On Wednesday 24 November 2010 21:47:32 Robert Haas wrote: > On Wed, Nov 24, 2010 at 3:14 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > > Robert Haas <robertmh...@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)?
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