/* Maxmemory directive handling (LRU eviction and other policies).
*
* ----------------------------------------------------------------------------
*
* Copyright (c) 2009-2016, Salvatore Sanfilippo <antirez at gmail dot com>
* All rights reserved.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions are met:
*
* * Redistributions of source code must retain the above copyright notice,
* this list of conditions and the following disclaimer.
* * Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* * Neither the name of Redis nor the names of its contributors may be used
* to endorse or promote products derived from this software without
* specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
* POSSIBILITY OF SUCH DAMAGE.
*/
#include "server.h"
#include "bio.h"
#include "atomicvar.h"
/* ----------------------------------------------------------------------------
* Data structures
* --------------------------------------------------------------------------*/
/* To improve the quality of the LRU approximation we take a set of keys
* that are good candidate for eviction across freeMemoryIfNeeded() calls.
*
* Entries inside the eviciton pool are taken ordered by idle time, putting
* greater idle times to the right (ascending order).
*
* When an LFU policy is used instead, a reverse frequency indication is used
* instead of the idle time, so that we still evict by larger value (larger
* inverse frequency means to evict keys with the least frequent accesses).
*
* Empty entries have the key pointer set to NULL. */
#define EVPOOL_SIZE 16
#define EVPOOL_CACHED_SDS_SIZE 255
struct evictionPoolEntry {
unsigned long long idle; /* Object idle time (inverse frequency for LFU) */
sds key; /* Key name. */
sds cached; /* Cached SDS object for key name. */
int dbid; /* Key DB number. */
};
static struct evictionPoolEntry *EvictionPoolLRU;
/* ----------------------------------------------------------------------------
* Implementation of eviction, aging and LRU
* --------------------------------------------------------------------------*/
/* Return the LRU clock, based on the clock resolution. This is a time
* in a reduced-bits format that can be used to set and check the
* object->lru field of redisObject structures. */
unsigned int getLRUClock(void) {
return (mstime()/LRU_CLOCK_RESOLUTION) & LRU_CLOCK_MAX;
}
/* This function is used to obtain the current LRU clock.
* If the current resolution is lower than the frequency we refresh the
* LRU clock (as it should be in production servers) we return the
* precomputed value, otherwise we need to resort to a system call. */
unsigned int LRU_CLOCK(void) {
unsigned int lruclock;
if (1000/server.hz <= LRU_CLOCK_RESOLUTION) {
lruclock = server.lruclock;
} else {
lruclock = getLRUClock();
}
return lruclock;
}
/* Given an object returns the min number of milliseconds the object was never
* requested, using an approximated LRU algorithm. */
unsigned long long estimateObjectIdleTime(robj *o) {
unsigned long long lruclock = LRU_CLOCK();
if (lruclock >= o->lru) {
return (lruclock - o->lru) * LRU_CLOCK_RESOLUTION;
} else {
return (lruclock + (LRU_CLOCK_MAX - o->lru)) *
LRU_CLOCK_RESOLUTION;
}
}
/* freeMemoryIfNeeded() gets called when 'maxmemory' is set on the config
* file to limit the max memory used by the server, before processing a
* command.
*
* The goal of the function is to free enough memory to keep Redis under the
* configured memory limit.
*
* The function starts calculating how many bytes should be freed to keep
* Redis under the limit, and enters a loop selecting the best keys to
* evict accordingly to the configured policy.
*
* If all the bytes needed to return back under the limit were freed the
* function returns C_OK, otherwise C_ERR is returned, and the caller
* should block the execution of commands that will result in more memory
* used by the server.
*
* ------------------------------------------------------------------------
*
* LRU approximation algorithm
*
* Redis uses an approximation of the LRU algorithm that runs in constant
* memory. Every time there is a key to expire, we sample N keys (with
* N very small, usually in around 5) to populate a pool of best keys to
* evict of M keys (the pool size is defined by EVPOOL_SIZE).
*
* The N keys sampled are added in the pool of good keys to expire (the one
* with an old access time) if they are better than one of the current keys
* in the pool.
*
* After the pool is populated, the best key we have in the pool is expired.
* However note that we don't remove keys from the pool when they are deleted
* so the pool may contain keys that no longer exist.
