From mboxrd@z Thu Jan 1 00:00:00 1970 Date: Sun, 22 Apr 2007 00:19:49 -0700 From: Andrew Morton Subject: Re: [PATCH 08/10] mm: count writeback pages per BDI Message-Id: <20070422001949.4d697fe5.akpm@linux-foundation.org> In-Reply-To: <1177153636.2934.43.camel@lappy> References: <20070420155154.898600123@chello.nl> <20070420155503.334628394@chello.nl> <20070421025525.042ed73a.akpm@linux-foundation.org> <1177153636.2934.43.camel@lappy> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org Return-Path: To: Peter Zijlstra Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org, miklos@szeredi.hu, neilb@suse.de, dgc@sgi.com, tomoki.sekiyama.qu@hitachi.com, nikita@clusterfs.com, trond.myklebust@fys.uio.no, yingchao.zhou@gmail.com List-ID: On Sat, 21 Apr 2007 13:07:16 +0200 Peter Zijlstra wrote: > On Sat, 2007-04-21 at 02:55 -0700, Andrew Morton wrote: > > On Fri, 20 Apr 2007 17:52:02 +0200 Peter Zijlstra wrote: > > > > > Count per BDI writeback pages. > > > > > > Signed-off-by: Peter Zijlstra > > > --- > > > 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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org