for (i = 0; i < 5; i++) {
dictEntry *de;
double swappability;
+ robj keyobj;
+ sds keystr;
if (maxtries) maxtries--;
de = dictGetRandomKey(db->dict);
+ keystr = dictGetEntryKey(de);
val = dictGetEntryVal(de);
- /* Only swap objects that are currently in memory.
- *
- * Also don't swap shared objects: not a good idea in general and
- * we need to ensure that the main thread does not touch the
- * object while the I/O thread is using it, but we can't
- * control other keys without adding additional mutex. */
- if (val->storage != REDIS_DS_MEMORY) {
+ initStaticStringObject(keyobj,keystr);
+
+ /* Don't remove objects that are currently target of a
+ * read or write operation. */
+ if (cacheScheduleIOGetFlags(db,&keyobj) != 0) {
if (maxtries) i--; /* don't count this try */
continue;
}
* When disk store is enabled, we need negative caching, that is, to remember
* keys that are for sure *not* on the disk key-value store.
*
- * This is useful for two reasons:
- *
- * 1) Without negative caching cache misses will cost us a disk lookup, even
- * if the same non existing key is accessed again and again. We negative
- * caching we remember that the key is not on disk, so if it's not in memory
- * and we have a negative cache entry, we don't try a disk access at all.
- *
- * 2) Negative caching is the way to fix a specific race condition. For instance
- * think at the following sequence of commands:
- *
- * SET foo bar
- * DEL foo
- * GET foo
- *
- * After the SET, we'll mark the value as dirty, so it will be flushed
- * on disk at some time. Later the key is deleted, so will be removed
- * from memory. Another job will be created to remove the key from the disk
- * store, but the removal is not synchronous, so may happen later in time.
- *
- * Finally we have a GET foo operation. This operation may result in
- * reading back a value from disk that is not updated data, as the deletion
- * operaiton against the disk KV store was still not completed, so we
- * read old data.
- *
- * Remembering that the given key is deleted is important. We can discard this
- * information once the key was really removed from the disk.
- *
- * So actually there are two kind of negative caching entries: entries that
- * can be evicted when we need to reclaim memory, and entries that will
- * not be evicted, for all the time we need this information to be available.
+ * This is usefuls because without negative caching cache misses will cost us
+ * a disk lookup, even if the same non existing key is accessed again and again.
*
- * The API allows to create both kind of negative caching. */
+ * With negative caching we remember that the key is not on disk, so if it's
+ * not in memory and we have a negative cache entry, we don't try a disk
+ * access at all.
+ */
+/* Returns true if the specified key may exists on disk, that is, we don't
+ * have an entry in our negative cache for this key */
int cacheKeyMayExist(redisDb *db, robj *key) {
return dictFind(db->io_negcache,key) == NULL;
}
+/* Set the specified key as an entry that may possibily exist on disk, that is,
+ * remove the negative cache entry for this key if any. */
void cacheSetKeyMayExist(redisDb *db, robj *key) {
dictDelete(db->io_negcache,key);
}
+/* Set the specified key as non existing on disk, that is, create a negative
+ * cache entry for this key. */
void cacheSetKeyDoesNotExist(redisDb *db, robj *key) {
- struct dictEntry *de;
-
- /* Don't overwrite negative cached entries with val set to 0, as this
- * entries were created with cacheSetKeyDoesNotExistRemember(). */
- de = dictFind(db->io_negcache,key);
- if (de != NULL && dictGetEntryVal(de) == NULL) return;
-
if (dictReplace(db->io_negcache,key,(void*)time(NULL))) {
incrRefCount(key);
}
}
-void cacheSetKeyDoesNotExistRemember(redisDb *db, robj *key) {
- if (dictReplace(db->io_negcache,key,NULL)) {
- incrRefCount(key);
- }
-}
-
/* ================== Disk store cache - Threaded I/O ====================== */
void freeIOJob(iojob *j) {
if (j->type == REDIS_IOJOB_LOAD) {
/* Create the key-value pair in the in-memory database */
if (j->val != NULL) {
- /* Note: the key may already be here if between the time
- * this key loading was scheduled and now there was the
- * need to blocking load the key for a key lookup.
