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mirror_zfs/module/zfs/vdev_cache.c
Paul Dagnelie ca5777793e Reduce loaded range tree memory usage 
This patch implements a new tree structure for ZFS, and uses it to 
store range trees more efficiently.

The new structure is approximately a B-tree, though there are some 
small differences from the usual characterizations. The tree has core 
nodes and leaf nodes; each contain data elements, which the elements 
in the core nodes acting as separators between its children. The 
difference between core and leaf nodes is that the core nodes have an 
array of children, while leaf nodes don't. Every node in the tree may 
be only partially full; in most cases, they are all at least 50% full 
(in terms of element count) except for the root node, which can be 
less full. Underfull nodes will steal from their neighbors or merge to 
remain full enough, while overfull nodes will split in two. The data 
elements are contained in tree-controlled buffers; they are copied 
into these on insertion, and overwritten on deletion. This means that 
the elements are not independently allocated, which reduces overhead, 
but also means they can't be shared between trees (and also that 
pointers to them are only valid until a side-effectful tree operation 
occurs). The overhead varies based on how dense the tree is, but is 
usually on the order of about 50% of the element size; the per-node 
overheads are very small, and so don't make a significant difference. 
The trees can accept arbitrary records; they accept a size and a 
comparator to allow them to be used for a variety of purposes.

The new trees replace the AVL trees used in the range trees today. 
Currently, the range_seg_t structure contains three 8 byte integers 
of payload and two 24 byte avl_tree_node_ts to handle its storage in 
both an offset-sorted tree and a size-sorted tree (total size: 64 
bytes). In the new model, the range seg structures are usually two 4 
byte integers, but a separate one needs to exist for the size-sorted 
and offset-sorted tree. Between the raw size, the 50% overhead, and 
the double storage, the new btrees are expected to use 8*1.5*2 = 24 
bytes per record, or 33.3% as much memory as the AVL trees (this is 
for the purposes of storing metaslab range trees; for other purposes, 
like scrubs, they use ~50% as much memory).

We reduced the size of the payload in the range segments by teaching 
range trees about starting offsets and shifts; since metaslabs have a 
fixed starting offset, and they all operate in terms of disk sectors, 
we can store the ranges using 4-byte integers as long as the size of 
the metaslab divided by the sector size is less than 2^32. For 512-byte
sectors, this is a 2^41 (or 2TB) metaslab, which with the default
settings corresponds to a 256PB disk. 4k sector disks can handle 
metaslabs up to 2^46 bytes, or 2^63 byte disks. Since we do not 
anticipate disks of this size in the near future, there should be 
almost no cases where metaslabs need 64-byte integers to store their 
ranges. We do still have the capability to store 64-byte integer ranges 
to account for cases where we are storing per-vdev (or per-dnode) trees, 
which could reasonably go above the limits discussed. We also do not 
store fill information in the compact version of the node, since it 
is only used for sorted scrub.

We also optimized the metaslab loading process in various other ways
to offset some inefficiencies in the btree model. While individual
operations (find, insert, remove_from) are faster for the btree than 
they are for the avl tree, remove usually requires a find operation, 
while in the AVL tree model the element itself suffices. Some clever 
changes actually caused an overall speedup in metaslab loading; we use 
approximately 40% less cpu to load metaslabs in our tests on Illumos.

Another memory and performance optimization was achieved by changing 
what is stored in the size-sorted trees. When a disk is heavily 
fragmented, the df algorithm used by default in ZFS will almost always 
find a number of small regions in its initial cursor-based search; it 
will usually only fall back to the size-sorted tree to find larger 
regions. If we increase the size of the cursor-based search slightly, 
and don't store segments that are smaller than a tunable size floor 
in the size-sorted tree, we can further cut memory usage down to 
below 20% of what the AVL trees store. This also results in further 
reductions in CPU time spent loading metaslabs.

The 16KiB size floor was chosen because it results in substantial memory 
usage reduction while not usually resulting in situations where we can't 
find an appropriate chunk with the cursor and are forced to use an 
oversized chunk from the size-sorted tree. In addition, even if we do 
have to use an oversized chunk from the size-sorted tree, the chunk 
would be too small to use for ZIL allocations, so it isn't as big of a 
loss as it might otherwise be. And often, more small allocations will 
follow the initial one, and the cursor search will now find the 
remainder of the chunk we didn't use all of and use it for subsequent 
allocations. Practical testing has shown little or no change in 
fragmentation as a result of this change.

If the size-sorted tree becomes empty while the offset sorted one still 
has entries, it will load all the entries from the offset sorted tree 
and disregard the size floor until it is unloaded again. This operation 
occurs rarely with the default setting, only on incredibly thoroughly 
fragmented pools.

