Re: [PATCH 4/8] vmscan: Do not writeback filesystem pages in direct reclaim

Mel Gorman
2010-07-22T09:19:52Z

On Thu, Jul 22, 2010 at 08:57:34AM +0900, KAMEZAWA Hiroyuki wrote:
> On Wed, 21 Jul 2010 15:27:10 +0100
> Mel Gorman <mel@csn.ul.ie> wrote:
>
> > On Wed, Jul 21, 2010 at 09:01:11PM +0900, KAMEZAWA Hiroyuki wrote:
>
> > > But, hmm, memcg will have to select to enter this rounine based on
> > > the result of 1st memory reclaim.
> > >
> >
> > It has the option of igoring pages being dirtied but I worry that the
> > container could be filled with dirty pages waiting for flushers to do
> > something.
>
> I'll prepare dirty_ratio for memcg. It's not easy but requested by I/O cgroup
> guys, too...
>

I can see why it might be difficult. Dirty pages are not being counted
on a per-container basis. It would require additional infrastructure to
count it or a lot of scanning.

>
> >
> > > >
> > > > - /*
> > > > - * The attempt at page out may have made some
> > > > - * of the pages active, mark them inactive again.
> > > > - */
> > > > - nr_active = clear_active_flags(&page_list, NULL);
> > > > - count_vm_events(PGDEACTIVATE, nr_active);
> > > > + while (nr_reclaimed < nr_taken && nr_dirty && dirty_retry--) {
> > > > + wakeup_flusher_threads(laptop_mode ? 0 : nr_dirty);
> > > > + congestion_wait(BLK_RW_ASYNC, HZ/10);
> > > >
> > >
> > > Congestion wait is required ?? Where the congestion happens ?
> > > I'm sorry you already have some other trick in other patch.
> > >
> >
> > It's to wait for the IO to occur.
> >
>
> 1 tick penalty seems too large. I hope we can have some waitqueue in future.
>

congestion_wait() if congestion occurs goes onto a waitqueue that is
woken if congestion clears. I didn't measure it this time around but I
doubt it waits for HZ/10 much of the time.

> > > > - nr_reclaimed += shrink_page_list(&page_list, sc, PAGEOUT_IO_SYNC);
> > > > + /*
> > > > + * The attempt at page out may have made some
> > > > + * of the pages active, mark them inactive again.
> > > > + */
> > > > + nr_active = clear_active_flags(&page_list, NULL);
> > > > + count_vm_events(PGDEACTIVATE, nr_active);
> > > > +
> > > > + nr_reclaimed += shrink_page_list(&page_list, sc,
> > > > + PAGEOUT_IO_SYNC, &nr_dirty);
> > > > + }
> > >
> > > Just a question. This PAGEOUT_IO_SYNC has some meanings ?
> > >
> >
> > Yes, in pageout it will wait on pages currently being written back to be
> > cleaned before trying to reclaim them.
> >
> Hmm. IIUC, this routine is called only when !current_is_kswapd() and
> pageout is done only whne current_is_kswapd(). So, this seems ....
> Wrong ?
>

Both direct reclaim and kswapd can reach shrink_inactive_list

Direct reclaim
do_try_to_free_pages
-> shrink_zones
-> shrink_zone
-> shrink_list
-> shrink_inactive list <--- the routine in question

Kswapd
balance_pgdat
-> shrink_zone
-> shrink_list
-> shrink_inactive_list

pageout() is still called by direct reclaim if the page is anon so it
will synchronously wait on those if PAGEOUT_IO_SYNC is set. For either
anon or file pages, if they are being currently written back, they will
be waited on in shrink_page_list() if PAGEOUT_IO_SYNC.

So it still has meaning. Did I miss something?

--
Mel Gorman
Part-time Phd Student Linux Technology Center
University of Limerick IBM Dublin Software Lab