usr/src/uts/common/fs/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, 2015 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 boolean_t zfs_prefetch_disable = B_FALSE;
  45
  46 /* max # of streams per zfetch */
  47 uint32_t        zfetch_max_streams = 8;
  48 /* min time before stream reclaim */
  49 uint32_t        zfetch_min_sec_reap = 2;
  50 /* max bytes to prefetch per stream (default 8MB) */
  51 uint32_t        zfetch_max_distance = 8 * 1024 * 1024;
  52 /* max number of bytes in an array_read in which we allow prefetching (1MB) */
  53 uint64_t        zfetch_array_rd_sz = 1024 * 1024;
  54
  55 typedef struct zfetch_stats {
  56         kstat_named_t zfetchstat_hits;
  57         kstat_named_t zfetchstat_misses;
  58         kstat_named_t zfetchstat_max_streams;
  59 } zfetch_stats_t;
  60
  61 static zfetch_stats_t zfetch_stats = {
  62         { "hits",                       KSTAT_DATA_UINT64 },
  63         { "misses",                     KSTAT_DATA_UINT64 },
  64         { "max_streams",                KSTAT_DATA_UINT64 },
  65 };
  66
  67 #define ZFETCHSTAT_BUMP(stat) \
  68         atomic_inc_64(&zfetch_stats.stat.value.ui64);
  69
  70 kstat_t         *zfetch_ksp;
  71
  72 void
  73 zfetch_init(void)
  74 {
  75         zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc",
  76             KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t),
  77             KSTAT_FLAG_VIRTUAL);
  78
  79         if (zfetch_ksp != NULL) {
  80                 zfetch_ksp->ks_data = &zfetch_stats;
  81                 kstat_install(zfetch_ksp);
  82         }
  83 }
  84
  85 void
  86 zfetch_fini(void)
  87 {
  88         if (zfetch_ksp != NULL) {
  89                 kstat_delete(zfetch_ksp);
  90                 zfetch_ksp = NULL;
  91         }
  92 }
  93
  94 /*
  95  * This takes a pointer to a zfetch structure and a dnode.  It performs the
  96  * necessary setup for the zfetch structure, grokking data from the
  97  * associated dnode.
  98  */
  99 void
 100 dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
 101 {
 102         if (zf == NULL)
 103                 return;
 104
 105         zf->zf_dnode = dno;
 106
 107         list_create(&zf->zf_stream, sizeof (zstream_t),
 108             offsetof(zstream_t, zs_node));
 109
 110         rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
 111 }
 112
 113 static void
 114 dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
 115 {
 116         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 117         list_remove(&zf->zf_stream, zs);
 118         mutex_destroy(&zs->zs_lock);
 119         kmem_free(zs, sizeof (*zs));
 120 }
 121
 122 /*
 123  * Clean-up state associated with a zfetch structure (e.g. destroy the
 124  * streams).  This doesn't free the zfetch_t itself, that's left to the caller.
 125  */
 126 void
 127 dmu_zfetch_fini(zfetch_t *zf)
 128 {
 129         zstream_t *zs;
 130
 131         ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));
 132
 133         rw_enter(&zf->zf_rwlock, RW_WRITER);
 134         while ((zs = list_head(&zf->zf_stream)) != NULL)
 135                 dmu_zfetch_stream_remove(zf, zs);
 136         rw_exit(&zf->zf_rwlock);
 137         list_destroy(&zf->zf_stream);
 138         rw_destroy(&zf->zf_rwlock);
 139
 140         zf->zf_dnode = NULL;
 141 }
 142
 143 /*
 144  * If there aren't too many streams already, create a new stream.
 145  * The "blkid" argument is the next block that we expect this stream to access.
 146  * While we're here, clean up old streams (which haven't been
 147  * accessed for at least zfetch_min_sec_reap seconds).
 148  */
 149 static void
 150 dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid)
 151 {
 152         zstream_t *zs_next;
 153         int numstreams = 0;
 154
 155         ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
 156
 157         /*
 158          * Clean up old streams.
 159          */
 160         for (zstream_t *zs = list_head(&zf->zf_stream);
 161             zs != NULL; zs = zs_next) {
 162                 zs_next = list_next(&zf->zf_stream, zs);
 163                 if (((gethrtime() - zs->zs_atime) / NANOSEC) >
 164                     zfetch_min_sec_reap)
 165                         dmu_zfetch_stream_remove(zf, zs);
 166                 else
 167                         numstreams++;
 168         }
 169
 170         /*
 171          * The maximum number of streams is normally zfetch_max_streams,
 172          * but for small files we lower it such that it's at least possible
 173          * for all the streams to be non-overlapping.
