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Re: testing HS/SR - 1 vs 2 performance

From: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Erik Rijkers <er(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-22 05:57:43
Message-ID: 4BCFE557.8040802@enterprisedb.com (view raw or flat)
Thread:
Lists: pgsql-hackers
Robert Haas wrote:
> On Wed, Apr 21, 2010 at 9:37 AM, Simon Riggs <simon(at)2ndquadrant(dot)com> wrote:
>> On Wed, 2010-04-21 at 15:27 +0300, Heikki Linnakangas wrote:
>>
>>> Given the discussion about the cyclic nature of XIDs, it would be good
>>> to add an assertion that when a new XID is added to the array, it is
>>>
>>> a) larger than the biggest value already in the array
>>> (TransactionIdFollows(new, head)), and
>>> b) not too far from the smallest value in the array to confuse binary
>>> search (TransactionIdFollows(new, tail)).
>> We discussed this in November. You convinced me it isn't possible for
>> older xids to stay in the standby because anti-wraparound vacuums would
>> conflict and kick them out. The primary can't run with wrapped xids and
>> neither can the standby. I think that is correct.
>>
>> Adding an assertion isn't going to do much because it's unlikely anybody
>> is going to be running for 2^31 transactions with asserts enabled.
>>
>> Worrying about things like this seems strange when real and negative
>> behaviours are right in our faces elsewhere. Performance and scalability
>> are real world concerns.
> 
> I think the assert is a good idea.  If there's no real problem here,
> the assert won't trip.  It's just a safety precaution.

Right. And assertions also act as documentation, they are a precise and
compact way to document invariants we assume to hold. A comment
explaining why the cyclic nature of XIDs is not a problem would be nice
too, in addition or instead of the assertions.

-- 
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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