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

From: "Andres Freund" <andres(at)2ndquadrant(dot)com>
> On 2014-02-18 00:43:54 +0900, MauMau wrote:
>> I'm worried about the big increase in max latency. Do you know the
>> cause?
>> More frequent checkpoints caused by increased WAL volume thanks to
>> enhanced
>> performance?
>
> I don't see much evidence of increased latency there? You can't really
> compare the latency when the throughput is significantly different.

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.

>> Although I'm not sure this is related to what I'm asking, the following
>> code
>> fragment in WALInsertSlotAcquireOne() catched my eyes. Shouldn't the if
>> condition be "slotno == -1" instead of "!="? I thought this part wants
>> to
>> make inserters to use another slot on the next insertion, when they fail
>> to
>> acquire the slot immediately. Inserters pass slotno == -1. I'm sorry if
>> I
>> misunderstood the code.
>
> I think you're right.

Thanks for your confirmation. I'd be glad if the fix could bring any
positive impact on max latency.

Regards
MauMau