Re: Do you know the reason for increased max latency due to xlog scaling?

On 2014-02-18 20:49:06 +0900, MauMau wrote:
> From: "Andres Freund" <andres(at)2ndquadrant(dot)com>
> >On 2014-02-18 01:35:52 +0900, MauMau wrote:
> >>For example, please see the max latencies of test set 2 (PG 9.3) and test
> >>set 4 (xlog scaling with padding). They are 207.359 and 1219.422
> >>respectively. The throughput is of course greatly improved, but I think
> >>the
> >>response time should not be sacrificed as much as possible. There are
> >>some
> >>users who are sensitive to max latency, such as stock exchange and online
> >>games.
> >
> >You need to compare both at the same throughput to have any meaningful
> >comparison.
>
> I'm sorry for my lack of understanding, but could you tell me why you think
> so? When the user upgrades to 9.4 and runs the same workload, he would
> experience vastly increased max latency --- or in other words, greater
> variance in response times.

No, the existing data indicates no such thing. When they upgrade they
will have the *same* throughput as before. The datapoints you indicate
that there's an increase in latency, but it's there while processing
several time as much data! The highest throughput of set 2 is 3223,
while the highest for set 4 is 14145.
To get interesting latency comparison you'd need to use pgbench --rate
and use a rate *both* versions can sustain.

Greetings,

Andres Freund

--
Andres Freund http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services