Re: Should we increase the default vacuum_cost_limit?

On 10/03/2019 06:55, Tom Lane wrote:
> Andrew Dunstan <andrew.dunstan@2ndquadrant.com> writes:
>> On 3/9/19 4:28 AM, David Rowley wrote:
>>> I agree that vacuum_cost_delay might not be granular enough, however.
>>> If we're going to change the vacuum_cost_delay into microseconds, then
>>> I'm a little concerned that it'll silently break existing code that
>>> sets it.  Scripts that do manual off-peak vacuums are pretty common
>>> out in the wild.
>> Maybe we could leave the default units as msec but store it and allow
>> specifying as usec. Not sure how well the GUC mechanism would cope with
>> that.
> I took a quick look at that and I'm afraid it'd be a mess.  GUC doesn't
> really distinguish between a variable's storage unit, its default input
> unit, or its default output unit (as seen in e.g. pg_settings).  Perhaps
> we could split those into two or three distinct concepts, but it seems
> complicated and bug-prone.  Also I think we'd still be forced into
> making obviously-incompatible changes in what pg_settings shows for
> this variable, since what it shows right now is integer ms.  That last
> isn't a deal-breaker perhaps, but 100% compatibility isn't going to
> happen this way.
>
> The idea of converting vacuum_cost_delay into a float variable, while
> keeping its native unit as ms, seems probably more feasible from a
> compatibility standpoint.  There are two sub-possibilities:
>
> 1. Just do that and lose units support for the variable.  I don't
> think this is totally unreasonable, because up to now ms is the
> *only* workable unit for it:
>
> regression=# set vacuum_cost_delay = '1s';
> ERROR:  1000 is outside the valid range for parameter "vacuum_cost_delay" (0 .. 100)
>
> Still, it'd mean that anyone who'd been explicitly setting it with
> an "ms" qualifier would have to change their postgresql.conf entry.
>
> 2. Add support for units for float variables, too.  I don't think
> this'd be a huge amount of work, and we'd surely have other uses
> for it in the long run.
>
> I'm inclined to go look into #2.  Anybody think this is a bad idea?
>
> 			regards, tom lane
>
Hope about  keeping the default unit of ms, but converting it to a 
'double' for input, but storing it as int (or long?) number of 
nanoseconds.  Gives finer grain of control withouthaving to specify a 
unit, while still allowing calculations to be fast?


Cheers,
Gavin