mdw@git.distorted.org.uk Git - mLib/blob - sub.c

   1 /* -*-c-*-
   2  *
   3  * $Id: sub.c,v 1.2 1999/05/05 18:50:31 mdw Exp $
   4  *
   5  * Allocation of known-size blocks
   6  *
   7  * (c) 1998 Straylight/Edgeware
   8  */
   9
  10 /*----- Licensing notice --------------------------------------------------*
  11  *
  12  * This file is part of the mLib utilities library.
  13  *
  14  * mLib is free software; you can redistribute it and/or modify
  15  * it under the terms of the GNU Library General Public License as
  16  * published by the Free Software Foundation; either version 2 of the
  17  * License, or (at your option) any later version.
  18  *
  19  * mLib is distributed in the hope that it will be useful,
  20  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  21  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  22  * GNU Library General Public License for more details.
  23  *
  24  * You should have received a copy of the GNU Library General Public
  25  * License along with mLib; if not, write to the Free Software
  26  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
  27  */
  28
  29 /*----- Revision history --------------------------------------------------*
  30  *
  31  * $Log: sub.c,v $
  32  * Revision 1.2  1999/05/05 18:50:31  mdw
  33  * Change licensing conditions to LGPL.
  34  *
  35  * Revision 1.1.1.1  1998/06/17 23:44:42  mdw
  36  * Initial version of mLib
  37  *
  38  */
  39
  40 /*----- The big idea ------------------------------------------------------*
  41  *
  42  * This file provides an extra layer over @malloc@.  It provides fast
  43  * turnover for small blocks, and tries to minimise the per-block overhead.
  44  *
  45  * To do its job, @alloc@ must place an extra restriction on you: you must
  46  * know the size of a block when you free it.  Usually you'll have this
  47  * information encoded in some way either in the block or in the thing that
  48  * referenced it, so this isn't a hardship.
  49  *
  50  * It works fairly simply.  If a request for a big block (as defined by the
  51  * constants below) comes in, it gets sent on to @malloc@ unmolested.  For
  52  * small blocks, it goes straight to a `bin' -- a list containing free blocks
  53  * of exactly that size, or the nearest bigger size we can manage.  If the
  54  * bin is empty, a `chunk' is allocated from @malloc@: this has enough room
  55  * for lots of blocks of the requested size, so it ets split up and each
  56  * individual small block is added to the bin list.  The first block in the
  57  * bin list is then removed and given to the caller.  In this way, @malloc@
  58  * only stores its information once for lots of little blocks, so we save
  59  * memory.  Because I know where the correct bin is just from the block size,
  60  * and I don't need to do any searching at all in the usual case (because the
  61  * list isn't empty) I can get a speed advantage too.
  62  *
  63  * This code is almost certainly not ANSI conformant, although I'm not
  64  * actually sure.  If some kind soul would let me know how seriously I've
  65  * violated the standard, and whether this is easily fixable, I'd be
  66  * grateful.
  67  */
  68
  69 /*----- Header files ------------------------------------------------------*/
  70
  71 /* --- ANSI headers --- */
  72
  73 #include <stdio.h>
  74 #include <stdlib.h>
  75 #include <string.h>
  76
  77 /* --- Local headers --- */
  78
  79 #undef TRACK_ENABLE                     /* Can't track suballoc routines */
  80 #include "alloc.h"
  81
  82 /*----- Configuration and tuning ------------------------------------------*/
  83
  84 /* --- The largest block I'll handle here --- *
  85  *
  86  * Anything larger will be handed on to @malloc@.
  87  */
  88
  89 #define SUB_MAXBIN 256
  90
  91 /* --- Preferred chunk size --- *
  92  *
  93  * When a bin is empty, I'll allocate a large chunk of approximately this
  94  * size and divvy it up into small bin-sized blocks.
  95  */
  96
  97 #define SUB_CHUNK 4096
  98
  99 /*----- Other useful macros -----------------------------------------------*/
 100
 101 /* --- The granularity of bin buffers --- *
 102  *
 103  * All blocks allocated by the binner are a multiple of this size.  I've
 104  * chosen @void *@ because I need to store @void *@ things in here.
 105  */
 106
 107 #define SUB_GRANULE sizeof(void *)
 108
 109 /* --- Finding the right bin for a given size --- *
 110  *
 111  * This chooses the correct bin for an allocation.  Input is the size of
 112  * block wanted; result is the bin index.
