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

From: "Andres Freund" <andres@2ndquadrant.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