diff options
Diffstat (limited to 'src/mem/cache/prefetch/signature_path.cc')
| -rw-r--r-- | src/mem/cache/prefetch/signature_path.cc | 275 |
1 files changed, 275 insertions, 0 deletions
diff --git a/src/mem/cache/prefetch/signature_path.cc b/src/mem/cache/prefetch/signature_path.cc new file mode 100644 index 000000000..a0356b1d1 --- /dev/null +++ b/src/mem/cache/prefetch/signature_path.cc @@ -0,0 +1,275 @@ +/** + * Copyright (c) 2018 Metempsy Technology Consulting + * All rights reserved. + * + * Redistribution and use in source and binary forms, with or without + * modification, are permitted provided that the following conditions are + * met: redistributions of source code must retain the above copyright + * notice, this list of conditions and the following disclaimer; + * redistributions in binary form must reproduce the above copyright + * notice, this list of conditions and the following disclaimer in the + * documentation and/or other materials provided with the distribution; + * neither the name of the copyright holders nor the names of its + * contributors may be used to endorse or promote products derived from + * this software without specific prior written permission. + * + * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS + * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT + * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR + * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT + * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, + * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT + * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, + * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY + * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT + * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE + * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. + * + * Authors: Javier Bueno + */ + +#include "mem/cache/prefetch/signature_path.hh" + +#include <cassert> + +#include "debug/HWPrefetch.hh" +#include "mem/cache/prefetch/associative_set_impl.hh" +#include "params/SignaturePathPrefetcher.hh" + +SignaturePathPrefetcher::SignaturePathPrefetcher( + const SignaturePathPrefetcherParams *p) + : QueuedPrefetcher(p), + stridesPerPatternEntry(p->strides_per_pattern_entry), + signatureShift(p->signature_shift), + signatureBits(p->signature_bits), + maxCounterValue(p->max_counter_value), + prefetchConfidenceThreshold(p->prefetch_confidence_threshold), + lookaheadConfidenceThreshold(p->lookahead_confidence_threshold), + signatureTable(p->signature_table_assoc, p->signature_table_entries, + p->signature_table_indexing_policy, + p->signature_table_replacement_policy), + patternTable(p->pattern_table_assoc, p->pattern_table_entries, + p->pattern_table_indexing_policy, + p->pattern_table_replacement_policy, + PatternEntry(stridesPerPatternEntry)) +{ +} + +SignaturePathPrefetcher::PatternStrideEntry & +SignaturePathPrefetcher::PatternEntry::getStrideEntry(stride_t stride, + uint8_t max_counter_value) +{ + PatternStrideEntry *pstride_entry = findStride(stride); + if (pstride_entry == nullptr) { + // Specific replacement algorithm for this table, + // pick the entry with the lowest counter value, + // then decrease the counter of all entries + + // If all counters have the max value, this will be the pick + PatternStrideEntry *victim_pstride_entry = &(strideEntries[0]); + + uint8_t current_counter = max_counter_value; + for (auto &entry : strideEntries) { + if (entry.counter < current_counter) { + victim_pstride_entry = &entry; + current_counter = entry.counter; + } + if (entry.counter > 0) { + entry.counter -= 1; + } + } + pstride_entry = victim_pstride_entry; + pstride_entry->counter = 0; + pstride_entry->stride = stride; + } + return *pstride_entry; +} + +void +SignaturePathPrefetcher::addPrefetch(Addr ppn, stride_t block, + bool is_secure, std::vector<AddrPriority> &addresses) +{ + /** + * block is relative to the provided ppn. Assuming a page size of 4kB and + * a block size of 64B, the range of the stride of this prefetcher is + * -63,63 (pageBytes/blkSize) as the last accessed block also ranges from + * 0,63, the block value is expected to be between -63 and 126 + * Negative block means that we are accessing the lower contiguous page, + * 64 or greater point to the next contiguous. + */ + assert(block > -((stride_t)(pageBytes/blkSize))); + assert(block < 2*((stride_t)(pageBytes/blkSize))); + + Addr pf_ppn; + stride_t pf_block; + if (block < 0) { + pf_ppn = ppn - 1; + pf_block = block + (pageBytes/blkSize); + } else if (block >= (pageBytes/blkSize)) { + pf_ppn = ppn + 1; + pf_block = block - (pageBytes/blkSize); + } else { + pf_ppn = ppn; + pf_block = block; + } + + Addr new_addr = pf_ppn * pageBytes; + new_addr += pf_block * (Addr)blkSize; + + DPRINTF(HWPrefetch, "Queuing prefetch to %#x.