1 /* $NetBSD: uvm_readahead.c,v 1.16 2023/09/23 18:21:12 ad Exp $ */ 2 3 /*- 4 * Copyright (c)2003, 2005, 2009 YAMAMOTO Takashi, 5 * All rights reserved. 6 * 7 * Redistribution and use in source and binary forms, with or without 8 * modification, are permitted provided that the following conditions 9 * are met: 10 * 1. Redistributions of source code must retain the above copyright 11 * notice, this list of conditions and the following disclaimer. 12 * 2. Redistributions in binary form must reproduce the above copyright 13 * notice, this list of conditions and the following disclaimer in the 14 * documentation and/or other materials provided with the distribution. 15 * 16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 17 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 18 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 19 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 26 * SUCH DAMAGE. 27 */ 28 29 /* 30 * uvm_object read-ahead 31 * 32 * TODO: 33 * - tune. 34 * - handle multiple streams. 35 * - find a better way to deal with PGO_LOCKED pager requests. 36 * (currently just ignored) 37 * - consider the amount of memory in the system. 38 * - consider the speed of the underlying device. 39 * - consider filesystem block size / block layout. 40 */ 41 42 #include <sys/cdefs.h> 43 __KERNEL_RCSID(0, "$NetBSD: uvm_readahead.c,v 1.16 2023/09/23 18:21:12 ad Exp $"); 44 45 #include <sys/param.h> 46 #include <sys/kmem.h> 47 48 #include <uvm/uvm.h> 49 #include <uvm/uvm_readahead.h> 50 51 #if defined(READAHEAD_DEBUG) 52 #define DPRINTF(a) printf a 53 #else /* defined(READAHEAD_DEBUG) */ 54 #define DPRINTF(a) /* nothing */ 55 #endif /* defined(READAHEAD_DEBUG) */ 56 57 /* 58 * uvm_ractx: read-ahead context. 59 */ 60 61 struct uvm_ractx { 62 int ra_flags; 63 #define RA_VALID 1 64 off_t ra_winstart; /* window start offset */ 65 size_t ra_winsize; /* window size */ 66 off_t ra_next; /* next offset to read-ahead */ 67 }; 68 69 #if defined(sun2) || defined(sun3) 70 /* XXX: on sun2 and sun3 MAXPHYS is 0xe000 */ 71 #undef MAXPHYS 72 #define MAXPHYS 0x8000 /* XXX */ 73 #endif 74 75 #define RA_WINSIZE_INIT MAXPHYS /* initial window size */ 76 #define RA_WINSIZE_MAX (MAXPHYS * 16) /* max window size */ 77 #define RA_WINSIZE_SEQENTIAL RA_WINSIZE_MAX /* fixed window size used for 78 SEQUENTIAL hint */ 79 #define RA_MINSIZE (MAXPHYS * 2) /* min size to start i/o */ 80 #define RA_IOCHUNK MAXPHYS /* read-ahead i/o chunk size */ 81 82 static off_t ra_startio(struct uvm_object *, off_t, size_t); 83 static struct uvm_ractx *ra_allocctx(void); 84 static void ra_freectx(struct uvm_ractx *); 85 86 /* 87 * uvm_ra_init: initialize readahead module. 88 */ 89 90 void 91 uvm_ra_init(void) 92 { 93 94 } 95 96 static struct uvm_ractx * 97 ra_allocctx(void) 98 { 99 100 return kmem_alloc(sizeof(struct uvm_ractx), KM_NOSLEEP); 101 } 102 103 static void 104 ra_freectx(struct uvm_ractx *ra) 105 { 106 107 kmem_free(ra, sizeof(struct uvm_ractx)); 108 } 109 110 /* 111 * ra_startio: start i/o for read-ahead. 112 * 113 * => start i/o for each RA_IOCHUNK sized chunk. 