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.



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