src/mem/cache/compressors/base.cc

   1 /*
   2  * Copyright (c) 2018-2020 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
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   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,
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  22  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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  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 <climits>
  37 #include <cmath>
  38 #include <cstdint>
  39 #include <string>
  40
  41 #include "base/trace.hh"
  42 #include "debug/CacheComp.hh"
  43 #include "mem/cache/tags/super_blk.hh"
  44 #include "params/BaseCacheCompressor.hh"
  45
  46 namespace Compressor {
  47
  48 // Uncomment this line if debugging compression
  49 //#define DEBUG_COMPRESSION
  50
  51 Base::CompressionData::CompressionData()
  52     : _size(0)
  53 {
  54 }
  55
  56 Base::CompressionData::~CompressionData()
  57 {
  58 }
  59
  60 void
  61 Base::CompressionData::setSizeBits(std::size_t size)
  62 {
  63     _size = size;
  64 }
  65
  66 std::size_t
  67 Base::CompressionData::getSizeBits() const
  68 {
  69     return _size;
  70 }
  71
  72 std::size_t
  73 Base::CompressionData::getSize() const
  74 {
  75     return std::ceil(_size/8);
  76 }
  77
  78 Base::Base(const Params &p)
  79   : SimObject(p), blkSize(p.block_size), chunkSizeBits(p.chunk_size_bits),
  80     sizeThreshold((blkSize * p.size_threshold_percentage) / 100),
  81     stats(*this)
  82 {
  83     fatal_if(64 % chunkSizeBits,
  84         "64 must be a multiple of the chunk granularity.");
  85
  86     fatal_if(blkSize < sizeThreshold, "Compressed data must fit in a block");
  87 }
  88
  89 std::vector<Base::Chunk>
  90 Base::toChunks(const uint64_t* data) const
  91 {
  92     // Number of chunks in a 64-bit value
  93     const unsigned num_chunks_per_64 =
  94         (sizeof(uint64_t) * CHAR_BIT) / chunkSizeBits;
  95
  96     // Turn a 64-bit array into a chunkSizeBits-array
  97     std::vector<Chunk> chunks((blkSize * CHAR_BIT) / chunkSizeBits, 0);
  98     for (int i = 0; i < chunks.size(); i++) {
  99         const int index_64 = std::floor(i / (double)num_chunks_per_64);
 100         const unsigned start = i % num_chunks_per_64;
 101         chunks[i] = bits(data[index_64],
 102             (start + 1) * chunkSizeBits - 1, start * chunkSizeBits);
 103     }
 104
 105     return chunks;
 106 }
 107
 108 void
 109 Base::fromChunks(const std::vector<Chunk>& chunks, uint64_t* data) const
 110 {
 111     // Number of chunks in a 64-bit value
 112     const unsigned num_chunks_per_64 =
 113         (sizeof(uint64_t) * CHAR_BIT) / chunkSizeBits;
 114
 115     // Turn a chunkSizeBits-array into a 64-bit array
 116     std::memset(data, 0, blkSize);
 117     for (int i = 0; i < chunks.size(); i++) {
 118         const int index_64 = std::floor(i / (double)num_chunks_per_64);
 119         const unsigned start = i % num_chunks_per_64;
 120         replaceBits(data[index_64], (start + 1) * chunkSizeBits - 1,
 121             start * chunkSizeBits, chunks[i]);
 122     }
 123 }
 124
 125 std::unique_ptr<Base::CompressionData>
 126 Base::compress(const uint64_t* data, Cycles& comp_lat, Cycles& decomp_lat)
 127 {
 128     // Apply compression
 129     std::unique_ptr<CompressionData> comp_data =
 130         compress(toChunks(data), comp_lat, decomp_lat);
 131
 132     // If we are in debug mode apply decompression just after the compression.
