VM: Where do we stand?

["Duraid Madina" <duraid@fl.net.au>]

I'm sure at least some of you will immediately recognize these words:

>
>Swap allocation is terrible.  Linux uses a linear array which it scans
>looking for a free swap block.  It does a relatively simple swap
>cluster cache, but eats the full linear scan if that fails which can be
>terribly nasty.  The swap clustering algorithm is a piece of crap, 
>too -- once swap becomes fragmented, the linux swapper falls on its
face.
>
>It does read-ahead based on the swapblk which wouldn't be bad if it
>clustered writes by object or didn't have a fragmentation problem.
>As it stands, their read clustering is useless.  Swap deallocation is 
>fast since they are using a simple reference count array.
>
>File read-ahead is half-hazard at best.
>
>The paging queues ( determing the age of the page and whether to 
>free or clean it) need to be written... the algorithms being used
>are terrible.
>
> * For the nominal page scan, it is using a one-hand clock algorithm.  
>   All I can say is:  Oh my god!  Are they nuts?  That was abandoned
>   a decade ago.  The priority mechanism they've implemented is nearly
>   useless.
>
> * To locate pages to swap out, it takes a pass through the task list. 
>   Ostensibly it locates the task with the largest RSS to then try to
>   swap pages out from rather then select pages that are not in use.
>   From my read of the code, it also botches this badly.
>
>Linux does not appear to do any page coloring whatsoever, but it would
>not be hard to add it in.
>

	Where does Linux stand, three years on? An O(1) scheduler is
nice, but I tell you what'd be even nicer...

	coming out for some food, (it's dark out)
	Duraid