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/*
* CDDL HEADER START
*
* The contents of this file are subject to the terms of the
* Common Development and Distribution License (the "License").
* You may not use this file except in compliance with the License.
*
* You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
* or http://www.opensolaris.org/os/licensing.
* See the License for the specific language governing permissions
* and limitations under the License.
*
* When distributing Covered Code, include this CDDL HEADER in each
* file and include the License file at usr/src/OPENSOLARIS.LICENSE.
* If applicable, add the following below this CDDL HEADER, with the
* fields enclosed by brackets "[]" replaced with your own identifying
* information: Portions Copyright [yyyy] [name of copyright owner]
*
* CDDL HEADER END
*/
/*
* Copyright 2009 Sun Microsystems, Inc. All rights reserved.
* Use is subject to license terms.
*/
/*
* Copyright (c) 2013, 2015 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
#include <sys/dnode.h>
#include <sys/dmu_objset.h>
#include <sys/dmu_zfetch.h>
#include <sys/dmu.h>
#include <sys/dbuf.h>
#include <sys/kstat.h>
/*
* This tunable disables predictive prefetch. Note that it leaves "prescient"
* prefetch (e.g. prefetch for zfs send) intact. Unlike predictive prefetch,
* prescient prefetch never issues i/os that end up not being needed,
* so it can't hurt performance.
*/
boolean_t zfs_prefetch_disable = B_FALSE;
/* max # of streams per zfetch */
uint32_t zfetch_max_streams = 8;
/* min time before stream reclaim */
uint32_t zfetch_min_sec_reap = 2;
/* max bytes to prefetch per stream (default 8MB) */
uint32_t zfetch_max_distance = 8 * 1024 * 1024;
/* max bytes to prefetch indirects for per stream (default 64MB) */
uint32_t zfetch_max_idistance = 64 * 1024 * 1024;
/* max number of bytes in an array_read in which we allow prefetching (1MB) */
uint64_t zfetch_array_rd_sz = 1024 * 1024;
typedef struct zfetch_stats {
kstat_named_t zfetchstat_hits;
kstat_named_t zfetchstat_misses;
kstat_named_t zfetchstat_max_streams;
} zfetch_stats_t;
static zfetch_stats_t zfetch_stats = {
{ "hits", KSTAT_DATA_UINT64 },
{ "misses", KSTAT_DATA_UINT64 },
{ "max_streams", KSTAT_DATA_UINT64 },
};
#define ZFETCHSTAT_BUMP(stat) \
atomic_inc_64(&zfetch_stats.stat.value.ui64);
kstat_t *zfetch_ksp;
void
zfetch_init(void)
{
zfetch_ksp = kstat_create("zfs", 0, "zfetchstats", "misc",
KSTAT_TYPE_NAMED, sizeof (zfetch_stats) / sizeof (kstat_named_t),
KSTAT_FLAG_VIRTUAL);
if (zfetch_ksp != NULL) {
zfetch_ksp->ks_data = &zfetch_stats;
kstat_install(zfetch_ksp);
}
}
void
zfetch_fini(void)
{
if (zfetch_ksp != NULL) {
kstat_delete(zfetch_ksp);
zfetch_ksp = NULL;
}
}
/*
* This takes a pointer to a zfetch structure and a dnode. It performs the
* necessary setup for the zfetch structure, grokking data from the
* associated dnode.
*/
void
dmu_zfetch_init(zfetch_t *zf, dnode_t *dno)
{
if (zf == NULL)
return;
zf->zf_dnode = dno;
list_create(&zf->zf_stream, sizeof (zstream_t),
offsetof(zstream_t, zs_node));
rw_init(&zf->zf_rwlock, NULL, RW_DEFAULT, NULL);
}
static void
dmu_zfetch_stream_remove(zfetch_t *zf, zstream_t *zs)
{
ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
list_remove(&zf->zf_stream, zs);
mutex_destroy(&zs->zs_lock);
kmem_free(zs, sizeof (*zs));
}
/*
* Clean-up state associated with a zfetch structure (e.g. destroy the
* streams). This doesn't free the zfetch_t itself, that's left to the caller.
