Re: Re: [COMMITTERS] pgsql: Avoid extra locks in GetSnapshotData if old_snapshot_threshold <

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2016-04-16 17:52:44 -0400, Tom Lane wrote:
>> That's more than a 5X penalty, which seems like it would make the
>> feature unusable; unless there is an argument that that's an extreme
>> case that wouldn't be representative of most real-world usage.
>> Which there may well be; I've not been following this thread carefully.

> The 4 % was with the feature disabled (in comparison to before it's
> introduction), we're not sure where that's coming from. But the 5x - and
> that was just on a mid-sized box - is with the feature enabled.

128 processors is a mid-sized box? Or if you didn't have 128 processors,
why are you testing "-c 128 -j 128" cases?

More seriously, the complaints here seem to center on performance in a
read-only workload; but I don't actually see why you'd want to turn on
this feature in a read-only, or even read-mostly, workload. It exists for
the benefit of people who are trying to keep their pg_xlog/ directories
reasonably sized, no? That doesn't sound very read-only-ish to me.

regards, tom lane