On 25 April 2017 at 22:07, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Craig Ringer <craig@2ndquadrant.com> writes:
>> On 25 Apr. 2017 13:37, "Heikki Linnakangas" <hlinnaka@iki.fi> wrote:
>>> For some data shared memory structures, that store no pointers, we wouldn't
>>> need to insist that they are mapped to the same address in every backend,
>>> though. In particular, shared_buffers. It wouldn't eliminate the problem,
>>> though, only make it less likely, so we'd still need to retry when it does
>>> happen.
>
>> Good point. Simply splitting out shared_buffers into a moveable segment
>> would make a massive difference. Much less chance of losing the dice roll
>> for mapping the fixed segment.
>
>> Should look at what else could be made cheaply relocatable too.
>
> I don't think it's worth spending any effort on. We need the retry
> code anyway, and making it near impossible to hit that would only mean
> that it's very poorly tested. The results upthread say that it isn't
> going to be hit often enough to be a performance problem, so why worry?
Good point. Deal with it if it becomes an issue.
That said, I didn't see if any of those tests covered really big
shared_buffers. That could become an issue down the track at least.
-- Craig Ringer http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training & Services
