RedisDay London 2018 - What happens when Redis runs out of memory
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RedisDay London 2018 - What happens when Redis runs out of memory
- 1. What happens when Redis runs out of memory Elena Kolevska
- 3. In computing, a cache is a hardware or software component that stores data so that future requests for that data can be served faster The Architecture of Computer Hardware and Systems Software, An Information Technology Approach ~ Irv Englander
- 4. A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. The more requests that can be served from the cache, the faster the system performs.
- 5. 5 2009 2018 March 29th First commit
- 6. Pre Redis 2.2 New data Expired data
- 7. 7 2009 2018 March 29th First commit May 2009 maxmemory implemented
- 8. Pre Redis 2.2 maxmemory + volatile-ttl
- 9. Pre Redis 2.2 maxmemory + volatile-ttl •sample 3 random volatile keys, expire the one with shortest TTL •If no volatile keys - return an error
- 10. "Why delete objects if there is no need to? And why use more memory than needed?" ~ Salvatore Sanfilippo
- 11. Why use extra bits for storing the TTL if there is no need to? ~ Salvatore Sanfilippo
- 12. 12 LRU = Least Recently Used
- 13. 13 LRU = Least Recently Used •If you’ve used an item from your cache recently - you’re more likely to use it again. •The longer you haven’t used an item, the smaller the chance you’ll need it again
- 14. 14 Since LRU is itself an approximation of what we want to achieve, how about approximating LRU itself? Salvatore Sanfilipo, Blog post
- 15. 15 Sample 3 random keys and evict the one with the highest idle time
- 16. 16 Sample X random keys and evict the one with the highest idle time maxmemory-samples
- 17. Commit: 165346ca29972817b1245e689315edeba1fe369b [165346ca] Author: antirez <antirez@gmail.com> Date: October 14, 2010 at 20:22:21 GMT+1 # MAXMEMORY POLICY: how Redis will select what to remove when maxmemory # is reached? You can select among five behaviors: # # volatile-lru -> remove the key with an expire set using an LRU algorithm # allkeys-lru -> remove any key accordingly to the LRU algorithm # volatile-random -> remove a random key with an expire set # allkeys->random -> remove a random key, any key # volatile-ttl -> remove the key with the nearest expire time (minor TTL) # # maxmemory-policy volatile-lru
- 18. 18 2009 2018 March 29th First commit May 2009 maxmemory implemented October 2010 LRU implemented v 2.2 alpha 3
- 19. About a month later the noeviction policy was added
- 20. 20 2009 2018 March 29th First commit May 2009 maxmemory implemented October 2010 LRU implemented November 2010 noeviction policy added v 2.2 alpha 3 v 2.2 alpha 5
- 21. ...if you look at this algorithm *across* its executions, you can see how we are trashing a lot of interesting data Salvatore Sanfilipo, Blog post
- 22. First rule of Fight Club is: observe your algorithms with naked eyes Salvatore Sanfilipo, Blog post
- 24. 24 LRU v2 • Use a pool of best candidates for eviction (maxmemory-samples) • All sampled keys are compared against the keys in the pool and if they're more adapt to eviction compared to a key in the pool - they switch places •After all N keys have benn compared, the best candidate is evicted from the pool
- 26. 26 2009 2018 March 29th First commit May 2009 maxmemory implemented October 2010 LRU implemented November 2010 noeviction policy added v 2.2 alpha 3 v 2.2 alpha 5 March 2014 - LRU eviction pool - maxmemory-samples = 5 - LRU field changed to 24 bits (v 2.8.8) v 3.0 Default policy noeviction v 2.8.8
- 28. 28 2009 2018 March 29th First commit May 2009 maxmemory implemented October 2010 LRU implemented November 2010 noeviction policy added July 2016 Cross-database eviction March 2014 - LRU eviction pool - maxmemory-samples = 5 - LRU field changed to 24 bits (v 2.8.8) v 3.0 Default policy noeviction v 2.8.8 v 2.2 alpha 3 v 2.2 alpha 5
- 29. r this subsystem of Redis was stimulated again at that point. I wan Salvatore Sanfilipo, Blog post
- 30. probability of being accessed in the future, that are the keys *mos Salvatore Sanfilipo, Blog post
- 31. 31 LFU = Least Frequently Used
- 32. 32 LFU = Least Frequently Used • Track access (implement access counter) • Track time (so you can know the frequency)
- 33. 33 LRU counter space <-> LFU counters space
- 34. 34 24 bits 16 bits Last decrease time 8 bits LOG C
- 35. 35 24 bits reduced-precision unix time less significant bits of the UNIX time converted to minutes Todo: show the "math" 16 bits Last decrease time 8 bits LOG C
- 36. 36 24 bits 16 bits Last decrease time 8 bits LOG C Morris (logarithmic) counter+--------+------------+------------+------------+------------+------------+ | factor | 100 hits | 1000 hits | 100K hits | 1M hits | 10M hits | +--------+------------+------------+------------+------------+------------+ | 0 | 104 | 255 | 255 | 255 | 255 | +--------+------------+------------+------------+------------+------------+ | 1 | 18 | 49 | 255 | 255 | 255 | +--------+------------+------------+------------+------------+------------+ | 10 | 10 | 18 | 142 | 255 | 255 | +--------+------------+------------+------------+------------+------------+ | 100 | 8 | 11 | 49 | 143 | 255 | +--------+------------+------------+------------+------------+------------+
- 37. 37 2009 2018 March 29th First commit May 2009 maxmemory implemented October 2010 LRU implemented November 2010 noeviction policy added July 2016 Cross-database eviction March 2014 - LRU eviction pool - maxmemory-samples = 5 - LRU field changed to 24 bits (v 2.8.8) v 3.0 Default policy noeviction v 2.8.8 July 2014 - LFU Implementation - Volatile-ttl uses the pool v 4.0 v 2.2 alpha 3 v 2.2 alpha 5
- 38. maxmemory-policy ★ noeviction ★ allkeys-random ★ volatile-random ★ volatile-ttl ★ allkeys-lru ★ volatile-lru ★ allkeys-lfu ★ volatile-lfu
- 39. Thank you!