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.