qed-l2-cache.c (6122B)
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 "qemu/osdep.h" 54#include "trace.h" 55#include "qed.h" 56 57/* Each L2 holds 2GB so this let's us fully cache a 100GB disk */ 58#define MAX_L2_CACHE_SIZE 50 59 60/** 61 * Initialize the L2 cache 62 */ 63void qed_init_l2_cache(L2TableCache *l2_cache) 64{ 65 QTAILQ_INIT(&l2_cache->entries); 66 l2_cache->n_entries = 0; 67} 68 69/** 70 * Free the L2 cache 71 */ 72void qed_free_l2_cache(L2TableCache *l2_cache) 73{ 74 CachedL2Table *entry, *next_entry; 75 76 QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) { 77 qemu_vfree(entry->table); 78 g_free(entry); 79 } 80} 81 82/** 83 * Allocate an uninitialized entry from the cache 84 * 85 * The returned entry has a reference count of 1 and is owned by the caller. 86 * The caller must allocate the actual table field for this entry and it must 87 * be freeable using qemu_vfree(). 88 */ 89CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache) 90{ 91 CachedL2Table *entry; 92 93 entry = g_malloc0(sizeof(*entry)); 94 entry->ref++; 95 96 trace_qed_alloc_l2_cache_entry(l2_cache, entry); 97 98 return entry; 99} 100 101/** 102 * Decrease an entry's reference count and free if necessary when the reference 103 * count drops to zero. 104 * 105 * Called with table_lock held. 106 */ 107void qed_unref_l2_cache_entry(CachedL2Table *entry) 108{ 109 if (!entry) { 110 return; 111 } 112 113 entry->ref--; 114 trace_qed_unref_l2_cache_entry(entry, entry->ref); 115 if (entry->ref == 0) { 116 qemu_vfree(entry->table); 117 g_free(entry); 118 } 119} 120 121/** 122 * Find an entry in the L2 cache. This may return NULL and it's up to the 123 * caller to satisfy the cache miss. 124 * 125 * For a cached entry, this function increases the reference count and returns 126 * the entry. 127 * 128 * Called with table_lock held. 129 */ 130CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset) 131{ 132 CachedL2Table *entry; 133 134 QTAILQ_FOREACH(entry, &l2_cache->entries, node) { 135 if (entry->offset == offset) { 136 trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref); 137 entry->ref++; 138 return entry; 139 } 140 } 141 return NULL; 142} 143 144/** 145 * Commit an L2 cache entry into the cache. This is meant to be used as part of 146 * the process to satisfy a cache miss. A caller would allocate an entry which 147 * is not actually in the L2 cache and then once the entry was valid and 148 * present on disk, the entry can be committed into the cache. 149 * 150 * Since the cache is write-through, it's important that this function is not 151 * called until the entry is present on disk and the L1 has been updated to 152 * point to the entry. 153 * 154 * N.B. This function steals a reference to the l2_table from the caller so the 155 * caller must obtain a new reference by issuing a call to 156 * qed_find_l2_cache_entry(). 157 * 158 * Called with table_lock held. 159 */ 160void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table) 161{ 162 CachedL2Table *entry; 163 164 entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset); 165 if (entry) { 166 qed_unref_l2_cache_entry(entry); 167 qed_unref_l2_cache_entry(l2_table); 168 return; 169 } 170 171 /* Evict an unused cache entry so we have space. If all entries are in use 172 * we can grow the cache temporarily and we try to shrink back down later. 173 */ 174 if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) { 175 CachedL2Table *next; 176 QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) { 177 if (entry->ref > 1) { 178 continue; 179 } 180 181 QTAILQ_REMOVE(&l2_cache->entries, entry, node); 182 l2_cache->n_entries--; 183 qed_unref_l2_cache_entry(entry); 184 185 /* Stop evicting when we've shrunk back to max size */ 186 if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) { 187 break; 188 } 189 } 190 } 191 192 l2_cache->n_entries++; 193 QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node); 194}