module/zfs/dmu_zfetch.c

   1 /*
   2  * CDDL HEADER START
   3  *
   4  * The contents of this file are subject to the terms of the
   5  * Common Development and Distribution License (the "License").
   6  * You may not use this file except in compliance with the License.
   7  *
   8  * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
   9  * or http://www.opensolaris.org/os/licensing.
  10  * See the License for the specific language governing permissions
  11  * and limitations under the License.
  12  *
  13  * When distributing Covered Code, include this CDDL HEADER in each
  14  * file and include the License file at usr/src/OPENSOLARIS.LICENSE.
  15  * If applicable, add the following below this CDDL HEADER, with the
  16  * fields enclosed by brackets "[]" replaced with your own identifying
  17  * information: Portions Copyright [yyyy] [name of copyright owner]
  18  *
  19  * CDDL HEADER END
  20  */
  21 /*
  22  * Copyright 2009 Sun Microsystems, Inc.  All rights reserved.
  23  * Use is subject to license terms.
  24  */
  25
  26 /*
  27  * Copyright (c) 2013, 2017 by Delphix. All rights reserved.
  28  */
  29
  30 #include <sys/zfs_context.h>
  31 #include <sys/dnode.h>
  32 #include <sys/dmu_objset.h>
  33 #include <sys/dmu_zfetch.h>
  34 #include <sys/dmu.h>
  35 #include <sys/dbuf.h>
  36 #include <sys/kstat.h>
  37
  38 /*
  39  * This tunable disables predictive prefetch.  Note that it leaves "prescient"
  40  * prefetch (e.g. prefetch for zfs send) intact.  Unlike predictive prefetch,
  41  * prescient prefetch never issues i/os that end up not being needed,
  42  * so it can't hurt performance.
  43  */
  44
  45 int zfs_prefetch_disable = B_FALSE;
  46
  47 /* max # of streams per zfetch */
  48 unsigned int    zfetch_max_streams = 8;
  49 /* min time before stream reclaim */
  50 unsigned int    zfetch_min_sec_reap = 2;
  51 /* max bytes to prefetch per stream (default 8MB) */
  52 unsigned int    zfetch_max_distance = 8 * 1024 * 1024;
  53 /* max bytes to prefetch indirects for per stream (default 64MB) */
  54 unsigned int    zfetch_max_idistance = 64 * 1024 * 1024;
  55 /* max number of bytes in an array_read in which we allow prefetching (1MB) */
  56 unsigned long   zfetch_array_rd_sz = 1024 * 1024;
  57
  58 typedef struct zfetch_stats {
  59         kstat_named_t zfetchstat_hits;
  60         kstat_named_t zfetchstat_misses;
  61         kstat_named_t zfetchstat_max_streams;
  62 } zfetch_stats_t;
  63
  64 static zfetch_stats_t zfetch_stats = {
  65         { "hits",                       KSTAT_DATA_UINT64 },
  66         { "misses",                     KSTAT_DATA_UINT64 },
  67         { "max_streams",                KSTAT_DATA_UINT64 },
  68 };
  69
  70 #define ZFETCHSTAT_BUMP(stat) \
  71         atomic_inc_64(&zfetch_stats.stat.value.ui64);
  72
  73 kstat_t         *zfetch_ksp;
  74
  75 void
  76 zfetch_init(void)
  77 {
  78         zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc",
  79             KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t),
  80             KSTAT_FLAG_VIRTUAL);
  81
  82         if (zfetch_ksp != NULL) {
  83                 zfetch_ksp->ks_data = &zfetch_stats;
  84                 kstat_install(zfetch_ksp);
  85         }
  86 }
  87
  88 void
  89 zfetch_fini(void)
  90 {
  91         if (zfetch_ksp != NULL) {
  92                 kstat_delete(zfetch_ksp);
  93                 zfetch_ksp = NULL;
  94         }
  95 }
  96
  97 /*
  98  * This takes a pointer to a zfetch structure and a dnode.  It performs the
  99  * necessary setup for the zfetch structure, grokking data from the
 100  * associated dnode.
