On Tue, Jan 14, 2014 at 12:15 PM, Claudio Freire <klaussfre...@gmail.com> wrote: > On Tue, Jan 14, 2014 at 2:12 PM, Robert Haas <robertmh...@gmail.com> wrote: >> In terms of avoiding double-buffering, here's my thought after reading >> what's been written so far. Suppose we read a page into our buffer >> pool. Until the page is clean, it would be ideal for the mapping to >> be shared between the buffer cache and our pool, sort of like >> copy-on-write. That way, if we decide to evict the page, it will >> still be in the OS cache if we end up needing it again (remember, the >> OS cache is typically much larger than our buffer pool). But if the >> page is dirtied, then instead of copying it, just have the buffer pool >> forget about it, because at that point we know we're going to write >> the page back out anyway before evicting it. >> >> This would be pretty similar to copy-on-write, except without the >> copying. It would just be forget-from-the-buffer-pool-on-write. > > But... either copy-on-write or forget-on-write needs a page fault, and > thus a page mapping. > > Is a page fault more expensive than copying 8k?
I don't know either. I wasn't thinking so much that it would save CPU time as that it would save memory. Consider a system with 32GB of RAM. If you set shared_buffers=8GB, then in the worst case you've got 25% of your RAM wasted storing pages that already exist, dirtied, in shared_buffers. It's easy to imagine scenarios in which that results in lots of extra I/O, so that the CPU required to do the accounting comes to seem cheap by comparison. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers