src/mem/cache/compressors/base.cc

   1 /*
   2  * Copyright (c) 2018 Inria
   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: Daniel Carvalho
  29  */
  30
  31 /** @file
  32  * Definition of a basic cache compressor.
  33  */
  34
  35 #include "mem/cache/compressors/base.hh"
  36
  37 #include <algorithm>
  38 #include <cmath>
  39 #include <cstdint>
  40 #include <string>
  41
  42 #include "debug/CacheComp.hh"
  43 #include "mem/cache/tags/super_blk.hh"
  44 #include "params/BaseCacheCompressor.hh"
  45
  46 // Uncomment this line if debugging compression
  47 //#define DEBUG_COMPRESSION
  48
  49 BaseCacheCompressor::CompressionData::CompressionData()
  50     : _size(0)
  51 {
  52 }
  53
  54 BaseCacheCompressor::CompressionData::~CompressionData()
  55 {
  56 }
  57
  58 void
  59 BaseCacheCompressor::CompressionData::setSizeBits(std::size_t size)
  60 {
  61     _size = size;
  62 }
  63
  64 std::size_t
  65 BaseCacheCompressor::CompressionData::getSizeBits() const
  66 {
  67     return _size;
  68 }
  69
  70 std::size_t
  71 BaseCacheCompressor::CompressionData::getSize() const
  72 {
  73     return std::ceil(_size/8);
  74 }
  75
  76 BaseCacheCompressor::BaseCacheCompressor(const Params *p)
  77   : SimObject(p), blkSize(p->block_size), sizeThreshold(p->size_threshold),
  78     stats(*this)
  79 {
  80     fatal_if(blkSize < sizeThreshold, "Compressed data must fit in a block");
  81 }
  82
  83 void
  84 BaseCacheCompressor::compress(const uint64_t* data, Cycles& comp_lat,
  85                               Cycles& decomp_lat, std::size_t& comp_size_bits)
  86 {
  87     // Apply compression
  88     std::unique_ptr<CompressionData> comp_data =
  89         compress(data, comp_lat, decomp_lat);
  90
  91     // If we are in debug mode apply decompression just after the compression.
  92     // If the results do not match, we've got an error
  93     #ifdef DEBUG_COMPRESSION
  94     uint64_t decomp_data[blkSize/8];
  95
  96     // Apply decompression
  97     decompress(comp_data.get(), decomp_data);
  98
  99     // Check if decompressed line matches original cache line
 100     fatal_if(std::memcmp(data, decomp_data, blkSize),
 101              "Decompressed line does not match original line.");
 102     #endif
 103
 104     // Get compression size. If compressed size is greater than the size
 105     // threshold, the compression is seen as unsuccessful
 106     comp_size_bits = comp_data->getSizeBits();
 107     if (comp_size_bits >= sizeThreshold * 8) {
 108         comp_size_bits = blkSize * 8;
 109     }
 110
 111     // Update stats
 112     stats.compressions++;
 113     stats.compressionSizeBits += comp_size_bits;
 114     stats.compressionSize[std::ceil(std::log2(comp_size_bits))]++;
 115
 116     // Print debug information
 117     DPRINTF(CacheComp, "Compressed cache line from %d to %d bits. " \
 118             "Compression latency: %llu, decompression latency: %llu\n",
 119             blkSize*8, comp_size_bits, comp_lat, decomp_lat);
 120 }
 121
 122 Cycles
 123 BaseCacheCompressor::getDecompressionLatency(const CacheBlk* blk)
 124 {
 125     const CompressionBlk* comp_blk = static_cast<const CompressionBlk*>(blk);
 126
 127     // If block is compressed, return its decompression latency
 128     if (comp_blk && comp_blk->isCompressed()){
 129         const Cycles decomp_lat = comp_blk->getDecompressionLatency();
 130         DPRINTF(CacheComp, "Decompressing block: %s (%d cycles)\n",
 131                 comp_blk->print(), decomp_lat);
 132         stats.decompressions += 1;
 133         return decomp_lat;
 134     }
 135
 136     // Block is not compressed, so there is no decompression latency
 137     return Cycles(0);
 138 }
 139
 140 void
 141 BaseCacheCompressor::setDecompressionLatency(CacheBlk* blk, const Cycles lat)
 142 {
 143     // Sanity check
 144     assert(blk != nullptr);
 145
 146     // Assign latency
 147     static_cast<CompressionBlk*>(blk)->setDecompressionLatency(lat);
 148 }
 149
 150 void
 151 BaseCacheCompressor::setSizeBits(CacheBlk* blk, const std::size_t size_bits)
 152 {
 153     // Sanity check
 154     assert(blk != nullptr);
 155
 156     // Assign size
 157     static_cast<CompressionBlk*>(blk)->setSizeBits(size_bits);
 158 }
 159
 160 BaseCacheCompressor::BaseCacheCompressorStats::BaseCacheCompressorStats(
 161     BaseCacheCompressor& _compressor)
 162   : Stats::Group(&_compressor), compressor(_compressor),
 163     compressions(this, "compressions",
 164         "Total number of compressions"),
 165     compressionSize(this, "compression_size",
 166         "Number of blocks that were compressed to this power of two size"),
 167     compressionSizeBits(this, "compression_size_bits",
 168         "Total compressed data size, in bits"),
 169     avgCompressionSizeBits(this, "avg_compression_size_bits",
 170         "Average compression size, in bits"),
 171     decompressions(this, "total_decompressions",
 172         "Total number of decompressions")
 173 {
 174 }
 175
 176 void
 177 BaseCacheCompressor::BaseCacheCompressorStats::regStats()
 178 {
 179     Stats::Group::regStats();
 180
 181     compressionSize.init(std::log2(compressor.blkSize*8) + 1);
 182     for (unsigned i = 0; i <= std::log2(compressor.blkSize*8); ++i) {
 183         std::string str_i = std::to_string(1 << i);
 184         compressionSize.subname(i, str_i);
 185         compressionSize.subdesc(i,
 186             "Number of blocks that compressed to fit in " + str_i + " bits");
 187     }
 188
 189     avgCompressionSizeBits.flags(Stats::total | Stats::nozero | Stats::nonan);
 190     avgCompressionSizeBits = compressionSizeBits / compressions;
 191 }
 192