Messages in this thread | | | Date | Sun, 16 Aug 2009 13:15:02 +0800 | From | Wu Fengguang <> | Subject | Re: [RFC] respect the referenced bit of KVM guest pages? |
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On Sun, Aug 16, 2009 at 11:53:00AM +0800, Rik van Riel wrote: > Wu Fengguang wrote: > > On Fri, Aug 07, 2009 at 05:09:55AM +0800, Jeff Dike wrote: > >> Side question - > >> Is there a good reason for this to be in shrink_active_list() > >> as opposed to __isolate_lru_page? > >> > >> if (unlikely(!page_evictable(page, NULL))) { > >> putback_lru_page(page); > >> continue; > >> } > >> > >> Maybe we want to minimize the amount of code under the lru lock or > >> avoid duplicate logic in the isolate_page functions. > > > > I guess the quick test means to avoid the expensive page_referenced() > > call that follows it. But that should be mostly one shot cost - the > > unevictable pages are unlikely to cycle in active/inactive list again > > and again. > > Please read what putback_lru_page does. > > It moves the page onto the unevictable list, so that > it will not end up in this scan again.
Yes it does. I said 'mostly' because there is a small hole that an unevictable page may be scanned but still not moved to unevictable list: when a page is mapped in two places, the first pte has the referenced bit set, the _second_ VMA has VM_LOCKED bit set, then page_referenced() will return 1 and shrink_page_list() will move it into active list instead of unevictable list. Shall we fix this rare case?
> >> But if there are important mlock-heavy workloads, this could make the > >> scan come up empty, or at least emptier than we might like. > > > > Yes, if the above 'if' block is removed, the inactive lists might get > > more expensive to reclaim. > > Why?
Without the 'if' block, an unevictable page may well be deactivated into inactive list (and some time later be moved to unevictable list from there), increasing the inactive list's scanned:reclaimed ratio.
Thanks, Fengguang
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