 133     // If the results do not match, we've got an error
 134     #ifdef DEBUG_COMPRESSION
 135     uint64_t decomp_data[blkSize/8];
 136
 137     // Apply decompression
 138     decompress(comp_data.get(), decomp_data);
 139
 140     // Check if decompressed line matches original cache line
 141     fatal_if(std::memcmp(data, decomp_data, blkSize),
 142              "Decompressed line does not match original line.");
 143     #endif
 144
 145     // Get compression size. If compressed size is greater than the size
 146     // threshold, the compression is seen as unsuccessful
 147     std::size_t comp_size_bits = comp_data->getSizeBits();
 148     if (comp_size_bits > sizeThreshold * CHAR_BIT) {
 149         comp_size_bits = blkSize * CHAR_BIT;
 150         comp_data->setSizeBits(comp_size_bits);
 151         stats.failedCompressions++;
 152     }
 153
 154     // Update stats
 155     stats.compressions++;
 156     stats.compressionSizeBits += comp_size_bits;
 157     if (comp_size_bits != 0) {
 158         stats.compressionSize[1 + std::ceil(std::log2(comp_size_bits))]++;
 159     } else {
 160         stats.compressionSize[0]++;
 161     }
 162
 163     // Print debug information
 164     DPRINTF(CacheComp, "Compressed cache line from %d to %d bits. " \
 165             "Compression latency: %llu, decompression latency: %llu\n",
 166             blkSize*8, comp_size_bits, comp_lat, decomp_lat);
 167
 168     return comp_data;
 169 }
 170
 171 Cycles
 172 Base::getDecompressionLatency(const CacheBlk* blk)
 173 {
 174     const CompressionBlk* comp_blk = static_cast<const CompressionBlk*>(blk);
 175
 176     // If block is compressed, return its decompression latency
 177     if (comp_blk && comp_blk->isCompressed()){
 178         const Cycles decomp_lat = comp_blk->getDecompressionLatency();
 179         DPRINTF(CacheComp, "Decompressing block: %s (%d cycles)\n",
 180                 comp_blk->print(), decomp_lat);
 181         stats.decompressions += 1;
 182         return decomp_lat;
 183     }
 184
 185     // Block is not compressed, so there is no decompression latency
 186     return Cycles(0);
 187 }
 188
 189 void
 190 Base::setDecompressionLatency(CacheBlk* blk, const Cycles lat)
 191 {
 192     // Sanity check
 193     assert(blk != nullptr);
 194
 195     // Assign latency
 196     static_cast<CompressionBlk*>(blk)->setDecompressionLatency(lat);
 197 }
 198
 199 void
 200 Base::setSizeBits(CacheBlk* blk, const std::size_t size_bits)
 201 {
 202     // Sanity check
 203     assert(blk != nullptr);
 204
 205     // Assign size
 206     static_cast<CompressionBlk*>(blk)->setSizeBits(size_bits);
 207 }
 208
 209 Base::BaseStats::BaseStats(Base& _compressor)
 210   : Stats::Group(&_compressor), compressor(_compressor),
 211     compressions(this, "compressions",
 212         "Total number of compressions"),
 213     failedCompressions(this, "failed_compressions",
 214         "Total number of failed compressions"),
 215     compressionSize(this, "compression_size",
 216         "Number of blocks that were compressed to this power of two size"),
 217     compressionSizeBits(this, "compression_size_bits",
 218         "Total compressed data size, in bits"),
 219     avgCompressionSizeBits(this, "avg_compression_size_bits",
 220         "Average compression size, in bits"),
 221     decompressions(this, "total_decompressions",
 222         "Total number of decompressions")
 223 {
 224 }
 225
 226 void
 227 Base::BaseStats::regStats()
 228 {
 229     Stats::Group::regStats();
 230
 231     // Values comprised are {0, 1, 2, 4, ..., blkSize}
 232     compressionSize.init(std::log2(compressor.blkSize*8) + 2);
 233     compressionSize.subname(0, "0");
 234     compressionSize.subdesc(0,
 235         "Number of blocks that compressed to fit in 0 bits");
 236     for (unsigned i = 0; i <= std::log2(compressor.blkSize*8); ++i) {
 237         std::string str_i = std::to_string(1 << i);
 238         compressionSize.subname(1+i, str_i);
 239         compressionSize.subdesc(1+i,
 240             "Number of blocks that compressed to fit in " + str_i + " bits");
 241     }
 242
 243     avgCompressionSizeBits.flags(Stats::total | Stats::nozero | Stats::nonan);
 244     avgCompressionSizeBits = compressionSizeBits / compressions;
 245 }
 246
 247 } // namespace Compressor