114 * => return offset to which we started i/o. 115 */ 116 117 static off_t 118 ra_startio(struct uvm_object *uobj, off_t off, size_t sz) 119 { 120 const off_t endoff = off + sz; 121 122 DPRINTF(("%s: uobj=%p, off=%" PRIu64 ", endoff=%" PRIu64 "\n", 123 __func__, uobj, off, endoff)); 124 125 KASSERT(rw_write_held(uobj->vmobjlock)); 126 127 /* 128 * Don't issue read-ahead if the last page of the range is already cached. 129 * The assumption is that since the access is sequential, the intermediate 130 * pages would have similar LRU stats, and hence likely to be still in cache 131 * too. This speeds up I/O using cache, since it avoids lookups and temporary 132 * allocations done by full pgo_get. 133 */ 134 struct vm_page *pg = uvm_pagelookup(uobj, trunc_page(endoff - 1)); 135 if (pg != NULL) { 136 DPRINTF(("%s: off=%" PRIu64 ", sz=%zu already cached\n", 137 __func__, off, sz)); 138 return endoff; 139 } 140 141 off = trunc_page(off); 142 while (off < endoff) { 143 const size_t chunksize = RA_IOCHUNK; 144 int error; 145 size_t donebytes; 146 int npages; 147 int orignpages; 148 size_t bytelen; 149 150 KASSERT((chunksize & (chunksize - 1)) == 0); 151 KASSERT((off & PAGE_MASK) == 0); 152 bytelen = ((off + chunksize) & -(off_t)chunksize) - off; 153 KASSERT((bytelen & PAGE_MASK) == 0); 154 npages = orignpages = bytelen >> PAGE_SHIFT; 155 KASSERT(npages != 0); 156 157 /* 158 * use UVM_ADV_RANDOM to avoid recursion. 159 */ 160 161 error = (*uobj->pgops->pgo_get)(uobj, off, NULL, 162 &npages, 0, VM_PROT_READ, UVM_ADV_RANDOM, PGO_NOTIMESTAMP); 163 rw_enter(uobj->vmobjlock, RW_WRITER); 164 DPRINTF(("%s: off=%" PRIu64 ", bytelen=%zu -> %d\n", 165 __func__, off, bytelen, error)); 166 if (error != 0 && error != EBUSY) { 167 if (error != EINVAL) { /* maybe past EOF */ 168 DPRINTF(("%s: error=%d\n", __func__, error)); 169 } 170 break; 171 } 172 KASSERT(orignpages == npages); 173 donebytes = orignpages << PAGE_SHIFT; 174 off += donebytes; 175 } 176 177 return off; 178 } 179 180 /* ------------------------------------------------------------ */ 181 182 /* 183 * uvm_ra_allocctx: allocate a context. 184 */ 185 186 struct uvm_ractx * 187 uvm_ra_allocctx(void) 188 { 189 struct uvm_ractx *ra; 190 191 ra = ra_allocctx(); 192 if (ra != NULL) { 193 ra->ra_flags = 0; 194 } 195 196 return ra; 197 } 198 199 /* 200 * uvm_ra_freectx: free a context. 201 */ 202 203 void 204 uvm_ra_freectx(struct uvm_ractx *ra) 205 { 206 207 KASSERT(ra != NULL); 208 ra_freectx(ra); 209 } 210 211 /* 212 * uvm_ra_request: update a read-ahead context and start i/o if appropriate. 213 * 214 * => called when [reqoff, reqoff+reqsize) is requested. 215 * => object must be locked by caller, will return locked. 216 */ 217 218 void 219 uvm_ra_request(struct uvm_ractx *ra, int advice, struct uvm_object *uobj, 220 off_t reqoff, size_t reqsize) 221 { 222 223 KASSERT(rw_write_held(uobj->vmobjlock)); 224 225 if (ra == NULL || advice == UVM_ADV_RANDOM) { 226 return; 227 } 228 229 if (advice == UVM_ADV_SEQUENTIAL) { 230 231 /* 232 * always do read-ahead with a large window. 