching Techniques
- 1. Memory Hierarchy CS4342 Advanced Computer Architecture Dilum Bandara Dilum.Bandara@uom.lk Slides adapted from “Computer Architecture, A Quantitative Approach” by John L. Hennessy and David A. Patterson, 5th Edition, 2012, Morgan Kaufmann Publishers
- 2. Processor-Memory Performance Gap 2 Gap grew 50% per year
- 3. Why Memory Hierarchy? Applications want unlimited amounts of memory with low latency Fast memory is more expensive per bit Solution Organize memory system into a hierarchy Entire addressable memory space available in largest, slowest memory Incrementally smaller & faster memories Temporal & spatial locality ensures that nearly all references can be found in smaller memories Gives illusion of a large, fast memory being presented to processor 3
- 5. Why Hierarchical Design? Becomes more crucial with multi-core processors Aggregate peak bandwidth grows with no of cores Intel Core i7 can generate 2 references per core per clock 4 cores and 3.2 GHz clock 25.6 billion 64-bit data references/second + 12.8 billion 128-bit instruction references = 409.6 GB/s DRAM bandwidth is only 6% of this (25 GB/s) Requires Multi-port, pipelined caches 2 levels of cache per core Shared third-level cache on chip 5
- 6. Core i7 Die & Major Components 6 Source: Intel
- 7. Performance vs. Power High-end microprocessors have >10 MB on-chip cache Consumes large amount of chip area & power budget Leakage current – when not operating Active current – when operating Major limiting factor for processors used in mobile devices 7
- 8. Definitions – Blocks Multiple blocks are moved between levels in the hierarchy Spatial locality efficiency Blocks are tagged with memory address Tags are searched parallel 8 Source: http://archive.arstechnica.com/paedia/c/caching/m-caching-5.html
- 9. Definitions – Associativity Defines where blocks can be placed in a cache 9
- 10. Pentium 4 vs. Opteron Memory Hierarchy 10 CPU Pentium 4 (3.2 GHz) Opteron (2.8 GHz) Instruction Cache Trace Cache 8K micro-ops) 2-way associative, 64 KB, 64B block Data Cache 8-way associative, 16 KB, 64B block, inclusive in L2 2-way associative, 64 KB, 64B block, exclusive to L2 L2 Cache 8-way associative, 2 MB, 128B block 16-way associative, 1 MB, 64B block Prefetch 8 streams to L2 1 stream to L2 Memory 200 MHz x 64 bits 200 MHz x 128 bits
- 11. Definitions – Updating Cache Write-through Update cache block & all other levels below Use write buffers to speed up Write-back Update cache block Update lower level when replacing cached block Use write buffers to speed up 11
- 12. Definitions – Replacing Cached Blocks Cache replacement policies Random Least Recently Used (LRU) Need to track last access time Least Frequently Used (LFU) Need to track no of accesses First In First Out (FIFO) 12
- 13. Definitions – Cache Misses When required items is not found in cache Miss rate – fraction of cache accesses that result in a failure Types of misses Compulsory – 1st access to a block Capacity – limited cache capacity force blocked to be removed from a cache & later retrieved Conflict – if placement strategy is not fully associative Average memory access time = Hit time + Miss rate x Miss penalty 13
- 14. Definitions – Cache Misses (Cont.) Memory stall cycles = Instruction count x Fraction of memory access per instructions x Miss rate x Miss Penalty Fraction of memory access per instructions = Instruction memory access per instruction + Data memory access per instruction Example 50% instructions are load & store. Miss rate is 2% & penalty is 25 clock cycles. Suppose CPI is 1. How fast can this be if all instructions are cache hits? IC x (1 + 0.75) x CC = 1.75 IC x 1 x CC 14
- 15. Cache Performance Metrics Hit time Miss rate Miss penalty Cache bandwidth Power consumption 15
- 16. 6 Basic Cache Optimization Techniques 1. Larger block sizes Reduce compulsory misses Increase capacity & conflict misses Increase miss penalty Choosing a correct block size is challenging 2. Larger total cache capacity to reduce miss rate Reduce misses Increase hit time Increase power consumption & cost 3. Higher no of cache levels Reduce overall memory access time 16
- 17. 6 Basic Cache Optimization Techniques (Cont.) 4. Higher associativity Reduce conflict misses Increase hit time Increase power consumption 5. Giving priority to read misses over writes Allow reads to check write buffer Reduce miss penalty 6. Avoiding address translation in cache indexing Virtual to physical address mapping Reduce hit time 17
- 18. 10 Advanced Cache Optimization Techniques 5 categories 1. Reducing hit time 2. Increasing cache bandwidth 3. Reducing miss penalty 4. Reducing miss rate 5. Reducing miss penalty or miss rate via parallelism 18
