Re: one question about LRU mechanism

["liyu@LAN" <liyu@ccoss.com.cn>]

In 2.6.11.11, mm do not have function isolate_lru_pages(), but I
downloaded linux-2.6.11.12.tar.bz2 source tarball, and apply follow two
patches in order:

patch-2.6.12-rc6
2.6.12-rc6-mm1

Oh, Have any error in this process? patch program say it can not change
some files , and save some *.rej files. but these *rej do not include
mm/vmscan.c.

This new function called two times in shrink_cache() and
refill_inactive_zone(). 

The main part of isolate_lru_pages() is 

/************************************************************/
        while (scan++ < nr_to_scan && !list_empty(src)) {
                page = lru_to_page(src);
                prefetchw_prev_lru_page(page, src, flags);
                                                                                                    
                if (!TestClearPageLRU(page))
                        BUG();
                list_del(&page->lru);
                if (get_page_testone(page)) {
                        /*
                         * It is being freed elsewhere
                         */
                        __put_page(page);
                        SetPageLRU(page);
                        list_add(&page->lru, src);
                        continue;
                } else {
                        list_add(&page->lru, dst);
                        nr_taken++;
                }
        }
/************************************************************/

I think, this change that new function isolate_lru_pages() is one kind
of refactoring (method extract ??), not one essence change. 

the call:
                list_del(&page->lru);

as I known, just delete its argument from list, but not its previous
element. so, It is most newest page that just be appended to
active_list.

I think, may be, codes like this will be better.

/***************************************************/
        while (scan++ < nr_to_scan && !list_empty(src->prev)) {
                page = lru_to_page(src->prev);
                prefetchw_prev_lru_page(page, src->prev, flags);
                                                                                                    
                if (!TestClearPageLRU(page))
                        BUG();
                list_del(&page->lru);
        ......
/**************************************************/


This is just my flimsy perspective.



在 2005-06-14二的 22:25 -0700，William Lee Irwin III wrote：
> On Wed, Jun 15, 2005 at 01:12:56PM +0800, liyu@WAN wrote:
> > 	I am reading memory managment code of kernel 2.6.11.11.
> > I found LRU is implement as linked-stack in linux, include two important
> > data structure linked-list :
> > zone->active_list and zone->inactive_list. when kernel need reclaim some
> > pages, it will call function refiil_inactive_list() ultimately to move
> > some page from active_list to inactive_list.
> 
> The "LRU" bits there don't actually describe the homebrew algorithm.
> 
> 
> On Wed, Jun 15, 2005 at 01:12:56PM +0800, liyu@WAN wrote:
> > 	It's OK, but I have one question in my mind:
> > I found all function that append page to active_list, it just append
> > page to head of active_list (use inline function list_add() ), but
> > refill_inactive_list() also start scanning from head of active_list, I
> > think the better way scan active_list is start from rear of active_list
> > and scan though prev member of list_head at reclaim pages. Scanning
> > start from head of active_list may make thrashing more possibly, I
> > think. and, in my view, "head of active_list" is zone->active_list,
> > "rear of active_list" is zone->active_list.prev .
> > 	May be, I am failed understand mm? or what's wrong?
> 
> I'm looking at 2.6.12-rc6-mm1.
> 
> As far as I can tell new active pages are added to the ->next component
> of the head of the active list in lru_cache_add_active() and the ->next
> component of the head of the inactive list in lru_cache_add(). Then
> isolate_lru_pages() dequeues from ->prev of the head of the inactive
> list in shrink_cache() and isolate_lru_pages() dequeues from the ->prev
> of the active list in refill_inactive_zone().
> 
> 
> -- wli
> -
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