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Diffstat (limited to 'qemu/block/qed-l2-cache.c')
| -rw-r--r-- | qemu/block/qed-l2-cache.c | 188 |
1 files changed, 0 insertions, 188 deletions
diff --git a/qemu/block/qed-l2-cache.c b/qemu/block/qed-l2-cache.c deleted file mode 100644 index 5cba79465..000000000 --- a/qemu/block/qed-l2-cache.c +++ /dev/null @@ -1,188 +0,0 @@ -/* - * 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 "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. - */ -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. - */ -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(). - */ -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); -} |