- 19. Advanced Optimizations 1 Small & simple 1st level caches Recently size of L1 cache increased either slightly or not at all Critical timing path in a cache hit addressing tag memory, then comparing tags, then selecting correct set Direct-mapped caches can overlap tag compare & transmission of data Improve hit time Lower associativity reduces power because fewer cache lines are accessed 19
- 20. L1 Size & Associativity – Access Time 20
- 21. L1 Size & Associativity – Energy 21
- 22. Advanced Optimizations 2 Way Prediction Given access to the current block, predict which block to access next Improve hit time Mis-prediction increase hit time Prediction accuracy > 90% for 2-way > 80% for 4-way Instruction cache has better accuracy than Data cache First used on MIPS R10000 in mid-90s Used on ARM Cortex-A8 22
- 23. Advanced Optimizations 3 Pipeline cache access Enable L1 cache access to be multiple cycles Examples Pentium – 1 cycle Pentium Pro to Pentium III – 2 cycles Pentium 4 to Core i7 – 4 cycles Improve bandwidth Makes it easier to increase associativity Increase hit time Increases branch mis-prediction penalty 23
- 24. Advanced Optimizations 4 Nonblocking Caches Allow hits before previous misses complete “Hit under miss” “Hit under multiple miss” L2 must support this In general, processors can hide L1 miss penalty but not L2 miss penalty Increase bandwidth 24
- 25. Advanced Optimizations 5 Multibanked Caches Organize cache as independent banks to support simultaneous access Examples ARM Cortex-A8 supports 1-4 banks for L2 Intel i7 supports 4 banks for L1 & 8 banks for L2 Interleave banks according to block address Increase bandwidth 25
- 26. Advanced Optimizations 6 Critical Word First, Early Restart Critical word first Request missed word from memory first Send it to processor as soon as it arrives Early restart Request words in normal order Send missed work to processor as soon as it arrives Reduce miss penalty Effectiveness depends on block size & likelihood of another access to portion of the block that has not yet been fetched 26
- 27. Advanced Optimizations 7 - 10 Merging Write Buffer Reduce miss penalty Compiler Optimizations Examples Loop Interchange – Swap nested loops to access memory in sequential order Instead of accessing entire rows or columns, subdivide matrices into blocks Reduce miss rate Hardware Prefetching Fetch 2 blocks on miss Reduce miss penalty or miss rate Compiler Prefetching Reduce miss penalty or miss rate 27
- 29. Memory Technologies Performance metrics Latency is concern of cache Bandwidth is concern of multiprocessors & I/O Access time Time between read request & when desired word arrives Cycle time Minimum time between unrelated requests to memory DRAM used for main memory SRAM used for cache 29
- 30. Memory Technology (Cont.) Amdahl Memory capacity should grow linearly with processor speed Unfortunately, memory capacity & speed hasn’t kept pace with processors Some optimizations Multiple accesses to same row Synchronous DRAM (SDRAM) Added clock to DRAM interface Burst mode with critical word first Wider interfaces Double data rate (DDR) Multiple banks on each DRAM device 30
- 31. DRAM Optimizations 31 MB/sec = Clock rate x 2 x 8 bytes
- 32. DRAM Power Consumption Reducing power in DRAMs Lower voltage Low power mode (ignores clock, continues to refresh) 32
- 33. Flash Memory Type of EEPROM Must be erased (in blocks) before being overwritten Non volatile Limited no of write cycles Cheaper than DRAM, more expensive than disk Slower than SRAM, faster than disk 33
- 34. Modern Memory Hierarchy 34 Source: http://blog.teachbook.com.au/index.php/2012/02/memory-hierarchy/
- 35. Intel Optane Non-volatile Memory 35 Source: www.forbes.com/sites/tomcoughlin/2018/06/11/intel-optane-finally-on-dimms/#5792e114190b
- 36. Intel Optane (Cont.) 36 Source: www.anandtech.com/show/9541/intel-announces-optane-storage-brand-for-3d-xpoint-products
- 37. Virtual Memory Each process has its own address space Protection via virtual memory Keeps processes in their own memory space Role of architecture Provide user mode & supervisor mode Protect certain aspects of CPU state Provide mechanisms for switching between user mode & supervisor mode Provide mechanisms to limit memory accesses Provide TLB to translate addresses 37
- 38. Paging Hardware With TLB Parallel search on TLB Address translation (p, d) If p is in associative register, get frame # out Otherwise get frame # from page table in memory
- 39. Summary Caching techniques are continuing to evolve Combination of techniques are combined Cache sizes are unlikely to increase significantly Better performance when programs are optimized based on cache architecture 39