- *
- * Also we don't add a key that was deleted in the
- * meantime and should not be on disk either. */
- if (cacheKeyMayExist(j->db,j->key) &&
- dbAdd(j->db,j->key,j->val) == REDIS_OK)
- {
+ /* Note: it's possible that the key is already in memory
+ * due to a blocking load operation. */
+ if (dbAdd(j->db,j->key,j->val) == REDIS_OK) {
incrRefCount(j->val);
if (j->expire != -1) setExpire(j->db,j->key,j->expire);
}
* for this key. */
cacheSetKeyDoesNotExist(j->db,j->key);
}
- /* Handle clients waiting for this key to be loaded. */
+ cacheScheduleIODelFlag(j->db,j->key,REDIS_IO_LOADINPROG);
handleClientsBlockedOnSwappedKey(j->db,j->key);
freeIOJob(j);
} else if (j->type == REDIS_IOJOB_SAVE) {
if (j->val) {
- redisAssert(j->val->storage == REDIS_DS_SAVING);
- j->val->storage = REDIS_DS_MEMORY;
cacheSetKeyMayExist(j->db,j->key);
} else {
- /* Key deleted. Probably we have this key marked as
- * non existing, and impossible to evict, in our negative
- * cache entry. Add it as a normal negative cache entry. */
- cacheSetKeyMayExist(j->db,j->key);
+ cacheSetKeyDoesNotExist(j->db,j->key);
}
+ cacheScheduleIODelFlag(j->db,j->key,REDIS_IO_SAVEINPROG);
freeIOJob(j);
}
processed++;
if (j->val) j->expire = expire;
} else if (j->type == REDIS_IOJOB_SAVE) {
if (j->val) {
- redisAssert(j->val->storage == REDIS_DS_SAVING);
dsSet(j->db,j->key,j->val);
} else {
dsDel(j->db,j->key);
unlockThreadedIO();
}
-void cacheScheduleForFlush(redisDb *db, robj *key) {
- dirtykey *dk;
- dictEntry *de;
-
- de = dictFind(db->dict,key->ptr);
- if (de) {
- robj *val = dictGetEntryVal(de);
- if (val->storage == REDIS_DS_DIRTY)
- return;
- else
- val->storage = REDIS_DS_DIRTY;
+/* ============= Disk store cache - Scheduling of IO operations =============
+ *
+ * We use a queue and an hash table to hold the state of IO operations
+ * so that's fast to lookup if there is already an IO operation in queue
+ * for a given key.
+ *
+ * There are two types of IO operations for a given key:
+ * REDIS_IO_LOAD and REDIS_IO_SAVE.
+ *
+ * The function cacheScheduleIO() function pushes the specified IO operation
+ * in the queue, but avoid adding the same key for the same operation
+ * multiple times, thanks to the associated hash table.
+ *
+ * We take a set of flags per every key, so when the scheduled IO operation
+ * gets moved from the scheduled queue to the actual IO Jobs queue that
+ * is processed by the IO thread, we flag it as IO_LOADINPROG or
+ * IO_SAVEINPROG.
+ *
+ * So for every given key we always know if there is some IO operation
+ * scheduled, or in progress, for this key.
+ *
+ * NOTE: all this is very important in order to guarantee correctness of
+ * the Disk Store Cache. Jobs are always queued here. Load jobs are
+ * queued at the head for faster execution only in the case there is not
+ * already a write operation of some kind for this job.
+ *
+ * So we have ordering, but can do exceptions when there are no already
+ * operations for a given key. Also when we need to block load a given
+ * key, for an immediate lookup operation, we can check if the key can
+ * be accessed synchronously without race conditions (no IN PROGRESS
+ * operations for this key), otherwise we blocking wait for completion. */
+
+#define REDIS_IO_LOAD 1
+#define REDIS_IO_SAVE 2
+#define REDIS_IO_LOADINPROG 4
+#define REDIS_IO_SAVEINPROG 8
+
+void cacheScheduleIOAddFlag(redisDb *db, robj *key, long flag) {
+ struct dictEntry *de = dictFind(db->io_queued,key);
+
+ if (!de) {
+ dictAdd(db->io_queued,key,(void*)flag);
+ incrRefCount(key);
+ return;
+ } else {
+ long flags = (long) dictGetEntryVal(de);
+ flags |= flag;
+ dictGetEntryVal(de) = (void*) flags;
}
+}
+
+void cacheScheduleIODelFlag(redisDb *db, robj *key, long flag) {
+ struct dictEntry *de = dictFind(db->io_queued,key);
+ long flags;
+
+ redisAssert(de != NULL);
+ flags = (long) dictGetEntryVal(de);
+ redisAssert(flags & flag);
+ flags &= ~flag;
+ if (flags == 0) {
+ dictDelete(db->io_queued,key);
+ } else {
+ dictGetEntryVal(de) = (void*) flags;
+ }
+}
- redisLog(REDIS_DEBUG,"Scheduling key %s for saving (%s)",key->ptr,
- de ? "key exists" : "key does not exist");
- dk = zmalloc(sizeof(*dk));
- dk->db = db;
- dk->key = key;
+int cacheScheduleIOGetFlags(redisDb *db, robj *key) {
+ struct dictEntry *de = dictFind(db->io_queued,key);
+
+ return (de == NULL) ? 0 : ((long) dictGetEntryVal(de));
+}
+
