![[BACK]](/cvsweb/icons/back.gif){kind=icon}
Return to qed-l2-cache.c CVS log ![[TXT]](/cvsweb/icons/text.gif){kind=icon}
![[DIR]](/cvsweb/icons/dir.gif){kind=icon}
Up to [Qemu by Fabrice Bellard] / qemu / block|
Return to qed-l2-cache.c CVS log | Up to [Qemu by Fabrice Bellard] / qemu / block |
1.1 root 1: /*
2: * QEMU Enhanced Disk Format L2 Cache
3: *
4: * Copyright IBM, Corp. 2010
5: *
6: * Authors:
7: * Anthony Liguori <[email protected]>
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);
1.1.1.2 ! root 77: g_free(entry);
1.1 root 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:
1.1.1.2 ! root 92: entry = g_malloc0(sizeof(*entry));
1.1 root 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);
1.1.1.2 ! root 114: g_free(entry);
1.1 root 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: }
This archive runs on limited infrastructure. Preserving old code on modern bandwidth. Automated agents are requested to crawl responsibly.