signature_path.hh revision 13624
1/** 2 * Copyright (c) 2018 Metempsy Technology Consulting 3 * All rights reserved. 4 * 5 * Redistribution and use in source and binary forms, with or without 6 * modification, are permitted provided that the following conditions are 7 * met: redistributions of source code must retain the above copyright 8 * notice, this list of conditions and the following disclaimer; 9 * redistributions in binary form must reproduce the above copyright 10 * notice, this list of conditions and the following disclaimer in the 11 * documentation and/or other materials provided with the distribution; 12 * neither the name of the copyright holders nor the names of its 13 * contributors may be used to endorse or promote products derived from 14 * this software without specific prior written permission. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS 17 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT 18 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR 19 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT 20 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, 21 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT 22 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, 23 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY 24 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT 25 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE 26 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27 * 28 * Authors: Javier Bueno 29 */ 30 31 /** 32 * Implementation of the Signature Path Prefetcher 33 * 34 * References: 35 * Lookahead prefetching with signature path 36 * J Kim, PV Gratz, ALN Reddy 37 * The 2nd Data Prefetching Championship (DPC2) 38 * The filter feature described in the paper is not implemented, as it 39 * redundant prefetches are dropped by the cache. 40 */ 41 42#ifndef __MEM_CACHE_PREFETCH_SIGNATURE_PATH_HH__ 43#define __MEM_CACHE_PREFETCH_SIGNATURE_PATH_HH__ 44 45#include "mem/cache/prefetch/associative_set.hh" 46#include "mem/cache/prefetch/queued.hh" 47#include "mem/packet.hh" 48 49struct SignaturePathPrefetcherParams; 50 51class SignaturePathPrefetcher : public QueuedPrefetcher 52{ 53 protected: 54 /** Signature type */ 55 typedef uint16_t signature_t; 56 /** Stride type */ 57 typedef int16_t stride_t; 58 59 /** Number of strides stored in each pattern entry */ 60 const unsigned stridesPerPatternEntry; 61 /** Number of bits to shift when generating a new signature */ 62 const uint8_t signatureShift; 63 /** Size of the signature, in bits */ 64 const signature_t signatureBits; 65 /** Maximum pattern entries counter value */ 66 const uint8_t maxCounterValue; 67 /** Minimum confidence to issue a prefetch */ 68 const double prefetchConfidenceThreshold; 69 /** Minimum confidence to keep navigating lookahead entries */ 70 const double lookaheadConfidenceThreshold; 71 72 /** Signature entry data type */ 73 struct SignatureEntry : public TaggedEntry 74 { 75 /** Path signature */ 76 signature_t signature; 77 /** Last accessed block within a page */ 78 stride_t lastBlock; 79 SignatureEntry() : signature(0), lastBlock(0) 80 {} 81 }; 82 /** Signature table */ 83 AssociativeSet<SignatureEntry> signatureTable; 84 85 /** A stride entry with its counter */ 86 struct PatternStrideEntry 87 { 88 /** stride in a page in blkSize increments */ 89 stride_t stride; 90 /** counter value (max value defined by maxCounterValue) */ 91 uint8_t counter; 92 PatternStrideEntry() : stride(0), counter(0) 93 {} 94 }; 95 /** Pattern entry data type, a set of stride and counter entries */ 96 struct PatternEntry : public TaggedEntry 97 { 98 /** group of stides */ 99 std::vector<PatternStrideEntry> strideEntries; 100 /** use counter, used by SPPv2 */ 101 uint8_t counter; 102 PatternEntry(size_t num_strides) : strideEntries(num_strides), 103 counter(0) 104 {} 105 106 /** Reset the entries to their initial values */ 107 void reset() override 108 { 109 for (auto &entry : strideEntries) { 110 entry.counter = 0; 111 entry.stride = 0; 112 } 113 counter = 0; 114 } 115 116 /** 117 * Returns the entry with the desired stride 118 * @param stride the stride to find 119 * @result a pointer to the entry, if the stride was found, or nullptr, 120 * if the stride was not found 121 */ 122 PatternStrideEntry *findStride(stride_t stride) 123 { 124 PatternStrideEntry *found_entry = nullptr; 125 for (auto &entry : strideEntries) { 126 if (entry.stride == stride) { 127 found_entry = &entry; 128 break; 129 } 130 } 131 return found_entry; 132 } 133 134 /** 135 * Gets the entry with the provided stride, if there is no entry with 136 * the associated stride, it replaces one of them. 137 * @param stride the stride to find 138 * @param max_counter_value maximum value of the confidence counters, 139 * it is used when no strides are found and an entry needs to be 140 * replaced 141 * @result reference to the selected entry 142 */ 143 PatternStrideEntry &getStrideEntry(stride_t stride, 144 uint8_t max_counter_value); 145 }; 146 /** Pattern table */ 147 AssociativeSet<PatternEntry> patternTable; 148 149 /** 150 * Generates a new signature from an existing one and a new stride 151 * @param sig current signature 152 * @param str stride to add to the new signature 153 * @result the new signature 154 */ 155 inline signature_t updateSignature(signature_t sig, stride_t str) const { 156 sig <<= signatureShift; 157 sig ^= str; 158 sig &= mask(signatureBits); 159 return sig; 160 } 161 162 /** 163 * Generates an address to be prefetched. 