I've repeated the tests, but this time logged details for 5% of the transaction (instead of aggregating the data for each second). I've also made the tests shorter - just 12 hours instead of 24, to reduce the time needed to complete the benchmark.

Overall, this means ~300M transactions in total for the un-throttled case, so sample with ~15M transactions available when computing the following charts.

I've used the same commits as during the previous testing, i.e. a298a1e0 (before patches) and 23a27b03 (with patches).

One interesting difference is that while the "patched" version resulted in slightly better performance (8122 vs. 8000 tps), the "unpatched" version got considerably slower (6790 vs. 7725 tps) - that's ~13% difference, so not negligible. Not sure what's the cause - the configuration was exactly the same, there's nothing in the log and the machine was dedicated to the testing. The only explanation I have is that the unpatched code is a bit more unstable when it comes to this type of stress testing.

There results (including scripts for generating the charts) are here:


Attached are three charts - again, those are using CDF to illustrate the distributions and compare them easily:

1) regular-latency.png

The two curves intersect at ~4ms, where both CDF reach ~85%. For the shorter transactions, the old code is slightly faster (i.e. apparently there's some per-transaction overhead). For higher latencies though, the patched code is clearly winning - there are far fewer transactions over 6ms, which makes a huge difference. (Notice the x-axis is actually log-scale, so the tail on the old code is actually much longer than it might appear.)

2) throttled-latency.png

In the throttled case (i.e. when the system is not 100% utilized, so it's more representative of actual production use), the difference is quite clearly in favor of the new code.

3) throttled-schedule-lag.png

Mostly just an alternative view on the previous chart, showing how much later the transactions were scheduled. Again, the new code is winning.


Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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