 101  */
 102 void
 103 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
 104 {
 105         if (zf == NULL)
 106                 return;
 107
 108         zf->zf_dnode = dno;
 109
 110         list_create(&zf->zf_stream, sizeof (zstream_t),
 111             offsetof(zstream_t, zs_node));
 112
 113         mutex_init(&zf->zf_lock, NULL, MUTEX_DEFAULT, NULL);
 114 }
 115
 116 static void
 117 dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
 118 {
 119         ASSERT(MUTEX_HELD(&zf->zf_lock));
 120         list_remove(&zf->zf_stream, zs);
 121         mutex_destroy(&zs->zs_lock);
 122         kmem_free(zs, sizeof (*zs));
 123 }
 124
 125 /*
 126  * Clean-up state associated with a zfetch structure (e.g. destroy the
 127  * streams).  This doesn't free the zfetch_t itself, that's left to the caller.
 128  */
 129 void
 130 dmu_zfetch_fini(zfetch_t *zf)
 131 {
 132         zstream_t *zs;
 133
 134         mutex_enter(&zf->zf_lock);
 135         while ((zs = list_head(&zf->zf_stream)) != NULL)
 136                 dmu_zfetch_stream_remove(zf, zs);
 137         mutex_exit(&zf->zf_lock);
 138         list_destroy(&zf->zf_stream);
 139         mutex_destroy(&zf->zf_lock);
 140
 141         zf->zf_dnode = NULL;
 142 }
 143
 144 /*
 145  * If there aren't too many streams already, create a new stream.
 146  * The "blkid" argument is the next block that we expect this stream to access.
 147  * While we're here, clean up old streams (which haven't been
 148  * accessed for at least zfetch_min_sec_reap seconds).
 149  */
 150 static void
 151 dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid)
 152 {
 153         zstream_t *zs_next;
 154         int numstreams = 0;
 155
 156         ASSERT(MUTEX_HELD(&zf->zf_lock));
 157
 158         /*
 159          * Clean up old streams.
 160          */
 161         for (zstream_t *zs = list_head(&zf->zf_stream);
 162             zs != NULL; zs = zs_next) {
 163                 zs_next = list_next(&zf->zf_stream, zs);
 164                 if (((gethrtime() - zs->zs_atime) / NANOSEC) >
 165                     zfetch_min_sec_reap)
 166                         dmu_zfetch_stream_remove(zf, zs);
 167                 else
 168                         numstreams++;
 169         }
 170
 171         /*
 172          * The maximum number of streams is normally zfetch_max_streams,
 173          * but for small files we lower it such that it's at least possible
 174          * for all the streams to be non-overlapping.
 175          *
 176          * If we are already at the maximum number of streams for this file,
 177          * even after removing old streams, then don't create this stream.
 178          */
 179         uint32_t max_streams = MAX(1, MIN(zfetch_max_streams,
 180             zf->zf_dnode->dn_maxblkid * zf->zf_dnode->dn_datablksz /
 181             zfetch_max_distance));
 182         if (numstreams >= max_streams) {
 183                 ZFETCHSTAT_BUMP(zfetchstat_max_streams);
 184                 return;
 185         }
 186
 187         zstream_t *zs = kmem_zalloc(sizeof (*zs), KM_SLEEP);
 188         zs->zs_blkid = blkid;
 189         zs->zs_pf_blkid = blkid;
 190         zs->zs_ipf_blkid = blkid;
 191         zs->zs_atime = gethrtime();
 192         mutex_init(&zs->zs_lock, NULL, MUTEX_DEFAULT, NULL);
 193
 194         list_insert_head(&zf->zf_stream, zs);
 195 }
 196
 197 /*
 198  * This is the predictive prefetch entry point.  It associates dnode access
 199  * specified with blkid and nblks arguments with prefetch stream, predicts
 200  * further accesses based on that stats and initiates speculative prefetch.
 201  * fetch_data argument specifies whether actual data blocks should be fetched:
 202  *   FALSE -- prefetch only indirect blocks for predicted data blocks;
 203  *   TRUE -- prefetch predicted data blocks plus following indirect blocks.
