Re: checkpointer continuous flushing - V16
От | Fabien COELHO |
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Тема | Re: checkpointer continuous flushing - V16 |
Дата | |
Msg-id | alpine.DEB.2.10.1603011644380.18133@sto обсуждение исходный текст |
Ответ на | Re: checkpointer continuous flushing - V16 (Tomas Vondra <tomas.vondra@2ndquadrant.com>) |
Список | pgsql-hackers |
Hello Tomas, > One of the goals of this thread (as I understand it) was to make the overall > behavior smoother - eliminate sudden drops in transaction rate due to bursts > of random I/O etc. > > One way to look at this is in terms of how much the tps fluctuates, so let's > see some charts. I've collected per-second tps measurements (using the > aggregation built into pgbench) but looking at that directly is pretty > pointless because it's very difficult to compare two noisy lines jumping up > and down. > > So instead let's see CDF of the per-second tps measurements. I.e. we have > 3600 tps measurements, and given a tps value the question is what percentage > of the measurements is below this value. > > y = Probability(tps <= x) > > We prefer higher values, and the ideal behavior would be that we get exactly > the same tps every second. Thus an ideal CDF line would be a step line. Of > course, that's rarely the case in practice. But comparing two CDF curves is > easy - the line more to the right is better, at least for tps measurements, > where we prefer higher values. Very nice and interesting graphs! Alas not easy to interpret for the HDD, as there are better/worse variation all along the distribution, the lines cross one another, so how it fares overall is unclear. Maybe a simple indication would be to compute the standard deviation on the per second tps? The median maybe interesting as well. > I do have some more data, but those are the most interesting charts. The rest > usually shows about the same thing (or nothing). > > Overall, I'm not quite sure the patches actually achieve the intended goals. > On the 10k SAS drives I got better performance, but apparently much more > variable behavior. On SSDs, I get a bit worse results. Indeed. -- Fabien.
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