There are some other small changes to zdb to teach it to handle btrees, 
but nothing major.
                                           
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Reviewed by: Sebastien Roy seb@delphix.com
Reviewed-by: Igor Kozhukhov <igor@dilos.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #9181
2019-10-09 10:36:03 -07:00

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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, 2016 by Delphix. All rights reserved.
*/
#include <sys/zfs_context.h>
#include <sys/spa.h>
#include <sys/vdev_impl.h>
#include <sys/zio.h>
#include <sys/kstat.h>
#include <sys/abd.h>
/*
* Virtual device read-ahead caching.
*
* This file implements a simple LRU read-ahead cache. When the DMU reads
* a given block, it will often want other, nearby blocks soon thereafter.
* We take advantage of this by reading a larger disk region and caching
* the result. In the best case, this can turn 128 back-to-back 512-byte
* reads into a single 64k read followed by 127 cache hits; this reduces
* latency dramatically. In the worst case, it can turn an isolated 512-byte
* read into a 64k read, which doesn't affect latency all that much but is
* terribly wasteful of bandwidth. A more intelligent version of the cache
* could keep track of access patterns and not do read-ahead unless it sees
* at least two temporally close I/Os to the same region. Currently, only
* metadata I/O is inflated. A further enhancement could take advantage of
* more semantic information about the I/O. And it could use something
* faster than an AVL tree; that was chosen solely for convenience.
*
* There are five cache operations: allocate, fill, read, write, evict.
*
* (1) Allocate. This reserves a cache entry for the specified region.
* We separate the allocate and fill operations so that multiple threads
* don't generate I/O for the same cache miss.
*
* (2) Fill. When the I/O for a cache miss completes, the fill routine
* places the data in the previously allocated cache entry.
*
* (3) Read. Read data from the cache.
*
* (4) Write. Update cache contents after write completion.
*
* (5) Evict. When allocating a new entry, we evict the oldest (LRU) entry
* if the total cache size exceeds zfs_vdev_cache_size.
*/
/*
* These tunables are for performance analysis.
*/
/*
* All i/os smaller than zfs_vdev_cache_max will be turned into
* 1<<zfs_vdev_cache_bshift byte reads by the vdev_cache (aka software
* track buffer). At most zfs_vdev_cache_size bytes will be kept in each
* vdev's vdev_cache.
*
* TODO: Note that with the current ZFS code, it turns out that the
* vdev cache is not helpful, and in some cases actually harmful. It
* is better if we disable this. Once some time has passed, we should
* actually remove this to simplify the code. For now we just disable
* it by setting the zfs_vdev_cache_size to zero. Note that Solaris 11
* has made these same changes.
*/
int zfs_vdev_cache_max = 1<<14; /* 16KB */
int zfs_vdev_cache_size = 0;
int zfs_vdev_cache_bshift = 16;
#define VCBS (1 << zfs_vdev_cache_bshift) /* 64KB */
kstat_t *vdc_ksp = NULL;
typedef struct vdc_stats {
kstat_named_t vdc_stat_delegations;
kstat_named_t vdc_stat_hits;
kstat_named_t vdc_stat_misses;
} vdc_stats_t;
static vdc_stats_t vdc_stats = {
{ "delegations", KSTAT_DATA_UINT64 },
{ "hits", KSTAT_DATA_UINT64 },
{ "misses", KSTAT_DATA_UINT64 }
};
#define VDCSTAT_BUMP(stat) atomic_inc_64(&vdc_stats.stat.value.ui64);
static inline int
vdev_cache_offset_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
return (TREE_CMP(ve1->ve_offset, ve2->ve_offset));
}
static int
vdev_cache_lastused_compare(const void *a1, const void *a2)