233 */ 234 235 if ((ra->ra_flags & RA_VALID) == 0) { 236 ra->ra_winstart = ra->ra_next = 0; 237 ra->ra_flags |= RA_VALID; 238 } 239 if (reqoff < ra->ra_winstart) { 240 ra->ra_next = reqoff; 241 } 242 ra->ra_winsize = RA_WINSIZE_SEQENTIAL; 243 goto do_readahead; 244 } 245 246 /* 247 * a request with UVM_ADV_NORMAL hint. (ie. no hint) 248 * 249 * we keep a sliding window in order to determine: 250 * - if the previous read-ahead was successful or not. 251 * - how many bytes to read-ahead. 252 */ 253 254 /* 255 * if it's the first request for this context, 256 * initialize context and return. 257 */ 258 259 if ((ra->ra_flags & RA_VALID) == 0) { 260 initialize: 261 ra->ra_winstart = ra->ra_next = reqoff + reqsize; 262 ra->ra_winsize = RA_WINSIZE_INIT; 263 ra->ra_flags |= RA_VALID; 264 goto done; 265 } 266 267 /* 268 * if it isn't in our window, 269 * initialize context and return. 270 * (read-ahead miss) 271 */ 272 273 if (reqoff < ra->ra_winstart || 274 ra->ra_winstart + ra->ra_winsize < reqoff) { 275 276 /* 277 * ... unless we seem to be reading the same chunk repeatedly. 278 * 279 * XXX should have some margin? 280 */ 281 282 if (reqoff + reqsize == ra->ra_winstart) { 283 DPRINTF(("%s: %p: same block: off=%" PRIu64 284 ", size=%zd, winstart=%" PRIu64 "\n", 285 __func__, ra, reqoff, reqsize, ra->ra_winstart)); 286 goto done; 287 } 288 goto initialize; 289 } 290 291 /* 292 * it's in our window. (read-ahead hit) 293 * - start read-ahead i/o if appropriate. 294 * - advance and enlarge window. 295 */ 296 297 do_readahead: 298 299 /* 300 * don't bother to read-ahead behind current request. 301 */ 302 303 if (reqoff > ra->ra_next) { 304 ra->ra_next = reqoff; 305 } 306 307 /* 308 * try to make [reqoff, reqoff+ra_winsize) in-core. 309 * note that [reqoff, ra_next) is considered already done. 310 */ 311 312 if (reqoff + ra->ra_winsize > ra->ra_next) { 313 off_t raoff = MAX(reqoff, ra->ra_next); 314 size_t rasize = reqoff + ra->ra_winsize - ra->ra_next; 315 316 #if defined(DIAGNOSTIC) 317 if (rasize > RA_WINSIZE_MAX) { 318 printf("%s: corrupted context", __func__); 319 rasize = RA_WINSIZE_MAX; 320 } 321 #endif /* defined(DIAGNOSTIC) */ 322 323 /* 324 * issue read-ahead only if we can start big enough i/o. 325 * otherwise we end up with a stream of small i/o. 326 */ 327 328 if (rasize >= RA_MINSIZE) { 329 off_t next; 330 331 next = ra_startio(uobj, raoff, rasize); 332 ra->ra_next = next; 333 } 334 } 335 336 /* 337 * update window. 338 * 339 * enlarge window by reqsize, so that it grows in a predictable manner 340 * regardless of the size of each read(2). 341 */ 342 343 ra->ra_winstart = reqoff + reqsize; 344 ra->ra_winsize = MIN(RA_WINSIZE_MAX, ra->ra_winsize + reqsize); 345 346 done:; 347 } 348 349 int 350 uvm_readahead(struct uvm_object *uobj, off_t off, off_t size) 351 { 352 353 /* 354 * don't allow too much read-ahead. 355 */ 356 if (size > RA_WINSIZE_MAX) { 357 size = RA_WINSIZE_MAX; 358 } 359 rw_enter(uobj->vmobjlock, RW_WRITER); 360 ra_startio(uobj, off, size); 361 rw_exit(uobj->vmobjlock); 362 return 0; 363 } 364
Indexes created Sat Sep 20 22:09:52 GMT 2025