xref: /titanic_52/usr/src/uts/common/fs/zfs/dmu_zfetch.c (revision 71d45228ba245d505c3beae1d756e775616f6d5a)
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, 2014 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 /* number of bytes in a array_read at which we stop 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 }
283