Re: Possible Idea with filesystem buffering.

[Chris Mason <mason@suse.com>]

On Tuesday, January 22, 2002 11:32:09 PM +0300 Hans Reiser
<reiser@namesys.com> wrote:

>> Its not about the cost of a function call, it's what the FS does to make
>> that call useful.  Pretend for a second the VM tells the FS everything it
>> needs to know to age a page (whatever scheme the FS wants to use).
>> 
>> Then pretend the VM decides there's memory pressure, and tells the FS
>> subcache to start freeing ram.  So, the FS goes through its list of pages
>> and finds the most suitable one for flushing, but it has no idea how
>> suitable that page is in comparison with the pages that don't belong to
>> that FS (or even other pages from different mount points of the same FS
>> flavor).
>> 
> 
> Why does it need to know how suitable it is compared to the other
> subcaches?  It just ages X pages, and depends on the VM to determine how
> large X is.  The VM pressures subcaches in proportion to their size, it
> doesn't need to know  how suitable one page is compared to another, it
> just has a notion of push on everyone in proportion to their size.

If subcache A has 1000 pages that are very very active, and subcache B has
500 pages that never ever get used, should A get twice as much memory
pressure?  That's what we want to avoid, and I don't see how subcaches
allow it.

-chris