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