Re: Cost limited statements RFC

On Thu, Jun 6, 2013 at 3:34 PM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> On Fri, May 24, 2013 at 11:51 AM, Greg Smith <greg(at)2ndquadrant(dot)com> wrote:
>>
>> On 5/24/13 9:21 AM, Robert Haas wrote:
>>
>>> But I wonder if we wouldn't be better off coming up with a little more
>>> user-friendly API. Instead of exposing a cost delay, a cost limit,
>>> and various charges, perhaps we should just provide limits measured in
>>> KB/s, like dirty_rate_limit = <amount of data you can dirty per
>>> second, in kB> and read_rate_limit = <amount of data you can read into
>>> shared buffers per second, in kB>.
>>
>>
>> I already made and lost the argument for doing vacuum in KB/s units, so I
>> wasn't planning on putting that in the way of this one.
>
>
> I think the problem is that making that change would force people to relearn
> something that was already long established, and it was far from clear that
> the improvement, though real, was big enough to justify forcing people to do
> that. That objection would not apply to a new feature, as there would be
> nothing to re-learn. The other objection was that (at that time) we had
> some hope that the entire workings would be redone for 9.3, and it seemed
> unfriendly to re-name things in 9.2 without much change in functionality,
> and then redo them completely in 9.3.

Right. Also, IIRC, the limits didn't really mean what they purported
to mean. You set either a read or a dirty rate in KB/s, but what was
really limited was the combination of the two, and the relative
importance of the two factors was based on other settings in a
severely non-obvious way.

If we can see our way clear to ripping out the autovacuum costing
stuff and replacing them with a read rate limit and a dirty rate
limit, I'd be in favor of that. The current system limits the linear
combination of those with user-specified coefficients, which is more
powerful but less intuitive. If we need that, we'll have to keep it
the way it is, but I'm hoping we don't.

--
Robert Haas
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