Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com> wrote:
> Kevin Grittner wrote:
>> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>> So for example we might try resetting the search to the start of
the
>>> relation with probability 0.01.
>>
>> If I understand the heuristic you propose, and my math skill
>> haven't eroded too badly from lack of use, every 229 spots
>> considered would cause a 90% chance of reset. That means that the
>> odds of getting past 50,000 spots (the number of pages with
>> available free space at which I generally start to get worried)
>> without resetting is about 1 in 10^218 -- which is a risk I'm
>> willing to accept. ;-)
>
> The FSM currently uses a clock sweep kind of algorithm to hand out
> pages, and the clock hand is reset to 0 at every vacuum. The purpose
> of resetting the clock hand is precisely to bias towards
> lower-numbered pages, to allow truncation later on.
>
> That's a bit of an over-simplification, though, there's actually a
> separate "clock hand" on each FSM page (next_slot pointer).
>
> We probably could do with more bias. For example, we still prefer
> pages close to the page we last inserted to, by searching for free
> space in the same FSM page first, before starting the search from
> the top of the tree. And of course, each backend tries first to
> insert to the last page it inserted to before even consulting the
> FSM. A mode to disable those behaviors might indeed be useful, along
> with the random reset.
That flushes out the descriptions given by Tom, but doesn't really
make me think I misunderstood the heuristic or miscalculated the odds
of getting far from the start with a 1% probability of a reset on each
advance of the sweep. (Of course, it's possible I'm being dense and
taking statements to mean what I expect, despite evidence to the
contrary.)
The way I figure it, if there is a 0.01 chance to reset the sweep,
then there's a 0.99 percent chance to continue the sweep from the last
position. 0.99^229 is about 0.1, which means there is a 10% chance
not to have reset after that many attempts to advance. Every 229
attempts after that multiplies by 0.1, too. So, after 229*6 attempts
(for example) the odds of not having reset are about one in a million.
Am I saying something stupid here? (I used to be good at these sorts
of calculations....)
-Kevin