 204  */
 205 void
 206 dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data,
 207     boolean_t have_lock)
 208 {
 209         zstream_t *zs;
 210         int64_t pf_start, ipf_start, ipf_istart, ipf_iend;
 211         int64_t pf_ahead_blks, max_blks;
 212         int epbs, max_dist_blks, pf_nblks, ipf_nblks;
 213         uint64_t end_of_access_blkid;
 214         end_of_access_blkid = blkid + nblks;
 215         spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
 216
 217         if (zfs_prefetch_disable)
 218                 return;
 219         /*
 220          * If we haven't yet loaded the indirect vdevs' mappings, we
 221          * can only read from blocks that we carefully ensure are on
 222          * concrete vdevs (or previously-loaded indirect vdevs).  So we
 223          * can't allow the predictive prefetcher to attempt reads of other
 224          * blocks (e.g. of the MOS's dnode object).
 225          */
 226         if (!spa_indirect_vdevs_loaded(spa))
 227                 return;
 228
 229         /*
 230          * As a fast path for small (single-block) files, ignore access
 231          * to the first block.
 232          */
 233         if (blkid == 0)
 234                 return;
 235
 236         if (!have_lock)
 237                 rw_enter(&zf->zf_dnode->dn_struct_rwlock, RW_READER);
 238         mutex_enter(&zf->zf_lock);
 239
 240         /*
 241          * Find matching prefetch stream.  Depending on whether the accesses
 242          * are block-aligned, first block of the new access may either follow
 243          * the last block of the previous access, or be equal to it.
 244          */
 245         for (zs = list_head(&zf->zf_stream); zs != NULL;
 246             zs = list_next(&zf->zf_stream, zs)) {
 247                 if (blkid == zs->zs_blkid || blkid + 1 == zs->zs_blkid) {
 248                         mutex_enter(&zs->zs_lock);
 249                         /*
 250                          * zs_blkid could have changed before we
 251                          * acquired zs_lock; re-check them here.
 252                          */
 253                         if (blkid == zs->zs_blkid) {
 254                                 break;
 255                         } else if (blkid + 1 == zs->zs_blkid) {
 256                                 blkid++;
 257                                 nblks--;
 258                                 if (nblks == 0) {
 259                                         /* Already prefetched this before. */
 260                                         mutex_exit(&zs->zs_lock);
 261                                         mutex_exit(&zf->zf_lock);
 262                                         if (!have_lock) {
 263                                                 rw_exit(&zf->zf_dnode->
 264                                                     dn_struct_rwlock);
 265                                         }
 266                                         return;
 267                                 }
 268                                 break;
 269                         }
 270                         mutex_exit(&zs->zs_lock);
 271                 }
 272         }
 273
 274         if (zs == NULL) {
 275                 /*
 276                  * This access is not part of any existing stream.  Create
 277                  * a new stream for it.
 278                  */
 279                 ZFETCHSTAT_BUMP(zfetchstat_misses);
 280
 281                 dmu_zfetch_stream_create(zf, end_of_access_blkid);
 282                 mutex_exit(&zf->zf_lock);
 283                 if (!have_lock)
 284                         rw_exit(&zf->zf_dnode->dn_struct_rwlock);
 285                 return;
 286         }
 287
 288         /*
 289          * This access was to a block that we issued a prefetch for on
 290          * behalf of this stream. Issue further prefetches for this stream.
 291          *
 292          * Normally, we start prefetching where we stopped
 293          * prefetching last (zs_pf_blkid).  But when we get our first
 294          * hit on this stream, zs_pf_blkid == zs_blkid, we don't
 295          * want to prefetch the block we just accessed.  In this case,
 296          * start just after the block we just accessed.
 297          */
 298         pf_start = MAX(zs->zs_pf_blkid, end_of_access_blkid);
 299
 300         /*
 301          * Double our amount of prefetched data, but don't let the
 302          * prefetch get further ahead than zfetch_max_distance.