{
const vdev_cache_entry_t *ve1 = (const vdev_cache_entry_t *)a1;
const vdev_cache_entry_t *ve2 = (const vdev_cache_entry_t *)a2;
int cmp = TREE_CMP(ve1->ve_lastused, ve2->ve_lastused);
if (likely(cmp))
return (cmp);
/*
* Among equally old entries, sort by offset to ensure uniqueness.
*/
return (vdev_cache_offset_compare(a1, a2));
}
/*
* Evict the specified entry from the cache.
*/
static void
vdev_cache_evict(vdev_cache_t *vc, vdev_cache_entry_t *ve)
{
ASSERT(MUTEX_HELD(&vc->vc_lock));
ASSERT3P(ve->ve_fill_io, ==, NULL);
ASSERT3P(ve->ve_abd, !=, NULL);
avl_remove(&vc->vc_lastused_tree, ve);
avl_remove(&vc->vc_offset_tree, ve);
abd_free(ve->ve_abd);
kmem_free(ve, sizeof (vdev_cache_entry_t));
}
/*
* Allocate an entry in the cache. At the point we don't have the data,
* we're just creating a placeholder so that multiple threads don't all
* go off and read the same blocks.
*/
static vdev_cache_entry_t *
vdev_cache_allocate(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
uint64_t offset = P2ALIGN(zio->io_offset, VCBS);
vdev_cache_entry_t *ve;
ASSERT(MUTEX_HELD(&vc->vc_lock));
if (zfs_vdev_cache_size == 0)
return (NULL);
/*
* If adding a new entry would exceed the cache size,
* evict the oldest entry (LRU).
*/
if ((avl_numnodes(&vc->vc_lastused_tree) << zfs_vdev_cache_bshift) >
zfs_vdev_cache_size) {
ve = avl_first(&vc->vc_lastused_tree);
if (ve->ve_fill_io != NULL)
return (NULL);
ASSERT3U(ve->ve_hits, !=, 0);
vdev_cache_evict(vc, ve);
}
ve = kmem_zalloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
ve->ve_offset = offset;
ve->ve_lastused = ddi_get_lbolt();
ve->ve_abd = abd_alloc_for_io(VCBS, B_TRUE);
avl_add(&vc->vc_offset_tree, ve);
avl_add(&vc->vc_lastused_tree, ve);
return (ve);
}
static void
vdev_cache_hit(vdev_cache_t *vc, vdev_cache_entry_t *ve, zio_t *zio)
{
uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS);
ASSERT(MUTEX_HELD(&vc->vc_lock));
ASSERT3P(ve->ve_fill_io, ==, NULL);
if (ve->ve_lastused != ddi_get_lbolt()) {
avl_remove(&vc->vc_lastused_tree, ve);
ve->ve_lastused = ddi_get_lbolt();
avl_add(&vc->vc_lastused_tree, ve);
}
ve->ve_hits++;
abd_copy_off(zio->io_abd, ve->ve_abd, 0, cache_phase, zio->io_size);
}
/*
* Fill a previously allocated cache entry with data.
*/
static void
vdev_cache_fill(zio_t *fio)
{
vdev_t *vd = fio->io_vd;
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_entry_t *ve = fio->io_private;
zio_t *pio;
ASSERT3U(fio->io_size, ==, VCBS);
/*
* Add data to the cache.
*/
mutex_enter(&vc->vc_lock);
ASSERT3P(ve->ve_fill_io, ==, fio);
ASSERT3U(ve->ve_offset, ==, fio->io_offset);
ASSERT3P(ve->ve_abd, ==, fio->io_abd);
ve->ve_fill_io = NULL;
/*
* Even if this cache line was invalidated by a missed write update,
* any reads that were queued up before the missed update are still
* valid, so we can satisfy them from this line before we evict it.
*/
zio_link_t *zl = NULL;
while ((pio = zio_walk_parents(fio, &zl)) != NULL)
vdev_cache_hit(vc, ve, pio);
if (fio->io_error || ve->ve_missed_update)
vdev_cache_evict(vc, ve);
mutex_exit(&vc->vc_lock);
}
/*
* Read data from the cache. Returns B_TRUE cache hit, B_FALSE on miss.
*/
boolean_t
vdev_cache_read(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
vdev_cache_entry_t *ve, *ve_search;
uint64_t cache_offset = P2ALIGN(zio->io_offset, VCBS);
zio_t *fio;
ASSERTV(uint64_t cache_phase = P2PHASE(zio->io_offset, VCBS));
ASSERT3U(zio->io_type, ==, ZIO_TYPE_READ);
if (zio->io_flags & ZIO_FLAG_DONT_CACHE)
return (B_FALSE);
if (zio->io_size > zfs_vdev_cache_max)
return (B_FALSE);
/*
* If the I/O straddles two or more cache blocks, don't cache it.
*/
if (P2BOUNDARY(zio->io_offset, zio->io_size, VCBS))
return (B_FALSE);
