On Wed, Dec 9, 2015 at 11:31 AM, Konstantin Knizhnik <k.knizh...@postgrespro.ru> wrote: > I expect synchronization issues with implementation of this algorithm. It > seems to be hard to avoid some global critical section which can cause > significant performance degradation at MPP systems (see topic "Move > PinBuffer and UnpinBuffer to atomics").
The sketch updates don't really need to be globally consistent, anything that is faster than cycling of buffers through the LRU tier would be enough. In principle atomic increments could be used, but funneling all buffer hits onto a small amount of cachelines and then doing 4x the amount of writes would result in extremely nasty cache line ping-ponging. Caffeine implementation appears to solve this by batching the frequency sketch updates using a striped set of circular buffers. Re-implementing this is not exactly trivial and some prior experience with lock free programming seems well advised. Based on the paper it looks like this algorithm can work with a low quality primary eviction algorithm. Swapping our GCLOCK out for something simpler like plain CLOCK could simplify an atomics based PinBuffer approach. Another interesting avenue for research would be to use ideas in TinyLFU to implement a tier of "nailed" buffers that have backend local or striped pin-unpin accounting. But for checking if the replacement policy implemented by W-TinyLFU is good it isn't necessary to have a performant locking solution. I think a global lock would be good enough for a proof of concept that only evaluates cache hit ratios. Regards, Ants Aasma -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