*
* When we try to evict a key, and all the entries in the pool don't exist
* we populate it again. This time we'll be sure that the pool has at least
* one key that can be evicted, if there is at least one key that can be
* evicted in the whole database. */
/* Create a new eviction pool. */
void evictionPoolAlloc(void) {
struct evictionPoolEntry *ep;
int j;
ep = zmalloc(sizeof(*ep)*EVPOOL_SIZE);
for (j = 0; j < EVPOOL_SIZE; j++) {
ep[j].idle = 0;
ep[j].key = NULL;
ep[j].cached = sdsnewlen(NULL,EVPOOL_CACHED_SDS_SIZE);
ep[j].dbid = 0;
}
EvictionPoolLRU = ep;
}
/* This is an helper function for freeMemoryIfNeeded(), it is used in order
* to populate the evictionPool with a few entries every time we want to
* expire a key. Keys with idle time smaller than one of the current
* keys are added. Keys are always added if there are free entries.
*
* We insert keys on place in ascending order, so keys with the smaller
* idle time are on the left, and keys with the higher idle time on the
* right. */
void evictionPoolPopulate(int dbid, dict *sampledict, dict *keydict, struct evictionPoolEntry *pool) {
int j, k, count;
dictEntry *samples[server.maxmemory_samples];
count = dictGetSomeKeys(sampledict,samples,server.maxmemory_samples);
for (j = 0; j < count; j++) {
unsigned long long idle;
sds key;
robj *o;
dictEntry *de;
de = samples[j];
key = dictGetKey(de);
/* If the dictionary we are sampling from is not the main
* dictionary (but the expires one) we need to lookup the key
* again in the key dictionary to obtain the value object. */
if (server.maxmemory_policy != MAXMEMORY_VOLATILE_TTL) {
if (sampledict != keydict) de = dictFind(keydict, key);
o = dictGetVal(de);
}
/* Calculate the idle time according to the policy. This is called
* idle just because the code initially handled LRU, but is in fact
* just a score where an higher score means better candidate. */
if (server.maxmemory_policy & MAXMEMORY_FLAG_LRU) {
idle = estimateObjectIdleTime(o);
} else if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
/* When we use an LRU policy, we sort the keys by idle time
* so that we expire keys starting from greater idle time.
* However when the policy is an LFU one, we have a frequency
* estimation, and we want to evict keys with lower frequency
* first. So inside the pool we put objects using the inverted
* frequency subtracting the actual frequency to the maximum
* frequency of 255. */
idle = 255-LFUDecrAndReturn(o);
} else if (server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL) {
/* In this case the sooner the expire the better. */
idle = ULLONG_MAX - (long)dictGetVal(de);
} else {
serverPanic("Unknown eviction policy in evictionPoolPopulate()");
}
/* Insert the element inside the pool.
* First, find the first empty bucket or the first populated
* bucket that has an idle time smaller than our idle time. */
k = 0;
while (k < EVPOOL_SIZE &&
pool[k].key &&
pool[k].idle < idle) k++;
if (k == 0 && pool[EVPOOL_SIZE-1].key != NULL) {
/* Can't insert if the element is < the worst element we have
* and there are no empty buckets. */
continue;
} else if (k < EVPOOL_SIZE && pool[k].key == NULL) {
/* Inserting into empty position. No setup needed before insert. */
} else {
/* Inserting in the middle. Now k points to the first element
* greater than the element to insert. */
if (pool[EVPOOL_SIZE-1].key == NULL) {
/* Free space on the right? Insert at k shifting
* all the elements from k to end to the right. */
/* Save SDS before overwriting. */
sds cached = pool[EVPOOL_SIZE-1].cached;
memmove(pool+k+1,pool+k,
sizeof(pool[0])*(EVPOOL_SIZE-k-1));
pool[k].cached = cached;
} else {
/* No free space on right? Insert at k-1 */
k--;
/* Shift all elements on the left of k (included) to the
* left, so we discard the element with smaller idle time. */
sds cached = pool[0].cached; /* Save SDS before overwriting. */
if (pool[0].key != pool[0].cached) sdsfree(pool[0].key);
memmove(pool,pool+1,sizeof(pool[0])*k);
pool[k].cached = cached;
}
}
/* Try to reuse the cached SDS string allocated in the pool entry,
* because allocating and deallocating this object is costly
* (according to the profiler, not my fantasy. Remember:
* premature optimizbla bla bla bla. */
int klen = sdslen(key);
if (klen > EVPOOL_CACHED_SDS_SIZE) {
pool[k].key = sdsdup(key);
} else {
memcpy(pool[k].cached,key,klen+1);
sdssetlen(pool[k].cached,klen);
pool[k].key = pool[k].cached;
}
pool[k].idle = idle;
pool[k].dbid = dbid;
}
}
/* ----------------------------------------------------------------------------
* LFU (Least Frequently Used) implementation.