ASSERT3U(cache_phase + zio->io_size, <=, VCBS);
mutex_enter(&vc->vc_lock);
ve_search = kmem_alloc(sizeof (vdev_cache_entry_t), KM_SLEEP);
ve_search->ve_offset = cache_offset;
ve = avl_find(&vc->vc_offset_tree, ve_search, NULL);
kmem_free(ve_search, sizeof (vdev_cache_entry_t));
if (ve != NULL) {
if (ve->ve_missed_update) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
if ((fio = ve->ve_fill_io) != NULL) {
zio_vdev_io_bypass(zio);
zio_add_child(zio, fio);
mutex_exit(&vc->vc_lock);
VDCSTAT_BUMP(vdc_stat_delegations);
return (B_TRUE);
}
vdev_cache_hit(vc, ve, zio);
zio_vdev_io_bypass(zio);
mutex_exit(&vc->vc_lock);
VDCSTAT_BUMP(vdc_stat_hits);
return (B_TRUE);
}
ve = vdev_cache_allocate(zio);
if (ve == NULL) {
mutex_exit(&vc->vc_lock);
return (B_FALSE);
}
fio = zio_vdev_delegated_io(zio->io_vd, cache_offset,
ve->ve_abd, VCBS, ZIO_TYPE_READ, ZIO_PRIORITY_NOW,
ZIO_FLAG_DONT_CACHE, vdev_cache_fill, ve);
ve->ve_fill_io = fio;
zio_vdev_io_bypass(zio);
zio_add_child(zio, fio);
mutex_exit(&vc->vc_lock);
zio_nowait(fio);
VDCSTAT_BUMP(vdc_stat_misses);
return (B_TRUE);
}
/*
* Update cache contents upon write completion.
*/
void
vdev_cache_write(zio_t *zio)
{
vdev_cache_t *vc = &zio->io_vd->vdev_cache;
vdev_cache_entry_t *ve, ve_search;
uint64_t io_start = zio->io_offset;
uint64_t io_end = io_start + zio->io_size;
uint64_t min_offset = P2ALIGN(io_start, VCBS);
uint64_t max_offset = P2ROUNDUP(io_end, VCBS);
avl_index_t where;
ASSERT3U(zio->io_type, ==, ZIO_TYPE_WRITE);
mutex_enter(&vc->vc_lock);
ve_search.ve_offset = min_offset;
ve = avl_find(&vc->vc_offset_tree, &ve_search, &where);
if (ve == NULL)
ve = avl_nearest(&vc->vc_offset_tree, where, AVL_AFTER);
while (ve != NULL && ve->ve_offset < max_offset) {
uint64_t start = MAX(ve->ve_offset, io_start);
uint64_t end = MIN(ve->ve_offset + VCBS, io_end);
if (ve->ve_fill_io != NULL) {
ve->ve_missed_update = 1;
} else {
abd_copy_off(ve->ve_abd, zio->io_abd,
start - ve->ve_offset, start - io_start,
end - start);
}
ve = AVL_NEXT(&vc->vc_offset_tree, ve);
}
mutex_exit(&vc->vc_lock);
}
void
vdev_cache_purge(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_entry_t *ve;
mutex_enter(&vc->vc_lock);
while ((ve = avl_first(&vc->vc_offset_tree)) != NULL)
vdev_cache_evict(vc, ve);
mutex_exit(&vc->vc_lock);
}
void
vdev_cache_init(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
mutex_init(&vc->vc_lock, NULL, MUTEX_DEFAULT, NULL);
avl_create(&vc->vc_offset_tree, vdev_cache_offset_compare,
sizeof (vdev_cache_entry_t),
offsetof(struct vdev_cache_entry, ve_offset_node));
avl_create(&vc->vc_lastused_tree, vdev_cache_lastused_compare,
sizeof (vdev_cache_entry_t),
offsetof(struct vdev_cache_entry, ve_lastused_node));
}
void
vdev_cache_fini(vdev_t *vd)
{
vdev_cache_t *vc = &vd->vdev_cache;
vdev_cache_purge(vd);
avl_destroy(&vc->vc_offset_tree);
avl_destroy(&vc->vc_lastused_tree);
mutex_destroy(&vc->vc_lock);
}
void
vdev_cache_stat_init(void)
{
vdc_ksp = kstat_create("zfs", 0, "vdev_cache_stats", "misc",
KSTAT_TYPE_NAMED, sizeof (vdc_stats) / sizeof (kstat_named_t),
KSTAT_FLAG_VIRTUAL);
if (vdc_ksp != NULL) {
vdc_ksp->ks_data = &vdc_stats;
kstat_install(vdc_ksp);
}
}
void
vdev_cache_stat_fini(void)
{
if (vdc_ksp != NULL) {
kstat_delete(vdc_ksp);
vdc_ksp = NULL;
}
}
/* BEGIN CSTYLED */
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_max, INT, ZMOD_RW,
"Inflate reads small than max");
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_size, INT, ZMOD_RD,
"Total size of the per-disk cache");
ZFS_MODULE_PARAM(zfs_vdev, zfs_vdev_, cache_bshift, INT, ZMOD_RW,
"Shift size to inflate reads too");
/* END CSTYLED */
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