Re: Question about lazy_space_alloc() / linux over-commit

On Sat, Mar 07, 2015 at 12:46:42AM -0500, Tom Lane wrote:
> Noah Misch <noah(at)leadboat(dot)com> writes:
> > On Thu, Mar 05, 2015 at 03:28:12PM -0600, Jim Nasby wrote:
> >> I was thinking the simpler route of just repalloc'ing... the memcpy would
> >> suck, but much less so than the extra index pass. 64M gets us 11M tuples,
> >> which probably isn't very common.
>
> > +1. Start far below 64 MiB; grow geometrically using repalloc_huge(); cap
> > growth at vac_work_mem.
>
> +1 for repalloc'ing at need, but I'm not sure about the "start far below
> 64 MiB" part. 64MB is a pretty small amount on nearly any machine these
> days (and for anybody who thinks it isn't, that's why maintenance_work_mem
> is a tunable).

True; nothing would explode, especially since the allocation would be strictly
smaller than it is today. However, I can't think of a place in PostgreSQL
where a growable allocation begins so aggressively, nor a reason to break new
ground in that respect. For comparison, tuplestore/tuplesort start memtupsize
at 1 KiB. (One could make a separate case for that practice being wrong.)

> A different line of thought is that it would seem to make sense to have
> the initial allocation vary depending on the relation size. For instance,
> you could assume there might be 10 dead tuples per page, and hence try to
> alloc that much if it fits in vac_work_mem.

Sounds better than a fixed 64 MiB start, though I'm not sure it's better than
a fixed 256 KiB start.