X-Git-Url: https://git.saurik.com/redis.git/blobdiff_plain/aa81e4d5f4b4d942b9340fd2d401e7795fa9fd1e..d158dc28f6617fd517172e04d3f46ce53d88013e:/src/dscache.c diff --git a/src/dscache.c b/src/dscache.c index 5551420a..6bfcac1d 100644 --- a/src/dscache.c +++ b/src/dscache.c @@ -19,76 +19,33 @@ /* 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 @@ -110,7 +67,12 @@ * 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: @@ -214,17 +176,18 @@ int cacheFreeOneEntry(void) { 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; } @@ -237,10 +200,10 @@ int cacheFreeOneEntry(void) { } } 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); @@ -270,63 +233,63 @@ int dsCanTouchDiskStore(void) { * 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; } } @@ -379,37 +342,18 @@ void vmThreadedIOCompletedJob(aeEventLoop *el, int fd, void *privdata, 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++; @@ -467,7 +411,6 @@ void *IOThreadEntryPoint(void *arg) { 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); @@ -588,27 +531,109 @@ void dsCreateIOJob(int type, redisDb *db, robj *key, robj *val) { 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) { @@ -623,37 +648,67 @@ 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 */ } @@ -663,21 +718,18 @@ void cacheCron(void) { 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 @@ -722,7 +774,7 @@ int waitForSwappedKey(redisClient *c, robj *key) { /* 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; }