Friday, September 30, 2011

Cacheismo Memory Allocation

Cacheismo uses two different kind of memory allocators. One is called chunkpool which is used to store the items and another is called fallocator which is used for request processing. Both have different design goals. chunkpool is designed to be reasonably fast but very compact. It tries to minimize memory wastage. fallocator on the other hand doesn't care about memory wastage but tries to be very fast. Here are the details.

fallocator is used when we are allocating memory temporarily and it is certain that it will be freed very soon. For example when reading data from socket and parsing the request, many small strings are generated but these are released as soon as the request is over, which could be a matter of micro seconds.   Cacheismo creates a fallocator per connection. fallocator not only allocates memory but also keeps track of memory. Thus if because of bad code we miss freeing some memory, it will be reclaimed when the fallocator is destroyed, essentially when the connection is closed.

fallocator internally allocates a big buffer (4KB) and keeps track of how much of the buffer is used. Malloc simply returns buffer+used at the pointer and increments used by size of malloc.  If the current buffer cannot handle the request, new buffer is allocated and memory given from new buffer. Used buffers are kept in a linked list. On free, fallocator decreases the refcount on the buffer which was used to allocate that memory. If the refcount becomes zero, the buffer is freed from the used buffers list. The benefit of this refcount based memory management is that it keeps memory usage to minimum. So if you are doing some malloc/frees in a loop, they will not exhaust the memory because refcount mechanism will ensure that they are released. If the malloc size is bigger than 4KB, fallocator defaults to malloc but keeps track of the buffer.

Moreover the 4KB buffer are cached to avoid going to malloc every time. This scheme of things give us O(1) malloc and  O(1) free in normal case (when  not going to system malloc).   It avoids fragmentation of the memory but could result in more memory being used than required at any point in time. It also gives us some protection against memory leaks.

chunkpool is used for storing the cacheitems and the goal here is to maximize memory utilization. chunkpool imposes two constrains on the application. One, it never allocates more than 4KB of contiguous memory. That is, if the application needs 8KB of memory, it can at max only get 2 4KB blocks and not a single 8KB block. Applications needs to be designed around this limitation. The second one is not a constraint, but an advice.  Apart from malloc, chunkpool exposes another function called relaxedMalloc which can return memory smaller than what application asked for. Thus if application asks for 4KB of memory, it might return 256 bytes or 1KB if it doesn't have a 4KB block. If the returned memory is too small, application can free it and deal with out of memory condition or if possible use large number of small buffers instead of small number of large buffers.

Because of these restrictions, fragmentation of memory doesn't makes the system unusable, but only adds  more memory management overhead. Internally we have slabs of size 16 + n16 upto 4096 bytes. Initially all memory is allocated to the 4KB slab. If applications asks for say 1KB of memory we split the 4Kb buffer into two buffer on of 3Kb and another of 1Kb. The 1 KB buffer goes to the application and the 3KB is stored in the 3Kb slab. Cacheismo chunkpool uses skiplist to keep track of non empty slabs. At runtime if we found that the slab of asked malloc size is empty, it quickly find the next not empty slab, splits the buffer and returns memory to the application. On free, chunkpool tries to merge the free buffers to create bigger buffers.  One limitation that I will fix later is that chunkpool memory uses the first four bytes for memory management. The implication is that any free buffer can only merge with logical next buffer and not logical previous buffer. This would require storing buffer size also at the end of the buffer and not just at the start.  To circumvent this problem, chunkpool requires an extra GC call where it tries to merge 8MB of pages at a time.  This is called once every second.  This is triggered only if at least 12% of memory is free and average chunk size is less than 256 bytes other wise it is a no-op. Even though chunkpool is capable of addressing upto 64GB of memory, it is advisable not to go beyond say 8GB.

chunkpool is a very efficient allocation manager for cache data, when compared with slab allocator.  It saves user the trouble of first thinking about what slab sizes should be used and maintains high HIT Rate when object sizes are changed because of changes in application code.  Cacheismo is not faster than memcached when comparing raw TPS, but when you take HIT Rate into account, it saves you lots of db accesses and increases overall application performance.  You can download it from here.

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