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Re: testing HS/SR - 1 vs 2 performance

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, Erik Rijkers <er(at)xs4all(dot)nl>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: testing HS/SR - 1 vs 2 performance
Date: 2010-04-17 19:45:47
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Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> On Sat, 2010-04-17 at 11:13 -0400, Tom Lane wrote:
>> It'd be cheaper anyway to sort and search the
>> array using plain <, so why are you so eager to use
>> TransactionIdFollows?

> The array grows to the right and is laid out one xid per element,
> resulting in a sequence of values that are transactionid-monotonic.

How do you know that just adding items at the right will produce a
sorted array?  It seems quite unlikely to me that transactions can be
guaranteed to arrive at this code in XID order.  I think you need to do
an explicitly sorted insertion.

> ... Doing it this way means that we can
> add rows past the head of the array and then move the head atomically,
> so that we can make adding xids lock-free.

... and even without that issue, this seems like utter fantasy.  How
are you going to do that "atomically"?  Have you considered what will
happen on weak-memory-ordering machines like PPC, in particular?

			regards, tom lane

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