 113  */
 114
 115 #define SUB_BIN(x) (((x) + SUB_GRANULE - 1) / SUB_GRANULE)
 116
 117 /* --- Convert a bin back to the block size --- *
 118  *
 119  * This gives the size of block contained in a given bin.
 120  */
 121
 122 #define SUB_BINSZ(x) ((x) * SUB_GRANULE)
 123
 124 /* --- Number of bins required --- */
 125
 126 #define SUB_BINS (SUB_MAXBIN / SUB_GRANULE + 1)
 127
 128 /*----- Static variables --------------------------------------------------*/
 129
 130 static void *sub__bins[SUB_BINS];
 131 static size_t sub__sizes[SUB_BINS];
 132
 133 /*----- Main code ---------------------------------------------------------*/
 134
 135 /* --- @sub_alloc@ --- *
 136  *
 137  * Arguments:   @size_t s@ = size of chunk wanted
 138  *
 139  * Returns:     Pointer to a block at least as large as the one wanted.
 140  *
 141  * Use:         Allocates a small block of memory.  If there is no more
 142  *              memory left, the exception @EXC_NOMEM@ is raised.
 143  */
 144
 145 void *sub_alloc(size_t s)
 146 {
 147   int bin = SUB_BIN(s);
 148   void *p;
 149
 150   /* --- Handle oversize blocks --- */
 151
 152   if (bin >= SUB_BINS)
 153     return (xmalloc(s));
 154
 155   /* --- If the bin is empty, find some memory --- */
 156
 157   if (!sub__bins[bin]) {
 158     char *p, *q;
 159
 160     p = xmalloc(sub__sizes[bin]);
 161     q = p + sub__sizes[bin];
 162
 163     s = SUB_BINSZ(bin);
 164
 165     q -= s;
 166     *(void **)q = 0;
 167
 168     while (q > p) {
 169       q -= s;
 170       *(void **)q = q + s;
 171     }
 172
 173     sub__bins[bin] = p;
 174   }
 175
 176   /* --- Extract the first block in the list --- */
 177
 178   p = sub__bins[bin];
 179   sub__bins[bin] = *(void **)p;
 180   return (p);
 181 }
 182
 183 /* --- @sub_free@ --- *
 184  *
 185  * Arguments:   @void *p@ = address of block to free
 186  *              @size_t s@ = size of block
 187  *
 188  * Returns:     ---
 189  *
 190  * Use:         Frees a block allocated by @sub_alloc@.
 191  */
 192
 193 void sub_free(void *p, size_t s)
 194 {
 195   int bin = SUB_BIN(s);
 196
 197   if (bin >= SUB_BINS)
 198     free(p);
 199   else {
 200     *(void **)p = sub__bins[bin];
 201     sub__bins[bin] = p;
 202   }
 203 }
 204
 205 /* --- @sub_init@ --- *
 206  *
 207  * Arguments:   ---
 208  *
 209  * Returns:     ---
 210  *
 211  * Use:         Initialises the magic allocator.
 212  */
 213
 214 void sub_init(void)
 215 {
 216   int i;
 217
 218   /* --- Initialise the sizes bins --- */
 219
 220   for (i = 1; i < SUB_BINS; i++) {
 221     sub__sizes[i] = ((SUB_CHUNK + SUB_BINSZ(i) - 1) /
 222                      SUB_BINSZ(i) * SUB_BINSZ(i));
 223   }
 224 }
 225
 226 /*----- Debugging code ----------------------------------------------------*/
 227
 228 #ifdef TEST_RIG
 229
 230 #define BLOCKS 1024
 231 #define SIZE_MAX 2048
 232 #define ITERATIONS 500000
 233
 234 int main(void)
 235 {
 236   static void *block[BLOCKS];
 237   static size_t size[BLOCKS];
 238   size_t allocced = 0;
 239   int i;
 240   long count;
 241
 242   sub_init();
 243
 244   for (count = 0; count < ITERATIONS; count++) {
 245     i = rand() % BLOCKS;
 246     if (block[i]) {
 247       sub_free(block[i], size[i]);
 248       block[i] = 0;
 249       allocced -= size[i];
 250     } else {
 251       block[i] = sub_alloc(size[i] =
 252                            rand() % (SUB_MAXBIN - 128) + 128);
 253       allocced += size[i];
 254       memset(block[i], 0, size[i]);    /* trample allocated storage */
 255     }
 256   }
 257
 258   return (0);
 259 }
 260
 261 #endif
 262
 263 /*----- That's all, folks -------------------------------------------------*/