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Re: CLOG contention, part 2

From: Simon Riggs <simon(at)2ndQuadrant(dot)com>
To: Jeff Janes <jeff(dot)janes(at)gmail(dot)com>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: CLOG contention, part 2
Date: 2012-01-29 23:04:47
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Lists: pgsql-hackers
On Sun, Jan 29, 2012 at 9:41 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:

> If I cast to a int, then I see advancement:

I'll initialise it as 0, rather than -1 and then we don't have a
problem in any circumstance.

>> I've specifically designed the pgbench changes required to simulate
>> conditions of clog contention to help in the evaluation of this patch.
> Yep, I've used that one for the testing.

Most of the current patch is just bookkeeping to keep track of the
point when we can look at history in read only manner.

I've isolated the code better to allow you to explore various
implementation options. I don't see any performance difference between
any of them really, but you're welcome to look.

Please everybody note that the clog history doesn't even become active
until the first checkpoint, so this is dead code until we've hit the
first checkpoint cycle and completed a million transactions since
startup. So its designed to tune for real world situations, and is not
easy to benchmark. (Maybe we could start earlier, but having extra
code just for first few minutes seems waste of energy, especially
since we must hit million xids also).

 Simon Riggs         
 PostgreSQL Development, 24x7 Support, Training & Services

Attachment: clog_history.v3.patch
Description: text/x-diff (8.2 KB)

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