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

On Tue, Feb 18, 2014 at 3:49 AM, MauMau <maumau307(at)gmail(dot)com> 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

The tests shown have not tested that. The test is not running the same
workload on 9.4, but rather a vastly higher workload. If we were to
throttle the workload in 9.4 (using pgbench's new -R, for example) to the
same level it was in 9.3, we probably would not see the max latency
increase. But that was not tested, so we don't know for sure.

> --- or in other words, greater variance in response times. With my simple
> understanding, that sounds like a problem for response-sensitive users.
>

If you need the throughput provided by 9.4, then using 9.3 gets lower
variance simply be refusing to do 80% of the assigned work. If you don't
need the throughput provided by 9.4, then you probably have some natural
throttling in place.

If you want a real-world like test, you might try to crank up the -c and -j
to the limit in 9.3 in a vain effort to match 9.4's performance, and see
what that does to max latency. (After all, that is what a naive web app is
likely to do--continue to make more and more connections as requests come
in faster than they can finish.)

Cheers,

Jeff