1 #include "jemalloc/internal/jemalloc_preamble.h" 2 3 #include "jemalloc/internal/assert.h" 4 #include "jemalloc/internal/bit_util.h" 5 #include "jemalloc/internal/bitmap.h" 6 #include "jemalloc/internal/pages.h" 7 #include "jemalloc/internal/sc.h" 8 9 /* 10 * This module computes the size classes used to satisfy allocations. The logic 11 * here was ported more or less line-by-line from a shell script, and because of 12 * that is not the most idiomatic C. Eventually we should fix this, but for now 13 * at least the damage is compartmentalized to this file. 14 */ 15 16 size_t 17 reg_size_compute(int lg_base, int lg_delta, int ndelta) { 18 return (ZU(1) << lg_base) + (ZU(ndelta) << lg_delta); 19 } 20 21 /* Returns the number of pages in the slab. */ 22 static int 23 slab_size(int lg_page, int lg_base, int lg_delta, int ndelta) { 24 size_t page = (ZU(1) << lg_page); 25 size_t reg_size = reg_size_compute(lg_base, lg_delta, ndelta); 26 27 size_t try_slab_size = page; 28 size_t try_nregs = try_slab_size / reg_size; 29 size_t perfect_slab_size = 0; 30 bool perfect = false; 31 /* 32 * This loop continues until we find the least common multiple of the 33 * page size and size class size. Size classes are all of the form 34 * base + ndelta * delta == (ndelta + base/ndelta) * delta, which is 35 * (ndelta + ngroup) * delta. The way we choose slabbing strategies 36 * means that delta is at most the page size and ndelta < ngroup. So 37 * the loop executes for at most 2 * ngroup - 1 iterations, which is 38 * also the bound on the number of pages in a slab chosen by default. 39 * With the current default settings, this is at most 7. 40 */ 41 while (!perfect) { 42 perfect_slab_size = try_slab_size; 43 size_t perfect_nregs = try_nregs; 44 try_slab_size += page; 45 try_nregs = try_slab_size / reg_size; 46 if (perfect_slab_size == perfect_nregs * reg_size) { 47 perfect = true; 48 } 49 } 50 return (int)(perfect_slab_size / page); 51 } 52 53 static void 54 size_class( 55 /* Output. */ 56 sc_t *sc, 57 /* Configuration decisions. */ 58 int lg_max_lookup, int lg_page, int lg_ngroup, 59 /* Inputs specific to the size class. */ 60 int index, int lg_base, int lg_delta, int ndelta) { 61 sc->index = index; 62 sc->lg_base = lg_base; 63 sc->lg_delta = lg_delta; 64 sc->ndelta = ndelta; 65 size_t size = reg_size_compute(lg_base, lg_delta, ndelta); 66 sc->psz = (size % (ZU(1) << lg_page) == 0); 67 if (index == 0) { 68 assert(!sc->psz); 69 } 70 if (size < (ZU(1) << (lg_page + lg_ngroup))) { 71 sc->bin = true; 72 sc->pgs = slab_size(lg_page, lg_base, lg_delta, ndelta); 73 } else { 74 sc->bin = false; 75 sc->pgs = 0; 76 } 77 if (size <= (ZU(1) << lg_max_lookup)) { 78 sc->lg_delta_lookup = lg_delta; 79 } else { 80 sc->lg_delta_lookup = 0; 81 } 82 } 83 84 static void 85 size_classes( 86 /* Output. */ 87 sc_data_t *sc_data, 88 /* Determined by the system. */ 89 size_t lg_ptr_size, int lg_quantum, 90 /* Configuration decisions. */ 91 int lg_tiny_min, int