+void cacheScheduleIO(redisDb *db, robj *key, int type) {
+ ioop *op;
+ long flags;
+
+ if ((flags = cacheScheduleIOGetFlags(db,key)) & type) return;
+
+ redisLog(REDIS_DEBUG,"Scheduling key %s for %s",
+ key->ptr, type == REDIS_IO_LOAD ? "loading" : "saving");
+ cacheScheduleIOAddFlag(db,key,type);
+ op = zmalloc(sizeof(*op));
+ op->type = type;
+ op->db = db;
+ op->key = key;
incrRefCount(key);
- dk->ctime = time(NULL);
- listAddNodeTail(server.cache_flush_queue, dk);
+ op->ctime = time(NULL);
+
+ /* Give priority to load operations if there are no save already
+ * in queue for the same key. */
+ if (type == REDIS_IO_LOAD && !(flags & REDIS_IO_SAVE)) {
+ listAddNodeHead(server.cache_io_queue, op);
+ } else {
+ /* FIXME: probably when this happens we want to at least move
+ * the write job about this queue on top, and set the creation time
+ * to a value that will force processing ASAP. */
+ listAddNodeTail(server.cache_io_queue, op);
+ }
}
void cacheCron(void) {
topush = 100-jobs;
if (topush < 0) topush = 0;
- while((ln = listFirst(server.cache_flush_queue)) != NULL) {
- dirtykey *dk = ln->value;
+ while((ln = listFirst(server.cache_io_queue)) != NULL) {
+ ioop *op = ln->value;
if (!topush) break;
topush--;
- if ((now - dk->ctime) >= server.cache_flush_delay) {
+ if (op->type == REDIS_IO_LOAD ||
+ (now - op->ctime) >= server.cache_flush_delay)
+ {
struct dictEntry *de;
robj *val;
- redisLog(REDIS_DEBUG,"Creating IO Job to save key %s",dk->key->ptr);
-
- /* Lookup the key, in order to put the current value in the IO
- * Job and mark it as DS_SAVING.
- * Otherwise if the key does not exists we schedule a disk store
- * delete operation, setting the value to NULL. */
- de = dictFind(dk->db->dict,dk->key->ptr);
- if (de) {
- val = dictGetEntryVal(de);
- redisAssert(val->storage == REDIS_DS_DIRTY);
- val->storage = REDIS_DS_SAVING;
+ redisLog(REDIS_DEBUG,"Creating IO %s Job for key %s",
+ op->type == REDIS_IO_LOAD ? "load" : "save", op->key->ptr);
+
+ if (op->type == REDIS_IO_LOAD) {
+ dsCreateIOJob(REDIS_IOJOB_LOAD,op->db,op->key,NULL);
} else {
- /* Setting the value to NULL tells the IO thread to delete
- * the key on disk. */
- val = NULL;
+ /* Lookup the key, in order to put the current value in the IO
+ * Job. Otherwise if the key does not exists we schedule a disk
+ * store delete operation, setting the value to NULL. */
+ de = dictFind(op->db->dict,op->key->ptr);
+ if (de) {
+ val = dictGetEntryVal(de);
+ } else {
+ /* Setting the value to NULL tells the IO thread to delete
+ * the key on disk. */
+ val = NULL;
+ }
+ dsCreateIOJob(REDIS_IOJOB_SAVE,op->db,op->key,val);
}
- dsCreateIOJob(REDIS_IOJOB_SAVE,dk->db,dk->key,val);
- listDelNode(server.cache_flush_queue,ln);
- decrRefCount(dk->key);
- zfree(dk);
+ /* Mark the operation as in progress. */
+ cacheScheduleIODelFlag(op->db,op->key,op->type);
+ cacheScheduleIOAddFlag(op->db,op->key,
+ (op->type == REDIS_IO_LOAD) ? REDIS_IO_LOADINPROG :
+ REDIS_IO_SAVEINPROG);
+ /* Finally remove the operation from the queue.
+ * But we'll have trace of it in the hash table. */
+ listDelNode(server.cache_io_queue,ln);
+ decrRefCount(op->key);
+ zfree(op);
} else {
break; /* too early */
}
server.cache_max_memory)
{
if (cacheFreeOneEntry() == REDIS_ERR) break;
+ /* FIXME: also free negative cache entries here. */
}
}
-/* ============ Virtual Memory - Blocking clients on missing keys =========== */
+/* ========== Disk store cache - Blocking clients on missing keys =========== */
/* This function makes the clinet 'c' waiting for the key 'key' to be loaded.
- * If the key is already in memory we don't need to block, regardless
- * of the storage of the value object for this key:
- *
- * - If it's REDIS_DS_MEMORY we have the key in memory.
- * - If it's REDIS_DS_DIRTY they key was modified, but still in memory.
- * - if it's REDIS_DS_SAVING the key is being saved by an IO Job. When
- * the client will lookup the key it will block if the key is still
- * in this stage but it's more or less the best we can do.
+ * If the key is already in memory we don't need to block.
*
* FIXME: we should try if it's actually better to suspend the client
* accessing an object that is being saved, and awake it only when
/* Are we already loading the key from disk? If not create a job */
if (de == NULL)
- dsCreateIOJob(REDIS_IOJOB_LOAD,c->db,key,NULL);
+ cacheScheduleIO(c->db,key,REDIS_IO_LOAD);
return 1;
}
projects / redis.git / blobdiff