* We have 24 total bits of space in each object in order to implement
* an LFU (Least Frequently Used) eviction policy, since we re-use the
* LRU field for this purpose.
*
* We split the 24 bits into two fields:
*
* 16 bits 8 bits
* +----------------+--------+
* + Last decr time | LOG_C |
* +----------------+--------+
*
* LOG_C is a logarithmic counter that provides an indication of the access
* frequency. However this field must also be decremented otherwise what used
* to be a frequently accessed key in the past, will remain ranked like that
* forever, while we want the algorithm to adapt to access pattern changes.
*
* So the remaining 16 bits are used in order to store the "decrement time",
* a reduced-precision Unix time (we take 16 bits of the time converted
* in minutes since we don't care about wrapping around) where the LOG_C
* counter is halved if it has an high value, or just decremented if it
* has a low value.
*
* New keys don't start at zero, in order to have the ability to collect
* some accesses before being trashed away, so they start at COUNTER_INIT_VAL.
* The logarithmic increment performed on LOG_C takes care of COUNTER_INIT_VAL
* when incrementing the key, so that keys starting at COUNTER_INIT_VAL
* (or having a smaller value) have a very high chance of being incremented
* on access.
*
* During decrement, the value of the logarithmic counter is halved if
* its current value is greater than two times the COUNTER_INIT_VAL, otherwise
* it is just decremented by one.
* --------------------------------------------------------------------------*/
/* Return the current time in minutes, just taking the least significant
* 16 bits. The returned time is suitable to be stored as LDT (last decrement
* time) for the LFU implementation. */
unsigned long LFUGetTimeInMinutes(void) {
return (server.unixtime/60) & 65535;
}
/* Given an object last access time, compute the minimum number of minutes
* that elapsed since the last access. Handle overflow (ldt greater than
* the current 16 bits minutes time) considering the time as wrapping
* exactly once. */
unsigned long LFUTimeElapsed(unsigned long ldt) {
unsigned long now = LFUGetTimeInMinutes();
if (now >= ldt) return now-ldt;
return 65535-ldt+now;
}
/* Logarithmically increment a counter. The greater is the current counter value
* the less likely is that it gets really implemented. Saturate it at 255. */
uint8_t LFULogIncr(uint8_t counter) {
if (counter == 255) return 255;
double r = (double)rand()/RAND_MAX;
double baseval = counter - LFU_INIT_VAL;
if (baseval < 0) baseval = 0;
double p = 1.0/(baseval*server.lfu_log_factor+1);
if (r < p) counter++;
return counter;
}
/* If the object decrement time is reached decrement the LFU counter but
* do not update LFU fields of the object, we update the access time
* and counter in an explicit way when the object is really accessed.
* And we will times halve the counter according to the times of
* elapsed time than server.lfu_decay_time.
* Return the object frequency counter.
*
* This function is used in order to scan the dataset for the best object
* to fit: as we check for the candidate, we incrementally decrement the
* counter of the scanned objects if needed. */
unsigned long LFUDecrAndReturn(robj *o) {
unsigned long ldt = o->lru >> 8;
unsigned long counter = o->lru & 255;
unsigned long num_periods = server.lfu_decay_time ? LFUTimeElapsed(ldt) / server.lfu_decay_time : 0;
if (num_periods)
counter = (num_periods > counter) ? 0 : counter - num_periods;
return counter;
}
/* ----------------------------------------------------------------------------
* The external API for eviction: freeMemroyIfNeeded() is called by the
* server when there is data to add in order to make space if needed.
* --------------------------------------------------------------------------*/
/* We don't want to count AOF buffers and slaves output buffers as
* used memory: the eviction should use mostly data size. This function
* returns the sum of AOF and slaves buffer. */
size_t freeMemoryGetNotCountedMemory(void) {
size_t overhead = 0;
int slaves = listLength(server.slaves);
if (slaves) {
listIter li;
listNode *ln;
listRewind(server.slaves,&li);
while((ln = listNext(&li))) {
client *slave = listNodeValue(ln);
overhead += getClientOutputBufferMemoryUsage(slave);
}
}
if (server.aof_state != AOF_OFF) {
overhead += sdsalloc(server.aof_buf)+aofRewriteBufferSize();
}
return overhead;
}
/* Get the memory status from the point of view of the maxmemory directive:
* if the memory used is under the maxmemory setting then C_OK is returned.
