On 2014-02-18 19:12:32 +0200, Heikki Linnakangas wrote: > You're missing MauMau's point. In essence, he's comparing two systems with > the same number of clients, issuing queries as fast as they can, and one can > do 2000 TPS while the other one can do 10000 TPS. You would expect the > lower-throughput system to have a *higher* average latency. > Each query takes > longer, that's why the throughput is lower. If you look at the avg_latency > columns in the graphs (http://hlinnaka.iki.fi/xloginsert-scaling/padding/), > that's exactly what you see. > > But what MauMau is pointing out is that the *max* latency is much higher in > the system that can do 10000 TPS. So some queries are taking much longer, > even though in average the latency is lower. In an ideal, totally fair > system, each query would take the same amount of time to execute, and after > it's saturated, increasing the number of clients just makes that constant > latency higher.
Consider me being enthusiastically unenthusiastic about that fact. The change in throughput still makes this pretty uninteresting. There's so many things that are influenced by a factor 5 increase in throughput, that a change in max latency is really not saying much. There's also the point that with 5 times the throughput it's getting more likely to sleep while holding critical locks and such. > Yeah, I'm pretty sure that's because of the extra checkpoints. If you look > at the individual test graphs, there are clear spikes in latency, but the > latency is otherwise small. With a higher TPS, you reach checkpoint_segments > quicker; I should've eliminated that effect in the tests I ran... I don't think that'd be a good idea. The number of full page writes so greatly influences the WAL charactersistics, that changing checkpoint segments would make the tests much harder to compare. Greetings, Andres Freund -- Andres Freund http://www.2ndQuadrant.com/ PostgreSQL Development, 24x7 Support, Training & Services -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
