On Tue, Feb 18, 2014 at 3:49 AM, MauMau <maumau...@gmail.com> wrote: > From: "Andres Freund" <and...@2ndquadrant.com> > >> On 2014-02-18 01:35:52 +0900, MauMau wrote: >> >>> For example, please see the max latencies of test set 2 (PG 9.3) and test >>> set 4 (xlog scaling with padding). They are 207.359 and 1219.422 >>> respectively. The throughput is of course greatly improved, but I think >>> the >>> response time should not be sacrificed as much as possible. There are >>> some >>> users who are sensitive to max latency, such as stock exchange and online >>> games. >>> >> >> You need to compare both at the same throughput to have any meaningful >> comparison. >> > > I'm sorry for my lack of understanding, but could you tell me why you > think so? When the user upgrades to 9.4 and runs the same workload, he > would experience vastly increased max latency
The tests shown have not tested that. The test is not running the same workload on 9.4, but rather a vastly higher workload. If we were to throttle the workload in 9.4 (using pgbench's new -R, for example) to the same level it was in 9.3, we probably would not see the max latency increase. But that was not tested, so we don't know for sure. > --- or in other words, greater variance in response times. With my simple > understanding, that sounds like a problem for response-sensitive users. > If you need the throughput provided by 9.4, then using 9.3 gets lower variance simply be refusing to do 80% of the assigned work. If you don't need the throughput provided by 9.4, then you probably have some natural throttling in place. If you want a real-world like test, you might try to crank up the -c and -j to the limit in 9.3 in a vain effort to match 9.4's performance, and see what that does to max latency. (After all, that is what a naive web app is likely to do--continue to make more and more connections as requests come in faster than they can finish.) Cheers, Jeff
