Re: Vacuum: allow usage of more than 1GB of work mem

On Tue, Nov 22, 2016 at 4:53 AM, Claudio Freire <klaussfreire@gmail.com>
wrote:

> On Mon, Nov 21, 2016 at 2:15 PM, Masahiko Sawada <sawada.mshk@gmail.com>
> wrote:
> > On Fri, Nov 18, 2016 at 6:54 AM, Claudio Freire <klaussfreire@gmail.com>
> wrote:
> >> On Thu, Nov 17, 2016 at 6:34 PM, Robert Haas <robertmhaas@gmail.com>
> wrote:
> >>> On Thu, Nov 17, 2016 at 1:42 PM, Claudio Freire <
> klaussfreire@gmail.com> wrote:
> >>>> Attached is patch 0002 with pgindent applied over it
> >>>>
> >>>> I don't think there's any other formatting issue, but feel free to
> >>>> point a finger to it if I missed any
> >>>
> >>> Hmm, I had imagined making all of the segments the same size rather
> >>> than having the size grow exponentially.  The whole point of this is
> >>> to save memory, and even in the worst case you don't end up with that
> >>> many segments as long as you pick a reasonable base size (e.g. 64MB).
> >>
> >> Wastage is bound by a fraction of the total required RAM, that is,
> >> it's proportional to the amount of required RAM, not the amount
> >> allocated. So it should still be fine, and the exponential strategy
> >> should improve lookup performance considerably.
> >
> > I'm concerned that it could use repalloc for large memory area when
> > vacrelstats->dead_tuples.dead_tuple is bloated. It would be overhead
> > and slow.
>
> How large?
>
> That array cannot be very large. It contains pointers to
> exponentially-growing arrays, but the repalloc'd array itself is
> small: one struct per segment, each segment starts at 128MB and grows
> exponentially.
>
> In fact, IIRC, it can be proven that such a repalloc strategy has an
> amortized cost of O(log log n) per tuple. If it repallocd the whole
> tid array it would be O(log n), but since it handles only pointers to
> segments of exponentially growing tuples it becomes O(log log n).
>
> Furthermore, n there is still limited to MAX_INT, which means the cost
> per tuple is bound by O(log log 2^32) = 5. And that's an absolute
> worst case that's ignoring the 128MB constant factor which is indeed
> relevant.
>
> > What about using semi-fixed memroy space without repalloc;
> > Allocate the array of ItemPointerData array, and each ItemPointerData
> > array stores the dead tuple locations. The size of ItemPointerData
> > array starts with small size (e.g. 16MB or 32MB). After we used an
> > array up, we then allocate next segment with twice size as previous
> > segment.
>
> That's what the patch does.
>
> > But to prevent over allocating memory, it would be better to
> > set a limit of allocating size of ItemPointerData array to 512MB or
> > 1GB.
>
> There already is a limit to 1GB (the maximum amount palloc can allocate)
>
>
Moved to next CF with "needs review" status.

Regards,
Hari Babu
Fujitsu Australia