/* TODO:
*
- * - The WATCH helper will be used to signal the cache system
- * we need to flush a given key/dbid into disk, adding this key/dbid
- * pair into a server.ds_cache_dirty linked list AND hash table (so that we
- * don't add the same thing multiple times).
- *
- * - cron() checks if there are elements on this list. When there are things
- * to flush, we create an IO Job for the I/O thread.
- * NOTE: We disalbe object sharing when server.ds_enabled == 1 so objects
- * that are referenced an IO job for flushing on disk are marked as
- * o->storage == REDIS_DS_SAVING.
- *
- * - This is what we do on key lookup:
- * 1) The key already exists in memory. object->storage == REDIS_DS_MEMORY
- * or it is object->storage == REDIS_DS_DIRTY:
- * We don't do nothing special, lookup, return value object pointer.
- * 2) The key is in memory but object->storage == REDIS_DS_SAVING.
- * When this happens we block waiting for the I/O thread to process
- * this object. Then continue.
- * 3) The key is not in memory. We block to load the key from disk.
- * Of course the key may not be present at all on the disk store as well,
- * in such case we just detect this condition and continue, returning
- * NULL from lookup.
- *
- * - Preloading of needed keys:
- * 1) As it was done with VM, also with this new system we try preloading
- * keys a client is going to use. We block the client, load keys
- * using the I/O thread, unblock the client. Same code as VM more or less.
- *
- * - Reclaiming memory.
- * In cron() we detect our memory limit was reached. What we
- * do is deleting keys that are REDIS_DS_MEMORY, using LRU.
- *
- * If this is not enough to return again under the memory limits we also
- * start to flush keys that need to be synched on disk synchronously,
- * removing it from the memory. We do this blocking as memory limit is a
- * much "harder" barrirer in the new design.
- *
- * - IO thread operations are no longer stopped for sync loading/saving of
- * things. When a key is found to be in the process of being saved
- * we simply wait for the IO thread to end its work.
- *
- * Otherwise if there is to load a key without any IO thread operation
- * just started it is blocking-loaded in the lookup function.
+ * WARNING: most of the following todo items and design issues are no
+ * longer relevant with the new design. Here as a checklist to see if
+ * some old ideas still apply.
*
* - What happens when an object is destroyed?
*
- * If o->storage == REDIS_DS_MEMORY then we simply destory the object.
- * If o->storage == REDIS_DS_DIRTY we can still remove the object. It had
- * changes not flushed on disk, but is being removed so
- * who cares.
- * if o->storage == REDIS_DS_SAVING then the object is being saved so
- * it is impossible that its refcount == 1, must be at
- * least two. When the object is saved the storage will
- * be set back to DS_MEMORY.
+ * If the object is destroyed since semantically it was deleted or
+ * replaced with something new, we don't care if there was a SAVE
+ * job pending for it. Anyway when the IO JOb will be created we'll get
+ * the pointer of the current value.
*
- * - What happens when keys are deleted?
- *
- * We simply schedule a key flush operation as usually, but when the
- * IO thread will be created the object pointer will be set to NULL
- * so the IO thread will know that the work to do is to delete the key
- * from the disk store.
+ * If the object is already a REDIS_IO_SAVEINPROG object, then it is
+ * impossible that we get a decrRefCount() that will reach refcount of zero
+ * since the object is both in the dataset and in the io job entry.
*
* - What happens with MULTI/EXEC?
*
- * Good question.
+ * Good question. Without some kind of versioning with a global counter
+ * it is not possible to have trasactions on disk, but they are still
+ * useful since from the point of view of memory and client bugs it is
+ * a protection anyway. Also it's useful for WATCH.
*
- * - If dsSet() fails on the write thread log the error and reschedule the
- * key for flush.
+ * Btw there is to check what happens when WATCH gets combined to keys
+ * that gets removed from the object cache. Should be save but better
+ * to check.
*
- * - Check why INCR will not update the LRU info for the object.
+ * - Check if/why INCR will not update the LRU info for the object.
