Re: should we set hint bits without dirtying the page?

On 03.12.2010 04:54, Tom Lane wrote:
> Robert Haas<robertmhaas@gmail.com>  writes:
>> I then got to wondering whether we should even go a step further, and
>> simply decree that a page with only hint bit updates is not dirty and
>> won't be written, period.
>
> This sort of thing has been discussed before.  It seems fairly clear to
> me that any of these variations represents a performance tradeoff: some
> cases will get better and some will get worse.  I think we are not going
> to get far unless we can agree on a set of benchmark cases that we'll
> use to decide whether the tradeoff is a win or not.  How can we arrive
> at that?

It's pretty easy to come up with a test case where that would be a win. 
I'd like to see some benchmark results of the worst case, to see how 
much loss we're talking about at most. Robert described the worst case:

> Where it's a problem is
> when you have a huge table that you're scanning over and over again,
> especially if data in that table was loaded by many different, widely
> spaced XIDs that require looking at many different CLOG pages.

I'd like to add to that: "and the table is big enough to not fit in 
shared_buffers, but small enough to fit in OS cache".

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