 174          *
 175          * If we are already at the maximum number of streams for this file,
 176          * even after removing old streams, then don't create this stream.
 177          */
 178         uint32_t max_streams = MAX(1, MIN(zfetch_max_streams,
 179             zf->zf_dnode->dn_maxblkid * zf->zf_dnode->dn_datablksz /
 180             zfetch_max_distance));
 181         if (numstreams >= max_streams) {
 182                 ZFETCHSTAT_BUMP(zfetchstat_max_streams);
 183                 return;
 184         }
 185
 186         zstream_t *zs = kmem_zalloc(sizeof (*zs), KM_SLEEP);
 187         zs->zs_blkid = blkid;
 188         zs->zs_pf_blkid = blkid;
 189         zs->zs_atime = gethrtime();
 190         mutex_init(&zs->zs_lock, NULL, MUTEX_DEFAULT, NULL);
 191
 192         list_insert_head(&zf->zf_stream, zs);
 193 }
 194
 195 /*
 196  * This is the prefetch entry point.  It calls all of the other dmu_zfetch
 197  * routines to create, delete, find, or operate upon prefetch streams.
 198  */
 199 void
 200 dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks)
 201 {
 202         zstream_t *zs;
 203
 204         if (zfs_prefetch_disable)
 205                 return;
 206
 207         /*
 208          * As a fast path for small (single-block) files, ignore access
 209          * to the first block.
 210          */
 211         if (blkid == 0)
 212                 return;
 213
 214         rw_enter(&zf->zf_rwlock, RW_READER);
 215
 216         for (zs = list_head(&zf->zf_stream); zs != NULL;
 217             zs = list_next(&zf->zf_stream, zs)) {
 218                 if (blkid == zs->zs_blkid) {
 219                         mutex_enter(&zs->zs_lock);
 220                         /*
 221                          * zs_blkid could have changed before we
 222                          * acquired zs_lock; re-check them here.
 223                          */
 224                         if (blkid != zs->zs_blkid) {
 225                                 mutex_exit(&zs->zs_lock);
 226                                 continue;
 227                         }
 228                         break;
 229                 }
 230         }
 231
 232         if (zs == NULL) {
 233                 /*
 234                  * This access is not part of any existing stream.  Create
 235                  * a new stream for it.
 236                  */
 237                 ZFETCHSTAT_BUMP(zfetchstat_misses);
 238                 if (rw_tryupgrade(&zf->zf_rwlock))
 239                         dmu_zfetch_stream_create(zf, blkid + nblks);
 240                 rw_exit(&zf->zf_rwlock);
 241                 return;
 242         }
 243
 244         /*
 245          * This access was to a block that we issued a prefetch for on
 246          * behalf of this stream. Issue further prefetches for this stream.
 247          *
 248          * Normally, we start prefetching where we stopped
 249          * prefetching last (zs_pf_blkid).  But when we get our first
 250          * hit on this stream, zs_pf_blkid == zs_blkid, we don't
 251          * want to prefetch to block we just accessed.  In this case,
 252          * start just after the block we just accessed.
 253          */
 254         int64_t pf_start = MAX(zs->zs_pf_blkid, blkid + nblks);
 255
 256         /*
 257          * Double our amount of prefetched data, but don't let the
 258          * prefetch get further ahead than zfetch_max_distance.
 259          */
 260         int pf_nblks =
 261             MIN((int64_t)zs->zs_pf_blkid - zs->zs_blkid + nblks,
 262             zs->zs_blkid + nblks +
 263             (zfetch_max_distance >> zf->zf_dnode->dn_datablkshift) - pf_start);
 264
 265         zs->zs_pf_blkid = pf_start + pf_nblks;
 266         zs->zs_atime = gethrtime();
 267         zs->zs_blkid = blkid + nblks;
 268
 269         /*
 270          * dbuf_prefetch() issues the prefetch i/o
 271          * asynchronously, but it may need to wait for an
 272          * indirect block to be read from disk.  Therefore
 273          * we do not want to hold any locks while we call it.
 274          */
 275         mutex_exit(&zs->zs_lock);
 276         rw_exit(&zf->zf_rwlock);
 277         for (int i = 0; i < pf_nblks; i++) {
 278                 dbuf_prefetch(zf->zf_dnode, 0, pf_start + i,
 279                     ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
 280         }
 281         ZFETCHSTAT_BUMP(zfetchstat_hits);
 282 }