* Otherwise, if we are over the memory limit, the function returns
* C_ERR.
*
* The function may return additional info via reference, only if the
* pointers to the respective arguments is not NULL. Certain fields are
* populated only when C_ERR is returned:
*
* 'total' total amount of bytes used.
* (Populated both for C_ERR and C_OK)
*
* 'logical' the amount of memory used minus the slaves/AOF buffers.
* (Populated when C_ERR is returned)
*
* 'tofree' the amount of memory that should be released
* in order to return back into the memory limits.
* (Populated when C_ERR is returned)
*
* 'level' this usually ranges from 0 to 1, and reports the amount of
* memory currently used. May be > 1 if we are over the memory
* limit.
* (Populated both for C_ERR and C_OK)
*/
int getMaxmemoryState(size_t *total, size_t *logical, size_t *tofree, float *level) {
size_t mem_reported, mem_used, mem_tofree;
/* Check if we are over the memory usage limit. If we are not, no need
* to subtract the slaves output buffers. We can just return ASAP. */
mem_reported = zmalloc_used_memory();
if (total) *total = mem_reported;
/* We may return ASAP if there is no need to compute the level. */
int return_ok_asap = !server.maxmemory || mem_reported <= server.maxmemory;
if (return_ok_asap && !level) return C_OK;
/* Remove the size of slaves output buffers and AOF buffer from the
* count of used memory. */
mem_used = mem_reported;
size_t overhead = freeMemoryGetNotCountedMemory();
mem_used = (mem_used > overhead) ? mem_used-overhead : 0;
/* Compute the ratio of memory usage. */
if (level) {
if (!server.maxmemory) {
*level = 0;
} else {
*level = (float)mem_used / (float)server.maxmemory;
}
}
if (return_ok_asap) return C_OK;
/* Check if we are still over the memory limit. */
if (mem_used <= server.maxmemory) return C_OK;
/* Compute how much memory we need to free. */
mem_tofree = mem_used - server.maxmemory;
if (logical) *logical = mem_used;
if (tofree) *tofree = mem_tofree;
return C_ERR;
}
/* This function is periodically called to see if there is memory to free
* according to the current "maxmemory" settings. In case we are over the
* memory limit, the function will try to free some memory to return back
* under the limit.
*
* The function returns C_OK if we are under the memory limit or if we
* were over the limit, but the attempt to free memory was successful.
* Otehrwise if we are over the memory limit, but not enough memory
* was freed to return back under the limit, the function returns C_ERR. */
int freeMemoryIfNeeded(void) {
/* By default replicas should ignore maxmemory
* and just be masters exact copies. */
if (server.masterhost && server.repl_slave_ignore_maxmemory) return C_OK;
size_t mem_reported, mem_tofree, mem_freed;
mstime_t latency, eviction_latency;
long long delta;
int slaves = listLength(server.slaves);
/* When clients are paused the dataset should be static not just from the
* POV of clients not being able to write, but also from the POV of
* expires and evictions of keys not being performed. */
if (clientsArePaused()) return C_OK;
if (getMaxmemoryState(&mem_reported,NULL,&mem_tofree,NULL) == C_OK)
return C_OK;
mem_freed = 0;
if (server.maxmemory_policy == MAXMEMORY_NO_EVICTION)
goto cant_free; /* We need to free memory, but policy forbids. */
latencyStartMonitor(latency);
while (mem_freed < mem_tofree) {
int j, k, i, keys_freed = 0;
static unsigned int next_db = 0;
sds bestkey = NULL;
int bestdbid;
redisDb *db;
dict *dict;
dictEntry *de;
if (server.maxmemory_policy & (MAXMEMORY_FLAG_LRU|MAXMEMORY_FLAG_LFU) ||
server.maxmemory_policy == MAXMEMORY_VOLATILE_TTL)
{
struct evictionPoolEntry *pool = EvictionPoolLRU;
while(bestkey == NULL) {
unsigned long total_keys = 0, keys;
/* We don't want to make local-db choices when expiring keys,
* so to start populate the eviction pool sampling keys from
* every DB. */
for (i = 0; i < server.dbnum; i++) {
db = server.db+i;
dict = (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) ?
