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Re: 8.0.X and the ARC patent

From: Greg Stark <gsstark(at)mit(dot)edu>
To: pgsql-hackers(at)postgresql(dot)org
Subject: Re: 8.0.X and the ARC patent
Date: 2005-03-05 01:10:52
Message-ID: 87mztivqw3.fsf@stark.xeocode.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Simon Riggs <simon(at)2ndquadrant(dot)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


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