Simon Riggs <simon@2ndquadrant.com> writes:
> Amdahl's Law tells me that looking at the checkpoints is the next best
> action for tuning, since they add considerably to the average response
> time. Looking at the oprofile for the run as a whole is missing out the
> delayed transaction behaviour that occurs during checkpoints.
Even aside from the effect it has on average response time. I would point out
that many applications are governed by the worst case more than the average
throughput.
For a web server, for example (or any OLTP application in general), it doesn't
matter if the database can handle x transactions/s on average. What matters is
that 100% of the time the latency is below the actual rate of requests. If
every 30m latency suddenly spikes up beyond that, even for only a minute, then
it will fall behind in the requests. The user will effectively see a
completely unresponsive web server.
So I would really urge you to focus your attention on the maximum latency
figure. It's at least if not *more* important than the average throughput
number.
PS That's why I was pushing before for the idea that the server should try to
spread the I/O from one checkpoint out over more or less the time interval
between checkpoints. If it's been 30m since the last checkpoint then you have
about 30m to do the I/O for this checkpoint. (Though I would suggest a safety
factor of aiming to be finished within 50% of the time.)
--
greg