*
* - Fix/Check the following race condition: a key gets a DEL so there is
* a write operation scheduled against this key. Later the same key will
* not marked as cacheKeyDoesNotExist(), otherwise, again, we can load
* data from disk that should instead be deleted.
*
- * - dsSet() use rename(2) in order to avoid corruptions.
+ * - dsSet() should use rename(2) in order to avoid corruptions.
+ *
+ * - Don't add a LOAD if there is already a LOADINPROGRESS, or is this
+ * impossible since anyway the io_keys stuff will work as lock?
+ *
+ * - Serialize special encoded things in a raw form.
*/
/* Virtual Memory is composed mainly of two subsystems:
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;
}
}
}
if (best == NULL) {
- /* FIXME: If there are objects marked as DS_DIRTY or DS_SAVING
- * let's wait for this objects to be clear and retry...
- *
- * Object cache vm limit is considered an hard limit. */
+ /* FIXME: If there are objects that are in the write queue
+ * so we can't delete them we should block here, at the cost of
+ * slowness as the object cache memory limit is considered
+ * n hard limit. */
return REDIS_ERR;
}
key = dictGetEntryKey(best);
* 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.
+ * 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.
*
- * 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.
- *
- * 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);
+/* Remove one entry from negative cache using approximated LRU. */
+int negativeCacheEvictOneEntry(void) {
+ struct dictEntry *de;
+ robj *best = NULL;
+ redisDb *best_db = NULL;
+ time_t time, best_time = 0;
+ int j;
+
+ for (j = 0; j < server.dbnum; j++) {
+ redisDb *db = server.db+j;
+ int i;
+
+ if (dictSize(db->io_negcache) == 0) continue;
+ for (i = 0; i < 3; i++) {
+ de = dictGetRandomKey(db->io_negcache);
+ time = (time_t) dictGetEntryVal(de);
+
+ if (best == NULL || time < best_time) {
+ best = dictGetEntryKey(de);
+ best_db = db;
+ best_time = time;
+ }
+ }
+ }
+ if (best) {
+ dictDelete(best_db->io_negcache,best);
+ return REDIS_OK;
+ } else {
+ return REDIS_ERR;
}
}
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);
}
- } else {
- /* The key does not exist. Create a negative cache entry
- * 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);
- }
+ 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);
+
+ if (flags & flag) {
+ redisLog(REDIS_WARNING,"Adding the same flag again: was: %ld, addede: %ld",flags,flag);
+ redisAssert(!(flags & flag));
+ }
+ flags |= flag;
+ dictGetEntryVal(de) = (void*) flags;
}
+}
+
+void cacheScheduleIODelFlag(redisDb *db, robj *key, long flag) {
+ struct dictEntry *de = dictFind(db->io_queued,key);
+ long 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;
+ 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;
+ }
+}
+
+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;
+ if (topush > (signed)listLength(server.cache_io_queue))
+ topush = listLength(server.cache_io_queue);
- 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;
+ /* Don't add a SAVE job in queue if there is already
+ * a save in progress for the same key. */
+ if (op->type == REDIS_IO_SAVE &&
+ cacheScheduleIOGetFlags(op->db,op->key) & REDIS_IO_SAVEINPROG)
+ {
+ /* Move the operation at the end of the list of there
+ * are other operations. Otherwise break, nothing to do
+ * here. */
+ if (listLength(server.cache_io_queue) > 1) {
+ listDelNode(server.cache_io_queue,ln);
+ listAddNodeTail(server.cache_io_queue,op);
+ continue;
+ } else {
+ break;
+ }
+ }
+
+ 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 */
}
while (server.ds_enabled && zmalloc_used_memory() >
server.cache_max_memory)
{
- if (cacheFreeOneEntry() == REDIS_ERR) break;
+ int done = 0;
+
+ if (cacheFreeOneEntry() == REDIS_OK) done++;
+ if (negativeCacheEvictOneEntry() == REDIS_OK) done++;
+ if (done == 0) break; /* nothing more to free */
}
}
-/* ============ 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