164 * @param ppn page number to prefetch from 165 * @param last_block last accessed block within the page ppn 166 * @param delta difference, in number of blocks, from the last_block 167 * accessed to the block to prefetch. The block to prefetch is 168 * computed by this formula: 169 * ppn * pageBytes + (last_block + delta) * blkSize 170 * This value can be negative. 171 * @param path_confidence the confidence factor of this prefetch 172 * @param signature the current path signature 173 * @param is_secure whether this page is inside the secure memory area 174 * @param addresses addresses to prefetch will be added to this vector 175 */ 176 void addPrefetch(Addr ppn, stride_t last_block, stride_t delta, 177 double path_confidence, signature_t signature, 178 bool is_secure, 179 std::vector<AddrPriority> &addresses); 180 181 /** 182 * Obtains the SignatureEntry of the given page, if the page is not found, 183 * it allocates a new one, replacing an existing entry if needed 184 * It also provides the stride of the current block and the initial 185 * path confidence of the corresponding entry 186 * @param ppn physical page number of the page 187 * @param is_secure whether this page is inside the secure memory area 188 * @param block accessed block within the page 189 * @param miss if the entry is not found, this will be set to true 190 * @param stride set to the computed stride 191 * @param initial_confidence set to the initial confidence value 192 * @result a reference to the SignatureEntry 193 */ 194 SignatureEntry &getSignatureEntry(Addr ppn, bool is_secure, stride_t block, 195 bool &miss, stride_t &stride, double &initial_confidence); 196 /** 197 * Obtains the PatternEntry of the given signature, if the signature is 198 * not found, it allocates a new one, replacing an existing entry if needed 199 * @param signature the signature of the desired entry 200 * @result a reference to the PatternEntry 201 */ 202 PatternEntry& getPatternEntry(Addr signature); 203 204 /** 205 * Updates the pattern table with the provided signature and stride 206 * @param signature the signature to use to index the pattern table 207 * @param stride the stride to use to index the set of strides of the 208 * pattern table entry 209 */ 210 void updatePatternTable(Addr signature, stride_t stride); 211 212 /** 213 * Computes the lookahead path confidence of the provided pattern entry 214 * @param sig the PatternEntry to use 215 * @param lookahead PatternStrideEntry within the provided PatternEntry 216 * @return the computed confidence factor 217 */ 218 virtual double calculateLookaheadConfidence(PatternEntry const &sig, 219 PatternStrideEntry const &lookahead) const; 220 221 /** 222 * Computes the prefetch confidence of the provided pattern entry 223 * @param sig the PatternEntry to use 224 * @param entry PatternStrideEntry within the provided PatternEntry 225 * @return the computed confidence factor 226 */ 227 virtual double calculatePrefetchConfidence(PatternEntry const &sig, 228 PatternStrideEntry const &entry) const; 229 230 /** 231 * Increases the counter of a given PatternEntry/PatternStrideEntry 232 * @param pattern_entry the corresponding PatternEntry 233 * @param pstride_entry the PatternStrideEntry within the PatternEntry 234 */ 235 virtual void increasePatternEntryCounter(PatternEntry &pattern_entry, 236 PatternStrideEntry &pstride_entry); 237 238 /** 239 * Whenever a new SignatureEntry is allocated, it computes the new 240 * signature to be used with the new entry, the resulting stride and the 241 * initial path confidence of the new entry. 242 * @param current_block accessed block within the page of the associated 243 entry 244 * @param new_signature new signature of the allocated entry 245 * @param new_conf the initial path confidence of this entry 246 * @param new_stride the resulting current stride 247 */ 248 virtual void handleSignatureTableMiss(stride_t current_block, 249 signature_t &new_signature, double &new_conf, 250 stride_t &new_stride); 251 252 /** 253 * Auxiliar prefetch mechanism used at the end of calculatePrefetch. 254 * This prefetcher uses this to activate the next line prefetcher if 255 * no prefetch candidates have been found. 256 * @param ppn physical page number of the current accessed page 257 * @param current_block last accessed block within the page ppn 258 * @param is_secure whether this page is inside the secure memory area 259 * @param addresses the addresses to be prefetched are added to this vector 260 * @param updated_filter_entries set of addresses containing these that 261 * their filter has been updated, if this call updates a new entry 262 */ 263 virtual void auxiliaryPrefetcher(Addr ppn, stride_t current_block, 264 bool is_secure, std::vector<AddrPriority> &addresses); 265 266 /** 267 * Handles the situation when the lookahead process has crossed the 268 * boundaries of the current page. This is not fully described in the 269 * paper that was used to implement this code, however, the article 270 * describing the upgraded version of this prefetcher provides some 271 * details. For this prefetcher, there are no specific actions to be 272 * done. 273 * @param signature the lookahead signature that crossed the page 274 * @param delta the current stride that caused it 275 * @param last_offset the last accessed block within the page 276 * @param path_confidence the path confidence at the moment of crossing 277 */ 278 virtual void handlePageCrossingLookahead(signature_t signature, 279 stride_t last_offset, stride_t delta, double path_confidence) { 280 } 281 282 public: 283 SignaturePathPrefetcher(const SignaturePathPrefetcherParams* p); 284 ~SignaturePathPrefetcher() {} 285 void calculatePrefetch(const PrefetchInfo &pfi, 286 std::vector<AddrPriority> &addresses) override; 287}; 288 289#endif//__MEM_CACHE_PREFETCH_SIGNATURE_PATH_HH__ 290