\n", new_addr); + addresses.push_back(AddrPriority(new_addr, 0)); +} + +void +SignaturePathPrefetcher::updatePatternTable(Addr signature, stride_t stride) +{ + assert(stride != 0); + // The pattern table is indexed by signatures + PatternEntry &p_entry = getPatternEntry(signature); + PatternStrideEntry &ps_entry = p_entry.getStrideEntry(stride, + maxCounterValue); + if (ps_entry.counter < maxCounterValue) { + ps_entry.counter += 1; + } +} + +SignaturePathPrefetcher::SignatureEntry & +SignaturePathPrefetcher::getSignatureEntry(Addr ppn, bool is_secure, + stride_t block, bool &miss) +{ + SignatureEntry* signature_entry = signatureTable.findEntry(ppn, is_secure); + if (signature_entry != nullptr) { + signatureTable.accessEntry(signature_entry); + miss = false; + } else { + signature_entry = signatureTable.findVictim(ppn); + assert(signature_entry != nullptr); + + signatureTable.insertEntry(ppn, is_secure, signature_entry); + signature_entry->signature = block; + signature_entry->lastBlock = block; + miss = true; + } + return *signature_entry; +} + +SignaturePathPrefetcher::PatternEntry & +SignaturePathPrefetcher::getPatternEntry(Addr signature) +{ + PatternEntry* pattern_entry = patternTable.findEntry(signature, false); + if (pattern_entry != nullptr) { + // Signature found + patternTable.accessEntry(pattern_entry); + } else { + // Signature not found + pattern_entry = patternTable.findVictim(signature); + assert(pattern_entry != nullptr); + + patternTable.insertEntry(signature, false, pattern_entry); + } + return *pattern_entry; +} + +void +SignaturePathPrefetcher::calculatePrefetch(const PrefetchInfo &pfi, + std::vector<AddrPriority> &addresses) +{ + Addr request_addr = pfi.getAddr(); + Addr ppn = request_addr / pageBytes; + stride_t current_block = (request_addr % pageBytes) / blkSize; + stride_t stride; + bool is_secure = pfi.isSecure(); + + // Get the SignatureEntry of this page to: + // - compute the current stride + // - obtain the current signature of accesses + bool miss; + SignatureEntry &signature_entry = getSignatureEntry(ppn, is_secure, + current_block, miss); + if (miss) { + // No history for this page, can't continue + return; + } + + stride = current_block - signature_entry.lastBlock; + if (stride == 0) { + // Can't continue with a stride 0 + return; + } + + // Update the confidence of the current signature + updatePatternTable(signature_entry.signature, stride); + + // Update the current SignatureEntry signature and lastBlock + signature_entry.signature = + updateSignature(signature_entry.signature, stride); + signature_entry.lastBlock = current_block; + + signature_t current_signature = signature_entry.signature; + double current_confidence = 1.0; + stride_t current_stride = signature_entry.lastBlock; + + do { + // With the updated signature, attempt to generate prefetches + // - search the PatternTable and select all entries with enough + // confidence, these are prefetch candidates + // - select the entry with the highest counter as the "lookahead" + PatternEntry *current_pattern_entry = + patternTable.findEntry(current_signature, false); + PatternStrideEntry const *lookahead = nullptr; + if (current_pattern_entry != nullptr) { + uint8_t max_counter = 0; + for (auto const &entry : current_pattern_entry->strideEntries) { + //select the entry with the maximum counter value as lookahead + if (max_counter < entry.counter) { + max_counter = entry.counter; + lookahead = &entry; + } + double prefetch_confidence = + (double) entry.counter / maxCounterValue; + + if (prefetch_confidence >= prefetchConfidenceThreshold) { + assert(entry.stride != 0); + //prefetch candidate + addPrefetch(ppn, current_stride + entry.stride, + is_secure, addresses); + } + } + } + if (lookahead != nullptr) { + // If a lookahead was selected, compute its confidence using + // the counter of its entry and the accumulated confidence + // if the confidence is high enough, generate a new signature + double lookahead_confidence; + if (lookahead->counter == maxCounterValue) { + // maximum confidence is 0.95, guaranteeing that + // current confidence will eventually fall beyond + // the threshold + lookahead_confidence = 0.95; + } else { + lookahead_confidence = + ((double) lookahead->counter / maxCounterValue); + } + current_confidence *= lookahead_confidence; + current_signature = + updateSignature(current_signature, lookahead->stride); + current_stride += lookahead->stride; + } else { + current_confidence = 0.0; + } + // If the accumulated confidence is high enough, keep repeating + // this process with the updated signature + } + while (current_confidence > lookaheadConfidenceThreshold); + + if (addresses.empty()) { + // Enable the next line prefetcher if no prefetch candidates are found + addPrefetch(ppn, current_block + 1, is_secure, addresses); + } +} + +SignaturePathPrefetcher* +SignaturePathPrefetcherParams::create() +{ + return new SignaturePathPrefetcher(this); +} |