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Annotation of qemu/block/qed-l2-cache.c, revision 1.1

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);
        !            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: }

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