*/
void
dmu_zfetch_fini(zfetch_t *zf)
{
zstream_t *zs;
ASSERT(!RW_LOCK_HELD(&zf->zf_rwlock));
rw_enter(&zf->zf_rwlock, RW_WRITER);
while ((zs = list_head(&zf->zf_stream)) != NULL)
dmu_zfetch_stream_remove(zf, zs);
rw_exit(&zf->zf_rwlock);
list_destroy(&zf->zf_stream);
rw_destroy(&zf->zf_rwlock);
zf->zf_dnode = NULL;
}
/*
* If there aren't too many streams already, create a new stream.
* The "blkid" argument is the next block that we expect this stream to access.
* While we're here, clean up old streams (which haven't been
* accessed for at least zfetch_min_sec_reap seconds).
*/
static void
dmu_zfetch_stream_create(zfetch_t *zf, uint64_t blkid)
{
zstream_t *zs_next;
int numstreams = 0;
ASSERT(RW_WRITE_HELD(&zf->zf_rwlock));
/*
* Clean up old streams.
*/
for (zstream_t *zs = list_head(&zf->zf_stream);
zs != NULL; zs = zs_next) {
zs_next = list_next(&zf->zf_stream, zs);
if (((gethrtime() - zs->zs_atime) / NANOSEC) >
zfetch_min_sec_reap)
dmu_zfetch_stream_remove(zf, zs);
else
numstreams++;
}
/*
* The maximum number of streams is normally zfetch_max_streams,
* but for small files we lower it such that it's at least possible
* for all the streams to be non-overlapping.
*
* If we are already at the maximum number of streams for this file,
* even after removing old streams, then don't create this stream.
*/
uint32_t max_streams = MAX(1, MIN(zfetch_max_streams,
zf->zf_dnode->dn_maxblkid * zf->zf_dnode->dn_datablksz /
zfetch_max_distance));
if (numstreams >= max_streams) {
ZFETCHSTAT_BUMP(zfetchstat_max_streams);
return;
}
zstream_t *zs = kmem_zalloc(sizeof (*zs), KM_SLEEP);
zs->zs_blkid = blkid;
zs->zs_pf_blkid = blkid;
zs->zs_ipf_blkid = blkid;
zs->zs_atime = gethrtime();
mutex_init(&zs->zs_lock, NULL, MUTEX_DEFAULT, NULL);
list_insert_head(&zf->zf_stream, zs);
}
/*
* This is the predictive prefetch entry point. It associates dnode access
* specified with blkid and nblks arguments with prefetch stream, predicts
* further accesses based on that stats and initiates speculative prefetch.
* fetch_data argument specifies whether actual data blocks should be fetched:
* FALSE -- prefetch only indirect blocks for predicted data blocks;
* TRUE -- prefetch predicted data blocks plus following indirect blocks.
*/
void
dmu_zfetch(zfetch_t *zf, uint64_t blkid, uint64_t nblks, boolean_t fetch_data)
{
zstream_t *zs;
int64_t pf_start, ipf_start, ipf_istart, ipf_iend;
int64_t pf_ahead_blks, max_blks;
int epbs, max_dist_blks, pf_nblks, ipf_nblks;
uint64_t end_of_access_blkid = blkid + nblks;
spa_t *spa = zf->zf_dnode->dn_objset->os_spa;
if (zfs_prefetch_disable)
return;
/*
* If we haven't yet loaded the indirect vdevs' mappings, we
* can only read from blocks that we carefully ensure are on
* concrete vdevs (or previously-loaded indirect vdevs). So we
* can't allow the predictive prefetcher to attempt reads of other
* blocks (e.g. of the MOS's dnode obejct).
*/
if (!spa_indirect_vdevs_loaded(spa))
return;
/*
* As a fast path for small (single-block) files, ignore access
* to the first block.
*/
if (blkid == 0)
return;
rw_enter(&zf->zf_rwlock, RW_READER);
/*
* Find matching prefetch stream. Depending on whether the accesses
* are block-aligned, first block of the new access may either follow
* the last block of the previous access, or be equal to it.
*/
for (zs = list_head(&zf->zf_stream); zs != NULL;
zs = list_next(&zf->zf_stream, zs)) {
if (blkid == zs->zs_blkid || blkid + 1 == zs->zs_blkid) {
mutex_enter(&zs->zs_lock);
/*
* zs_blkid could have changed before we
* acquired zs_lock; re-check them here.