 303          */
 304         if (fetch_data) {
 305                 max_dist_blks =
 306                     zfetch_max_distance >> zf->zf_dnode->dn_datablkshift;
 307                 /*
 308                  * Previously, we were (zs_pf_blkid - blkid) ahead.  We
 309                  * want to now be double that, so read that amount again,
 310                  * plus the amount we are catching up by (i.e. the amount
 311                  * read just now).
 312                  */
 313                 pf_ahead_blks = zs->zs_pf_blkid - blkid + nblks;
 314                 max_blks = max_dist_blks - (pf_start - end_of_access_blkid);
 315                 pf_nblks = MIN(pf_ahead_blks, max_blks);
 316         } else {
 317                 pf_nblks = 0;
 318         }
 319
 320         zs->zs_pf_blkid = pf_start + pf_nblks;
 321
 322         /*
 323          * Do the same for indirects, starting from where we stopped last,
 324          * or where we will stop reading data blocks (and the indirects
 325          * that point to them).
 326          */
 327         ipf_start = MAX(zs->zs_ipf_blkid, zs->zs_pf_blkid);
 328         max_dist_blks = zfetch_max_idistance >> zf->zf_dnode->dn_datablkshift;
 329         /*
 330          * We want to double our distance ahead of the data prefetch
 331          * (or reader, if we are not prefetching data).  Previously, we
 332          * were (zs_ipf_blkid - blkid) ahead.  To double that, we read
 333          * that amount again, plus the amount we are catching up by
 334          * (i.e. the amount read now + the amount of data prefetched now).
 335          */
 336         pf_ahead_blks = zs->zs_ipf_blkid - blkid + nblks + pf_nblks;
 337         max_blks = max_dist_blks - (ipf_start - end_of_access_blkid);
 338         ipf_nblks = MIN(pf_ahead_blks, max_blks);
 339         zs->zs_ipf_blkid = ipf_start + ipf_nblks;
 340
 341         epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT;
 342         ipf_istart = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs;
 343         ipf_iend = P2ROUNDUP(zs->zs_ipf_blkid, 1 << epbs) >> epbs;
 344
 345         zs->zs_atime = gethrtime();
 346         zs->zs_blkid = end_of_access_blkid;
 347         mutex_exit(&zs->zs_lock);
 348         mutex_exit(&zf->zf_lock);
 349
 350         /*
 351          * dbuf_prefetch() is asynchronous (even when it needs to read
 352          * indirect blocks), but we still prefer to drop our locks before
 353          * calling it to reduce the time we hold them.
 354          */
 355
 356         for (int i = 0; i < pf_nblks; i++) {
 357                 dbuf_prefetch(zf->zf_dnode, 0, pf_start + i,
 358                     ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
 359         }
 360         for (int64_t iblk = ipf_istart; iblk < ipf_iend; iblk++) {
 361                 dbuf_prefetch(zf->zf_dnode, 1, iblk,
 362                     ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
 363         }
 364         if (!have_lock)
 365                 rw_exit(&zf->zf_dnode->dn_struct_rwlock);
 366         ZFETCHSTAT_BUMP(zfetchstat_hits);
 367 }
 368
 369 #if defined(_KERNEL)
 370 /* BEGIN CSTYLED */
 371 module_param(zfs_prefetch_disable, int, 0644);
 372 MODULE_PARM_DESC(zfs_prefetch_disable, "Disable all ZFS prefetching");
 373
 374 module_param(zfetch_max_streams, uint, 0644);
 375 MODULE_PARM_DESC(zfetch_max_streams, "Max number of streams per zfetch");
 376
 377 module_param(zfetch_min_sec_reap, uint, 0644);
 378 MODULE_PARM_DESC(zfetch_min_sec_reap, "Min time before stream reclaim");
 379
 380 module_param(zfetch_max_distance, uint, 0644);
 381 MODULE_PARM_DESC(zfetch_max_distance,
 382         "Max bytes to prefetch per stream (default 8MB)");
 383
 384 module_param(zfetch_array_rd_sz, ulong, 0644);
 385 MODULE_PARM_DESC(zfetch_array_rd_sz, "Number of bytes in a array_read");
 386 /* END CSTYLED */
 387 #endif