block/qed-l2-cache.c

   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         g_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 = g_malloc0(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         g_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     /* Evict an unused cache entry so we have space.  If all entries are in use
 165      * we can grow the cache temporarily and we try to shrink back down later.
 166      */
 167     if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {
 168         CachedL2Table *next;
 169         QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) {
 170             if (entry->ref > 1) {
 171                 continue;
 172             }
 173
 174             QTAILQ_REMOVE(&l2_cache->entries, entry, node);
 175             l2_cache->n_entries--;
 176             qed_unref_l2_cache_entry(entry);
 177
 178             /* Stop evicting when we've shrunk back to max size */
 179             if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) {
 180                 break;
 181             }
 182         }
 183     }
 184
 185     l2_cache->n_entries++;
 186     QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node);
 187 }