root / block / qed-l2-cache.c @ d2d979c6
History | View | Annotate | Download (5.4 kB)
1 |
/*
|
---|---|
2 |
* QEMU Enhanced Disk Format L2 Cache
|
3 |
*
|
4 |
* Copyright IBM, Corp. 2010
|
5 |
*
|
6 |
* Authors:
|
7 |
* Anthony Liguori <aliguori@us.ibm.com>
|
8 |
*
|
9 |
* This work is licensed under the terms of the GNU LGPL, version 2 or later.
|
10 |
* See the COPYING.LIB file in the top-level directory.
|
11 |
*
|
12 |
*/
|
13 |
|
14 |
/*
|
15 |
* L2 table cache usage is as follows:
|
16 |
*
|
17 |
* An open image has one L2 table cache that is used to avoid accessing the
|
18 |
* image file for recently referenced L2 tables.
|
19 |
*
|
20 |
* Cluster offset lookup translates the logical offset within the block device
|
21 |
* to a cluster offset within the image file. This is done by indexing into
|
22 |
* the L1 and L2 tables which store cluster offsets. It is here where the L2
|
23 |
* table cache serves up recently referenced L2 tables.
|
24 |
*
|
25 |
* If there is a cache miss, that L2 table is read from the image file and
|
26 |
* committed to the cache. Subsequent accesses to that L2 table will be served
|
27 |
* from the cache until the table is evicted from the cache.
|
28 |
*
|
29 |
* L2 tables are also committed to the cache when new L2 tables are allocated
|
30 |
* in the image file. Since the L2 table cache is write-through, the new L2
|
31 |
* table is first written out to the image file and then committed to the
|
32 |
* cache.
|
33 |
*
|
34 |
* Multiple I/O requests may be using an L2 table cache entry at any given
|
35 |
* time. That means an entry may be in use across several requests and
|
36 |
* reference counting is needed to free the entry at the correct time. In
|
37 |
* particular, an entry evicted from the cache will only be freed once all
|
38 |
* references are dropped.
|
39 |
*
|
40 |
* An in-flight I/O request will hold a reference to a L2 table cache entry for
|
41 |
* the period during which it needs to access the L2 table. This includes
|
42 |
* cluster offset lookup, L2 table allocation, and L2 table update when a new
|
43 |
* data cluster has been allocated.
|
44 |
*
|
45 |
* An interesting case occurs when two requests need to access an L2 table that
|
46 |
* is not in the cache. Since the operation to read the table from the image
|
47 |
* file takes some time to complete, both requests may see a cache miss and
|
48 |
* start reading the L2 table from the image file. The first to finish will
|
49 |
* commit its L2 table into the cache. When the second tries to commit its
|
50 |
* table will be deleted in favor of the existing cache entry.
|
51 |
*/
|
52 |
|
53 |
#include "trace.h" |
54 |
#include "qed.h" |
55 |
|
56 |
/* Each L2 holds 2GB so this let's us fully cache a 100GB disk */
|
57 |
#define MAX_L2_CACHE_SIZE 50 |
58 |
|
59 |
/**
|
60 |
* Initialize the L2 cache
|
61 |
*/
|
62 |
void qed_init_l2_cache(L2TableCache *l2_cache)
|
63 |
{ |
64 |
QTAILQ_INIT(&l2_cache->entries); |
65 |
l2_cache->n_entries = 0;
|
66 |
} |
67 |
|
68 |
/**
|
69 |
* Free the L2 cache
|
70 |
*/
|
71 |
void qed_free_l2_cache(L2TableCache *l2_cache)
|
72 |
{ |
73 |
CachedL2Table *entry, *next_entry; |
74 |
|
75 |
QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { |
76 |
qemu_vfree(entry->table); |
77 |
qemu_free(entry); |
78 |
} |
79 |
} |
80 |
|
81 |
/**
|
82 |
* Allocate an uninitialized entry from the cache
|
83 |
*
|
84 |
* The returned entry has a reference count of 1 and is owned by the caller.
|
85 |
* The caller must allocate the actual table field for this entry and it must
|
86 |
* be freeable using qemu_vfree().
|
87 |
*/
|
88 |
CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) |
89 |
{ |
90 |
CachedL2Table *entry; |
91 |
|
92 |
entry = qemu_mallocz(sizeof(*entry));
|
93 |
entry->ref++; |
94 |
|
95 |
trace_qed_alloc_l2_cache_entry(l2_cache, entry); |
96 |
|
97 |
return entry;
|
98 |
} |
99 |
|
100 |
/**
|
101 |
* Decrease an entry's reference count and free if necessary when the reference
|
102 |
* count drops to zero.
|
103 |
*/
|
104 |
void qed_unref_l2_cache_entry(CachedL2Table *entry)
|
105 |
{ |
106 |
if (!entry) {
|
107 |
return;
|
108 |
} |
109 |
|
110 |
entry->ref--; |
111 |
trace_qed_unref_l2_cache_entry(entry, entry->ref); |
112 |
if (entry->ref == 0) { |
113 |
qemu_vfree(entry->table); |
114 |
qemu_free(entry); |
115 |
} |
116 |
} |
117 |
|
118 |
/**
|
119 |
* Find an entry in the L2 cache. This may return NULL and it's up to the
|
120 |
* caller to satisfy the cache miss.
|
121 |
*
|
122 |
* For a cached entry, this function increases the reference count and returns
|
123 |
* the entry.
|
124 |
*/
|
125 |
CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) |
126 |
{ |
127 |
CachedL2Table *entry; |
128 |
|
129 |
QTAILQ_FOREACH(entry, &l2_cache->entries, node) { |
130 |
if (entry->offset == offset) {
|
131 |
trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); |
132 |
entry->ref++; |
133 |
return entry;
|
134 |
} |
135 |
} |
136 |
return NULL; |
137 |
} |
138 |
|
139 |
/**
|
140 |
* Commit an L2 cache entry into the cache. This is meant to be used as part of
|
141 |
* the process to satisfy a cache miss. A caller would allocate an entry which
|
142 |
* is not actually in the L2 cache and then once the entry was valid and
|
143 |
* present on disk, the entry can be committed into the cache.
|
144 |
*
|
145 |
* Since the cache is write-through, it's important that this function is not
|
146 |
* called until the entry is present on disk and the L1 has been updated to
|
147 |
* point to the entry.
|
148 |
*
|
149 |
* N.B. This function steals a reference to the l2_table from the caller so the
|
150 |
* caller must obtain a new reference by issuing a call to
|
151 |
* qed_find_l2_cache_entry().
|
152 |
*/
|
153 |
void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table)
|
154 |
{ |
155 |
CachedL2Table *entry; |
156 |
|
157 |
entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); |
158 |
if (entry) {
|
159 |
qed_unref_l2_cache_entry(entry); |
160 |
qed_unref_l2_cache_entry(l2_table); |
161 |
return;
|
162 |
} |
163 |
|
164 |
if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {
|
165 |
entry = QTAILQ_FIRST(&l2_cache->entries); |
166 |
QTAILQ_REMOVE(&l2_cache->entries, entry, node); |
167 |
l2_cache->n_entries--; |
168 |
qed_unref_l2_cache_entry(entry); |
169 |
} |
170 |
|
171 |
l2_cache->n_entries++; |
172 |
QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); |
173 |
} |