On Wednesday 24 November 2010 21:47:32 Robert Haas wrote:
> On Wed, Nov 24, 2010 at 3:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> > Robert Haas <robertmhaas@gmail.com> writes:
> >> Full results, and call graph, attached. The first obvious fact is
> >> that most of the memset overhead appears to be coming from
> >> InitCatCache.
> >
> > AFAICT that must be the palloc0 calls that are zeroing out (mostly)
> > the hash bucket headers. I don't see any real way to make that cheaper
> > other than to cut the initial sizes of the hash tables (and add support
> > for expanding them later, which is lacking in catcache ATM). Not
> > convinced that that creates any net savings --- it might just save
> > some cycles at startup in exchange for more cycles later, in typical
> > backend usage.
> >
> > Making those hashtables expansible wouldn't be a bad thing in itself,
> > mind you.
>
> The idea I had was to go the other way and say, hey, if these hash
> tables can't be expanded anyway, let's put them on the BSS instead of
> heap-allocating them. Any new pages we request from the OS will be
> zeroed anyway, but with palloc we then have to re-zero the allocated
> block anyway because palloc can return a memory that's been used,
> freed, and reused. However, for anything that only needs to be
> allocated once and never freed, and whose size can be known at compile
> time, that's not an issue.
>
> In fact, it wouldn't be that hard to relax the "known at compile time"
> constraint either. We could just declare:
>
> char lotsa_zero_bytes[NUM_ZERO_BYTES_WE_NEED];
>
> ...and then peel off chunks.
Won't this just cause loads of additional pagefaults after fork() when those
pages are used the first time and then a second time when first written to (to
copy it)?
Andres