On Tue, Jan 14, 2014 at 12:15 PM, Claudio Freire <klaussfreire@gmail.com> wrote:
> On Tue, Jan 14, 2014 at 2:12 PM, Robert Haas <robertmhaas@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
