Re: [Lsf-pc] Linux kernel impact on PostgreSQL performance

On Tue, Jan 14, 2014 at 2:17 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Tue, Jan 14, 2014 at 12:15 PM, Claudio Freire <klaussfreire(at)gmail(dot)com> wrote:
>> On Tue, Jan 14, 2014 at 2:12 PM, Robert Haas <robertmhaas(at)gmail(dot)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.

Not necessarily, you pay the CPU cost on each page fault (ie: first
write to the buffer at least), each time the page checks into the
shared buffers level.

It's like a tiered cache.

When promoting is expensive, one must be careful. The traffic to/from
the L0 (shared buffers) and L1 (page cache) will be considerable, even
if everything fits in RAM.

I guess it's the constant battle between inclusive and exclusive caches.