Saturday, September 24, 2011

Introducing Cacheismo

Cacheismo is scriptable object cache which can be used as a replacement for memcached. It supports memcache protocol (tcp and ascii only). It automatically manages slab sizes, no user configuration is required.  Slabs are intelligently managed and they reconfigure themselves as per  the usage. PUT never ever fails. Details follow:
- Efficient use of memory 
Slab allocator used in memcached would waste memory if slab sizes and object sizes are not in sync. For example when using slab sizes of 64/128/256/512/1024 bytes object with size 513 is stored in 1024 bytes slab, wasting almost half the memory. Cacheismo tries to  maintain memory used by object as close as possible to the object size
- Reclaimable Memory 
Every byte is reclaimable in cacheismo. PUT never fails. You will never have slab full issues. More on memory management.
- True LRU support. Dependable cache
Memcached will evict items from the slab in which object will be placed. Cacheismo will always use the LRU, irrespective of the size. This ensures that the most important data is always in the cache. No surprises. If something is not in the cache, it either expired or was least used.
- Server Side Scriptable objects
The best part of using cacheismo is that is it fully scriptable in lua. Instead of cluttering your code with workarounds for  memcached read/write semantics, you can move that logic inside  cacheismo.  See Use cases for cacheismo Rate Limiting Quota With Cacheismo Using Cacheismo Cluster For Real-time Analytics
Sample objects map, set, quota and sliding window counters written in lua can be found in scripts directory. You could create  your own objects and place them in the scripts directory. The interface for accessing scriptable objects is implemented via memcached get requests. For example:  get set:new:mykey   would create a new set object refered via myKey get set:put:myKey:a   would put key a in the set myKey get set:count:myKey   would return number of elements in the set get set:union:myKey1:myKey2   would return union of sets myKey1 and myKey2 See scripts/set.lua for other functions. I call this approach virtual keys. Reason for using it ... You can use existing memcached libraries to access cacheismo. Building - cacheismo depends on libevent and lua (luagit can  also be used). Complete implementation is in about 5000 lines  of code and fairly organised. 
Blue line is (memcachedTPS-cacheismoTPS)/memcachedTPS. This essentially measures how much faster is memcached compared to cacheismo. As you can see at max it is 20% faster.
Black line is (cacheismoHitRATE-memcachedHitRATE)/cacheismoHitRATE. This essentially means how much memory efficient is cacheismo compared to memcached. For the first half of the test case we don't see any difference. But in the second half cacheismo is at max about 80% efficient. The reason for this behavior is that memcached can't reassign slabs once they are used whereas cacheismo continues to adjust with changing object sizes. Over time memcached commits memory to some slabs and hence its hit rate decreases over time.
Get source from GitHub
Google Group
UPDATE: Cacheismo Next Steps

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