Suggested code change: slab.c : Moderately simple : reap based on last access time and show LAST access AGE

[Mitchell Erblich]

	* The basis of the below changes deal with STANDARD rules that caches
	are time dependent when dealing with their objects. We assume re-use
	shortly after frees and as time moves forward a lower percentage of  
objects
	will be re-used.

	If I understand the Linux SLAB implementation then ..

	SLAB Caches IMO should NORMALLY be reaped ONLY after X time has  
passed after
	the last object is freed and movement of the slab to the freelist.

	It is logical that on a freq alloc/free/alloc repeated sequences that a
	reasonable time has NOT passed and the drain_freelist() will release
	a slab that would be re-used for the next alloc.

	Secondly, if and when CERTAIN events are pending (ie: extremely low  
free memory)
	then the time since should be ignored and all available free slabs  
should be
	re-used / slab_destroy().

	Suggested something like code:

	To add flexibility : add a /proc variable for "X time"
	OR
	#define   FREE_SLAB_AFTER 10	/*secs */

	Thus ..

		in struct slab : add an entry : unsigned long went_free;   /* time  
slab went free */
		Use ALSO as last object inuse changes
		

	drain_freelist():

		Add a boolean arg to drain_freelist() FREE_SOME_EMPTYS or  
FREE_ALL_EMPTYS
		and FUNCTION_CALLERS
		and convert the tofree to the boolean
	
		
		/* After X secs have passed or FREE_ALL_EMPTYS, then destroy/re-use */
		if (  (! (time_after(jiffies, slabp->went_free ) &&  
(FREE_SOME_EMPTYS)))
		    continue;


	TODO:   set jiffy time ONLY when it when it changes  (slabp->inuse =  
0;) and ++ / --
		       slabp->went_free = jiffies;
		alloc_slabmgt();
		slab_get_object();
		slab_put_object();


	/* By adding a jiffies slab struct item */
	/* Adding debug set of the jiffies anytime the SLAB is accessed and a  
like drain
		function looking for leaks say every hour could be done.
	Locating these slabs could then SET WARNings, as it indicates a  
MEMORY LEAK?
	Could be done in check_slabp for age greater than 1 hr

	and s_show() can print the age of the slab .. convert jiffies...
	SHOWS age and last usage..

	Mitchell Erblich
	



	
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