Re: [RFC 1/2] deactive invalidated pages

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> Hi KOSAKI,
> 
> 2010/11/23 KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>:
> >> By Other approach, app developer uses POSIX_FADV_DONTNEED.
> >> But it has a problem. If kernel meets page is writing
> >> during invalidate_mapping_pages, it can't work.
> >> It is very hard for application programmer to use it.
> >> Because they always have to sync data before calling
> >> fadivse(..POSIX_FADV_DONTNEED) to make sure the pages could
> >> be discardable. At last, they can't use deferred write of kernel
> >> so that they could see performance loss.
> >> (http://insights.oetiker.ch/linux/fadvise.html)
> >
> > If rsync use the above url patch, we don't need your patch.
> > fdatasync() + POSIX_FADV_DONTNEED should work fine.
> 
> It works well. But it needs always fdatasync before calling fadvise.
> For small file, it hurt performance since we can't use the deferred write.

I doubt rsync need to call fdatasync. Why?

If rsync continue to do following loop, some POSIX_FADV_DONTNEED
may not drop some dirty pages. But they can be dropped at next loop's
POSIX_FADV_DONTNEED. Then, It doesn't make serious issue.

1) read
2) write
3) POSIX_FADV_DONTNEED
4) goto 1


Am I missing anything?


> > So, I think the core worth of previous PeterZ's patch is in readahead
> > based heuristics. I'm curious why you drop it.
> >
> 
> In previous peter's patch, it couldn't move active page into inactive list.
> So it's not what i want and I think invalidation is stronger hint than
> the readahead heuristic.
> But if we need it, I will add it in my series. It can help reclaiming
> unnecessary inactive page asap.
> but before that, I hope we make sure fadvise works well enough.

I've got it.Yeah, 1) implement manual  oepration 2) add automatic heuristic 
is right order. I think. we can easily test your one.



> >> In fact, invalidate is very big hint to reclaimer.
> >> It means we don't use the page any more. So let's move
> >> the writing page into inactive list's head.
> >
> > But, I agree this.
> 
> Thank you.
>
> >> +static void __pagevec_lru_deactive(struct pagevec *pvec)
> >> +{
> >> +     int i, lru, file;
> >> +
> >> +     struct zone *zone = NULL;
> >> +
> >> +     for (i = 0; i < pagevec_count(pvec); i++) {
> >> +             struct page *page = pvec->pages[i];
> >> +             struct zone *pagezone = page_zone(page);
> >> +
> >> +             if (pagezone != zone) {
> >> +                     if (zone)
> >> +                             spin_unlock_irq(&zone->lru_lock);
> >> +                     zone = pagezone;
> >> +                     spin_lock_irq(&zone->lru_lock);
> >> +             }
> >> +
> >> +             if (PageLRU(page)) {
> >> +                     if (PageActive(page)) {
> >> +                             file = page_is_file_cache(page);
> >> +                             lru = page_lru_base_type(page);
> >> +                             del_page_from_lru_list(zone, page,
> >> +                                             lru + LRU_ACTIVE);
> >> +                             ClearPageActive(page);
> >> +                             ClearPageReferenced(page);
> >> +                             add_page_to_lru_list(zone, page, lru);
> >> +                             __count_vm_event(PGDEACTIVATE);
> >> +
> >> +                             update_page_reclaim_stat(zone, page, file, 0);
> >
> > When PageActive is unset, we need to change cgroup lru too.
> 
> Doesn't add_page_to_lru_list/del_page_from_lru_list do it?

Grr, my fault. I've forgot to we changed add_page_to_lru_list.