db->dict : db->expires;
if ((keys = dictSize(dict)) != 0) {
evictionPoolPopulate(i, dict, db->dict, pool);
total_keys += keys;
}
}
if (!total_keys) break; /* No keys to evict. */
/* Go backward from best to worst element to evict. */
for (k = EVPOOL_SIZE-1; k >= 0; k--) {
if (pool[k].key == NULL) continue;
bestdbid = pool[k].dbid;
if (server.maxmemory_policy & MAXMEMORY_FLAG_ALLKEYS) {
de = dictFind(server.db[pool[k].dbid].dict,
pool[k].key);
} else {
de = dictFind(server.db[pool[k].dbid].expires,
pool[k].key);
}
/* Remove the entry from the pool. */
if (pool[k].key != pool[k].cached)
sdsfree(pool[k].key);
pool[k].key = NULL;
pool[k].idle = 0;
/* If the key exists, is our pick. Otherwise it is
* a ghost and we need to try the next element. */
if (de) {
bestkey = dictGetKey(de);
break;
} else {
/* Ghost... Iterate again. */
}
}
}
}
/* volatile-random and allkeys-random policy */
else if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM ||
server.maxmemory_policy == MAXMEMORY_VOLATILE_RANDOM)
{
/* When evicting a random key, we try to evict a key for
* each DB, so we use the static 'next_db' variable to
* incrementally visit all DBs. */
for (i = 0; i < server.dbnum; i++) {
j = (++next_db) % server.dbnum;
db = server.db+j;
dict = (server.maxmemory_policy == MAXMEMORY_ALLKEYS_RANDOM) ?
db->dict : db->expires;
if (dictSize(dict) != 0) {
de = dictGetRandomKey(dict);
bestkey = dictGetKey(de);
bestdbid = j;
break;
}
}
}
/* Finally remove the selected key. */
if (bestkey) {
db = server.db+bestdbid;
robj *keyobj = createStringObject(bestkey,sdslen(bestkey));
propagateExpire(db,keyobj,server.lazyfree_lazy_eviction);
/* We compute the amount of memory freed by db*Delete() alone.
* It is possible that actually the memory needed to propagate
* the DEL in AOF and replication link is greater than the one
* we are freeing removing the key, but we can't account for
* that otherwise we would never exit the loop.
*
* AOF and Output buffer memory will be freed eventually so
* we only care about memory used by the key space. */
delta = (long long) zmalloc_used_memory();
latencyStartMonitor(eviction_latency);
if (server.lazyfree_lazy_eviction)
dbAsyncDelete(db,keyobj);
else
dbSyncDelete(db,keyobj);
latencyEndMonitor(eviction_latency);
latencyAddSampleIfNeeded("eviction-del",eviction_latency);
latencyRemoveNestedEvent(latency,eviction_latency);
delta -= (long long) zmalloc_used_memory();
mem_freed += delta;
server.stat_evictedkeys++;
notifyKeyspaceEvent(NOTIFY_EVICTED, "evicted",
keyobj, db->id);
decrRefCount(keyobj);
keys_freed++;
/* When the memory to free starts to be big enough, we may
* start spending so much time here that is impossible to
* deliver data to the slaves fast enough, so we force the
* transmission here inside the loop. */
if (slaves) flushSlavesOutputBuffers();
/* Normally our stop condition is the ability to release
* a fixed, pre-computed amount of memory. However when we
* are deleting objects in another thread, it's better to
* check, from time to time, if we already reached our target
* memory, since the "mem_freed" amount is computed only
* across the dbAsyncDelete() call, while the thread can
* release the memory all the time. */
if (server.lazyfree_lazy_eviction && !(keys_freed % 16)) {
if (getMaxmemoryState(NULL,NULL,NULL,NULL) == C_OK) {
/* Let's satisfy our stop condition. */
mem_freed = mem_tofree;
}
}
}
if (!keys_freed) {
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("eviction-cycle",latency);
goto cant_free; /* nothing to free... */
}
}
latencyEndMonitor(latency);
latencyAddSampleIfNeeded("eviction-cycle",latency);
return C_OK;
cant_free:
/* We are here if we are not able to reclaim memory. There is only one
* last thing we can try: check if the lazyfree thread has jobs in queue
* and wait... */
while(bioPendingJobsOfType(BIO_LAZY_FREE)) {
if (((mem_reported - zmalloc_used_memory()) + mem_freed) >= mem_tofree)
break;
usleep(1000);
}
return C_ERR;
}
/* This is a wrapper for freeMemoryIfNeeded() that only really calls the
* function if right now there are the conditions to do so safely:
*
* - There must be no script in timeout condition.
* - Nor we are loading data right now.
*
*/
int freeMemoryIfNeededAndSafe(void) {
if (server.lua_timedout || server.loading) return C_OK;
return freeMemoryIfNeeded();
}