lg_max_lookup, int lg_page, int lg_ngroup) { 92 int ptr_bits = (1 << lg_ptr_size) * 8; 93 int ngroup = (1 << lg_ngroup); 94 int ntiny = 0; 95 int nlbins = 0; 96 int lg_tiny_maxclass = (unsigned)-1; 97 int nbins = 0; 98 int npsizes = 0; 99 100 int index = 0; 101 102 int ndelta = 0; 103 int lg_base = lg_tiny_min; 104 int lg_delta = lg_base; 105 106 /* Outputs that we update as we go. */ 107 size_t lookup_maxclass = 0; 108 size_t small_maxclass = 0; 109 int lg_large_minclass = 0; 110 size_t large_maxclass = 0; 111 112 /* Tiny size classes. */ 113 while (lg_base < lg_quantum) { 114 sc_t *sc = &sc_data->sc[index]; 115 size_class(sc, lg_max_lookup, lg_page, lg_ngroup, index, 116 lg_base, lg_delta, ndelta); 117 if (sc->lg_delta_lookup != 0) { 118 nlbins = index + 1; 119 } 120 if (sc->psz) { 121 npsizes++; 122 } 123 if (sc->bin) { 124 nbins++; 125 } 126 ntiny++; 127 /* Final written value is correct. */ 128 lg_tiny_maxclass = lg_base; 129 index++; 130 lg_delta = lg_base; 131 lg_base++; 132 } 133 134 /* First non-tiny (pseudo) group. */ 135 if (ntiny != 0) { 136 sc_t *sc = &sc_data->sc[index]; 137 /* 138 * See the note in sc.h; the first non-tiny size class has an 139 * unusual encoding. 140 */ 141 lg_base--; 142 ndelta = 1; 143 size_class(sc, lg_max_lookup, lg_page, lg_ngroup, index, 144 lg_base, lg_delta, ndelta); 145 index++; 146 lg_base++; 147 lg_delta++; 148 if (sc->psz) { 149 npsizes++; 150 } 151 if (sc->bin) { 152 nbins++; 153 } 154 } 155 while (ndelta < ngroup) { 156 sc_t *sc = &sc_data->sc[index]; 157 size_class(sc, lg_max_lookup, lg_page, lg_ngroup, index, 158 lg_base, lg_delta, ndelta); 159 index++; 160 ndelta++; 161 if (sc->psz) { 162 npsizes++; 163 } 164 if (sc->bin) { 165 nbins++; 166 } 167 } 168 169 /* All remaining groups. */ 170 lg_base = lg_base + lg_ngroup; 171 while (lg_base < ptr_bits - 1) { 172 ndelta = 1; 173 int ndelta_limit; 174 if (lg_base == ptr_bits - 2) { 175 ndelta_limit = ngroup - 1; 176 } else { 177 ndelta_limit = ngroup; 178 } 179 while (ndelta <= ndelta_limit) { 180 sc_t *sc = &sc_data->sc[index]; 181 size_class(sc, lg_max_lookup, lg_page, lg_ngroup, index, 182 lg_base, lg_delta, ndelta); 183 if (sc->lg_delta_lookup != 0) { 184 nlbins = index + 1; 185 /* Final written value is correct. */ 186 lookup_maxclass = (ZU(1) << lg_base) 187 + (ZU(ndelta) << lg_delta); 188 } 189 if (sc->psz) { 190 npsizes++; 191 } 192 if (sc->bin) { 193 nbins++; 194 /* Final written value is correct. */ 195 small_maxclass = (ZU(1) << lg_base) 196 + (ZU(ndelta) << lg_delta); 197 if (lg_ngroup > 0) { 198 lg_large_minclass = lg_base + 1; 199 } else { 200 lg_large_minclass = lg_base + 2; 201 } 202 } 203 large_maxclass = (ZU(1) << lg_base) 204 + (ZU(ndelta) << lg_delta); 205 index++; 206 ndelta++; 207 } 208 lg_base++; 209 lg_delta++; 210 } 211 /* Additional outputs. */ 212 int nsizes = index; 213 unsigned lg_ceil_nsizes = lg_ceil(nsizes); 214 215 /* Fill in the output data. */ 216 sc_data->ntiny = ntiny; 217 sc_data->nlbins = nlbins; 