qemu

qed-l2-cache.c (6149B)

---

 /*
  * QEMU Enhanced Disk Format L2 Cache
  *
  * Copyright IBM, Corp. 2010
  *
  * Authors:
  *  Anthony Liguori   <aliguori@us.ibm.com>
  *
  * This work is licensed under the terms of the GNU LGPL, version 2 or later.
  * See the COPYING.LIB file in the top-level directory.
  *
  */
 
 /*
  * L2 table cache usage is as follows:
  *
  * An open image has one L2 table cache that is used to avoid accessing the
  * image file for recently referenced L2 tables.
  *
  * Cluster offset lookup translates the logical offset within the block device
  * to a cluster offset within the image file.  This is done by indexing into
  * the L1 and L2 tables which store cluster offsets.  It is here where the L2
  * table cache serves up recently referenced L2 tables.
  *
  * If there is a cache miss, that L2 table is read from the image file and
  * committed to the cache.  Subsequent accesses to that L2 table will be served
  * from the cache until the table is evicted from the cache.
  *
  * L2 tables are also committed to the cache when new L2 tables are allocated
  * in the image file.  Since the L2 table cache is write-through, the new L2
  * table is first written out to the image file and then committed to the
  * cache.
  *
  * Multiple I/O requests may be using an L2 table cache entry at any given
  * time.  That means an entry may be in use across several requests and
  * reference counting is needed to free the entry at the correct time.  In
  * particular, an entry evicted from the cache will only be freed once all
  * references are dropped.
  *
  * An in-flight I/O request will hold a reference to a L2 table cache entry for
  * the period during which it needs to access the L2 table.  This includes
  * cluster offset lookup, L2 table allocation, and L2 table update when a new
  * data cluster has been allocated.
  *
  * An interesting case occurs when two requests need to access an L2 table that
  * is not in the cache.  Since the operation to read the table from the image
  * file takes some time to complete, both requests may see a cache miss and
  * start reading the L2 table from the image file.  The first to finish will
  * commit its L2 table into the cache.  When the second tries to commit its
  * table will be deleted in favor of the existing cache entry.
  */
 
 #include "qemu/osdep.h"
 #include "qemu/memalign.h"
 #include "trace.h"
 #include "qed.h"
 
 /* Each L2 holds 2GB so this let's us fully cache a 100GB disk */
 #define MAX_L2_CACHE_SIZE 50
 
 /**
  * Initialize the L2 cache
  */
 void qed_init_l2_cache(L2TableCache *l2_cache)
 {
     QTAILQ_INIT(&l2_cache->entries);
     l2_cache->n_entries = 0;
 }
 
 /**
  * Free the L2 cache
  */
 void qed_free_l2_cache(L2TableCache *l2_cache)
 {
     CachedL2Table *entry, *next_entry;
 
     QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next_entry) {
         qemu_vfree(entry->table);
         g_free(entry);
     }
 }
 
 /**
  * Allocate an uninitialized entry from the cache
  *
  * The returned entry has a reference count of 1 and is owned by the caller.
  * The caller must allocate the actual table field for this entry and it must
  * be freeable using qemu_vfree().
  */
 CachedL2Table *qed_alloc_l2_cache_entry(L2TableCache *l2_cache)
 {
     CachedL2Table *entry;
 
     entry = g_malloc0(sizeof(*entry));
     entry->ref++;
 
     trace_qed_alloc_l2_cache_entry(l2_cache, entry);
 
     return entry;
 }
 
 /**
  * Decrease an entry's reference count and free if necessary when the reference
  * count drops to zero.
  *
  * Called with table_lock held.
  */
 void qed_unref_l2_cache_entry(CachedL2Table *entry)
 {
     if (!entry) {
         return;
     }
 
     entry->ref--;
     trace_qed_unref_l2_cache_entry(entry, entry->ref);
     if (entry->ref == 0) {
         qemu_vfree(entry->table);
         g_free(entry);
     }
 }
 
 /**
  * Find an entry in the L2 cache.  This may return NULL and it's up to the
  * caller to satisfy the cache miss.
  *
  * For a cached entry, this function increases the reference count and returns
  * the entry.
  *
  * Called with table_lock held.
  */
 CachedL2Table *qed_find_l2_cache_entry(L2TableCache *l2_cache, uint64_t offset)
 {
     CachedL2Table *entry;
 
     QTAILQ_FOREACH(entry, &l2_cache->entries, node) {
         if (entry->offset == offset) {
             trace_qed_find_l2_cache_entry(l2_cache, entry, offset, entry->ref);
             entry->ref++;
             return entry;
         }
     }
     return NULL;
 }
 
 /**
  * Commit an L2 cache entry into the cache.  This is meant to be used as part of
  * the process to satisfy a cache miss.  A caller would allocate an entry which
  * is not actually in the L2 cache and then once the entry was valid and
  * present on disk, the entry can be committed into the cache.
  *
  * Since the cache is write-through, it's important that this function is not
  * called until the entry is present on disk and the L1 has been updated to
  * point to the entry.
  *
  * N.B. This function steals a reference to the l2_table from the caller so the
  * caller must obtain a new reference by issuing a call to
  * qed_find_l2_cache_entry().
  *
  * Called with table_lock held.
  */
 void qed_commit_l2_cache_entry(L2TableCache *l2_cache, CachedL2Table *l2_table)
 {
     CachedL2Table *entry;
 
     entry = qed_find_l2_cache_entry(l2_cache, l2_table->offset);
     if (entry) {
         qed_unref_l2_cache_entry(entry);
         qed_unref_l2_cache_entry(l2_table);
         return;
     }
 
     /* Evict an unused cache entry so we have space.  If all entries are in use
      * we can grow the cache temporarily and we try to shrink back down later.
      */
     if (l2_cache->n_entries >= MAX_L2_CACHE_SIZE) {
         CachedL2Table *next;
         QTAILQ_FOREACH_SAFE(entry, &l2_cache->entries, node, next) {
             if (entry->ref > 1) {
                 continue;
             }
 
             QTAILQ_REMOVE(&l2_cache->entries, entry, node);
             l2_cache->n_entries--;
             qed_unref_l2_cache_entry(entry);
 
             /* Stop evicting when we've shrunk back to max size */
             if (l2_cache->n_entries < MAX_L2_CACHE_SIZE) {
                 break;
             }
         }
     }
 
     l2_cache->n_entries++;
     QTAILQ_INSERT_TAIL(&l2_cache->entries, l2_table, node);
 }