Messages in this thread | | | Date | Sun, 22 Apr 2007 00:19:49 -0700 | From | Andrew Morton <> | Subject | Re: [PATCH 08/10] mm: count writeback pages per BDI |
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On Sat, 21 Apr 2007 13:07:16 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote: > > On Fri, 20 Apr 2007 17:52:02 +0200 Peter Zijlstra <a.p.zijlstra@chello.nl> wrote: > > > > > Count per BDI writeback pages. > > > > > > Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl> > > > --- > > > include/linux/backing-dev.h | 1 + > > > mm/page-writeback.c | 12 ++++++++++-- > > > 2 files changed, 11 insertions(+), 2 deletions(-) > > > > > > Index: linux-2.6/mm/page-writeback.c > > > =================================================================== > > > --- linux-2.6.orig/mm/page-writeback.c 2007-04-20 15:27:28.000000000 +0200 > > > +++ linux-2.6/mm/page-writeback.c 2007-04-20 15:28:10.000000000 +0200 > > > @@ -979,14 +979,18 @@ int test_clear_page_writeback(struct pag > > > int ret; > > > > > > if (mapping) { > > > + struct backing_dev_info *bdi = mapping->backing_dev_info; > > > unsigned long flags; > > > > > > write_lock_irqsave(&mapping->tree_lock, flags); > > > ret = TestClearPageWriteback(page); > > > - if (ret) > > > + if (ret) { > > > radix_tree_tag_clear(&mapping->page_tree, > > > page_index(page), > > > PAGECACHE_TAG_WRITEBACK); > > > + if (bdi_cap_writeback_dirty(bdi)) > > > + __dec_bdi_stat(bdi, BDI_WRITEBACK); > > > > Why do we test bdi_cap_writeback_dirty() here? > > > > If we remove that test, we end up accumulating statistics for > > non-writebackable backing devs, but does that matter? > > It would not, had I not cheated: > > +void bdi_init(struct backing_dev_info *bdi) > +{ > + int i; > + > + if (!(bdi_cap_writeback_dirty(bdi) || bdi_cap_account_dirty(bdi))) > + return; > + > + for (i = 0; i < NR_BDI_STAT_ITEMS; i++) > + percpu_counter_init(&bdi->bdi_stat[i], 0); > +} > +EXPORT_SYMBOL(bdi_init); > > > Probably the common > > case is writebackable backing-devs, so eliminating the test-n-branch might > > be a net microgain. > > Time vs space. Now we don't even have storage for those BDIs.. > > Don't particularly care on this point though, I just thought it might be > worthwhile to save on the percpu data.
It could be that we never call test_clear_page_writeback() against !bdi_cap_writeback_dirty() pages anwyay. I can't think why we would, but the relationships there aren't very clear. Does "don't account for dirty memory" imply "doesn't ever do writeback"? One would need to check, and it's perhaps a bit fragile.
It's worth checking though. Boy we're doing a lot of stuff in there nowadays.
OT: it might be worth looking into batching this work up - the predominant caller should be mpage_end_io_write(), and he has a whole bunch of pages which are usually all from the same file, all contiguous. It's pretty inefficient to be handling that data one-page-at-a-time, and some significant speedups may be available.
Instead, everyone seems to think that variable pagecache page size is the only way of improving things. Shudder.
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