On Wednesday 24 November 2010 22:18:08 Robert Haas wrote: > On Nov 24, 2010, at 4:05 PM, Andres Freund <and...@anarazel.de> wrote: > >>> 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)? > >> > >> Aren't we incurring those page faults anyway, for whatever memory > >> palloc is handing out? The heap is no different from bss; we just > >> move the pointer with sbrk(). > > > > Yes, but only once. Also scrubbing a page is faster than copying it... > > (and there were patches floating around to do that in advance, not sure > > if they got integrated into mainline linux) > I'm not following - can you elaborate? When forking the memory mapping of the process is copied - the actual pages are not. When a page is first accessed the page fault handler will setup a mapping to the "old" page and mark it as shared. When now written to it will fault again and copy the page.
In contrast if you access a page the first time after an sbrk (or mmap, doesn't matter) a new page will get scrubbed and and a mapping will get setup. Andres -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
