Re: [RFC 10/16] Variable Order Page Cache: Readahead fixups

[Christoph Lameter <clameter@sgi.com>]

On Mon, 21 May 2007, Fengguang Wu wrote:

> > I am not sure how to solve that one yet. With the above fix we stay at the 
> > 2M sized readahead. As the compound order increases so the number of pages
> > is reduced. We could keep the number of pages constant but then very high
> > orders may cause a excessive use of memory for readahead.
> 
> Do we need to support very high orders(i.e. >2MB)?

Yes actually we could potentially be using up to 1 TB page size on our 
new machines that can support several petabytes of RAM. But the read 
ahead is likely irrelevant in that case. And this is an extreme case that 
will be rarely used but a customer has required that we will be able to 
handle such a situation. I think 2-4 megabytes may be more typical.

> If not, we can define a MAX_PAGE_CACHE_SIZE=2MB, and limit page orders
> under that threshold. Now large readahead can be done in
> MAX_PAGE_CACHE_SIZE chunks.

Maybe we can just logarithmically decrease the pages for readahead? 
Readahead should possibly depend on the overall memory of the machine. If 
the machine has several terabytes of main memory then a couple megs of 
readahead may be necessary.

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