218 sc_data->nbins = nbins; 219 sc_data->nsizes = nsizes; 220 sc_data->lg_ceil_nsizes = lg_ceil_nsizes; 221 sc_data->npsizes = npsizes; 222 sc_data->lg_tiny_maxclass = lg_tiny_maxclass; 223 sc_data->lookup_maxclass = lookup_maxclass; 224 sc_data->small_maxclass = small_maxclass; 225 sc_data->lg_large_minclass = lg_large_minclass; 226 sc_data->large_minclass = (ZU(1) << lg_large_minclass); 227 sc_data->large_maxclass = large_maxclass; 228 229 /* 230 * We compute these values in two ways: 231 * - Incrementally, as above. 232 * - In macros, in sc.h. 233 * The computation is easier when done incrementally, but putting it in 234 * a constant makes it available to the fast paths without having to 235 * touch the extra global cacheline. We assert, however, that the two 236 * computations are equivalent. 237 */ 238 assert(sc_data->npsizes == SC_NPSIZES); 239 assert(sc_data->lg_tiny_maxclass == SC_LG_TINY_MAXCLASS); 240 assert(sc_data->small_maxclass == SC_SMALL_MAXCLASS); 241 assert(sc_data->large_minclass == SC_LARGE_MINCLASS); 242 assert(sc_data->lg_large_minclass == SC_LG_LARGE_MINCLASS); 243 assert(sc_data->large_maxclass == SC_LARGE_MAXCLASS); 244 245 /* 246 * In the allocation fastpath, we want to assume that we can 247 * unconditionally subtract the requested allocation size from 248 * a ssize_t, and detect passing through 0 correctly. This 249 * results in optimal generated code. For this to work, the 250 * maximum allocation size must be less than SSIZE_MAX. 251 */ 252 assert(SC_LARGE_MAXCLASS < SSIZE_MAX); 253 } 254 255 void 256 sc_data_init(sc_data_t *sc_data) { 257 size_classes(sc_data, LG_SIZEOF_PTR, LG_QUANTUM, SC_LG_TINY_MIN, 258 SC_LG_MAX_LOOKUP, LG_PAGE, SC_LG_NGROUP); 259 260 sc_data->initialized = true; 261 } 262 263 static void 264 sc_data_update_sc_slab_size(sc_t *sc, size_t reg_size, size_t pgs_guess) { 265 size_t min_pgs = reg_size / PAGE; 266 if (reg_size % PAGE != 0) { 267 min_pgs++; 268 } 269 /* 270 * BITMAP_MAXBITS is actually determined by putting the smallest 271 * possible size-class on one page, so this can never be 0. 272 */ 273 size_t max_pgs = BITMAP_MAXBITS * reg_size / PAGE; 274 275 assert(min_pgs <= max_pgs); 276 assert(min_pgs > 0); 277 assert(max_pgs >= 1); 278 if (pgs_guess < min_pgs) { 279 sc->pgs = (int)min_pgs; 280 } else if (pgs_guess > max_pgs) { 281 sc->pgs = (int)max_pgs; 282 } else { 283 sc->pgs = (int)pgs_guess; 284 } 285 } 286 287 void 288 sc_data_update_slab_size(sc_data_t *data, size_t begin, size_t end, int pgs) { 289 assert(data->initialized); 290 for (int i = 0; i < data->nsizes; i++) { 291 sc_t *sc = &data->sc[i]; 292 if (!sc->bin) { 293 break; 294 } 295 size_t reg_size = reg_size_compute(sc->lg_base, sc->lg_delta, 296 sc->ndelta); 297 if (begin <= reg_size && reg_size <= end) { 298 sc_data_update_sc_slab_size(sc, reg_size, pgs); 299 } 300 } 301 } 302 303 void 304 sc_boot(sc_data_t *data) { 305 sc_data_init(data); 306 } 307
Indexes created Sun Apr 19 00:22:55 UTC 2026