*/
if (blkid == zs->zs_blkid) {
break;
} else if (blkid + 1 == zs->zs_blkid) {
blkid++;
nblks--;
if (nblks == 0) {
/* Already prefetched this before. */
mutex_exit(&zs->zs_lock);
rw_exit(&zf->zf_rwlock);
return;
}
break;
}
mutex_exit(&zs->zs_lock);
}
}
if (zs == NULL) {
/*
* This access is not part of any existing stream. Create
* a new stream for it.
*/
ZFETCHSTAT_BUMP(zfetchstat_misses);
if (rw_tryupgrade(&zf->zf_rwlock))
dmu_zfetch_stream_create(zf, end_of_access_blkid);
rw_exit(&zf->zf_rwlock);
return;
}
/*
* This access was to a block that we issued a prefetch for on
* behalf of this stream. Issue further prefetches for this stream.
*
* Normally, we start prefetching where we stopped
* prefetching last (zs_pf_blkid). But when we get our first
* hit on this stream, zs_pf_blkid == zs_blkid, we don't
* want to prefetch the block we just accessed. In this case,
* start just after the block we just accessed.
*/
pf_start = MAX(zs->zs_pf_blkid, end_of_access_blkid);
/*
* Double our amount of prefetched data, but don't let the
* prefetch get further ahead than zfetch_max_distance.
*/
if (fetch_data) {
max_dist_blks =
zfetch_max_distance >> zf->zf_dnode->dn_datablkshift;
/*
* Previously, we were (zs_pf_blkid - blkid) ahead. We
* want to now be double that, so read that amount again,
* plus the amount we are catching up by (i.e. the amount
* read just now).
*/
pf_ahead_blks = zs->zs_pf_blkid - blkid + nblks;
max_blks = max_dist_blks - (pf_start - end_of_access_blkid);
pf_nblks = MIN(pf_ahead_blks, max_blks);
} else {
pf_nblks = 0;
}
zs->zs_pf_blkid = pf_start + pf_nblks;
/*
* Do the same for indirects, starting from where we stopped last,
* or where we will stop reading data blocks (and the indirects
* that point to them).
*/
ipf_start = MAX(zs->zs_ipf_blkid, zs->zs_pf_blkid);
max_dist_blks = zfetch_max_idistance >> zf->zf_dnode->dn_datablkshift;
/*
* We want to double our distance ahead of the data prefetch
* (or reader, if we are not prefetching data). Previously, we
* were (zs_ipf_blkid - blkid) ahead. To double that, we read
* that amount again, plus the amount we are catching up by
* (i.e. the amount read now + the amount of data prefetched now).
*/
pf_ahead_blks = zs->zs_ipf_blkid - blkid + nblks + pf_nblks;
max_blks = max_dist_blks - (ipf_start - end_of_access_blkid);
ipf_nblks = MIN(pf_ahead_blks, max_blks);
zs->zs_ipf_blkid = ipf_start + ipf_nblks;
epbs = zf->zf_dnode->dn_indblkshift - SPA_BLKPTRSHIFT;
ipf_istart = P2ROUNDUP(ipf_start, 1 << epbs) >> epbs;
ipf_iend = P2ROUNDUP(zs->zs_ipf_blkid, 1 << epbs) >> epbs;
zs->zs_atime = gethrtime();
zs->zs_blkid = end_of_access_blkid;
mutex_exit(&zs->zs_lock);
rw_exit(&zf->zf_rwlock);
/*
* dbuf_prefetch() is asynchronous (even when it needs to read
* indirect blocks), but we still prefer to drop our locks before
* calling it to reduce the time we hold them.
*/
for (int i = 0; i < pf_nblks; i++) {
dbuf_prefetch(zf->zf_dnode, 0, pf_start + i,
ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
}
for (int64_t iblk = ipf_istart; iblk < ipf_iend; iblk++) {
dbuf_prefetch(zf->zf_dnode, 1, iblk,
ZIO_PRIORITY_ASYNC_READ, ARC_FLAG_PREDICTIVE_PREFETCH);
}
ZFETCHSTAT_BUMP(zfetchstat_hits);
}