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Date: Tue, 10 Nov 2009 15:55:20 +0900 (JST) From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> To: Wu Fengguang <fengguang.wu@...el.com> Cc: kosaki.motohiro@...fujitsu.com, Peter Zijlstra <peterz@...radead.org>, Martin Knoblauch <spamtrap@...bisoft.de>, linux-kernel@...r.kernel.org Subject: Re: Likley stupid question on "throttle_vm_writeout" > On Mon, Nov 09, 2009 at 04:26:33PM +0100, Peter Zijlstra wrote: > > On Mon, 2009-11-09 at 07:15 -0800, Martin Knoblauch wrote: > > > Hi, (please CC me on replies) > > > > > > I have a likely stupid question on the function "throttle_vm_writeout". Looking at the code I find: > > > > > > if (global_page_state(NR_UNSTABLE_NFS) + > > > global_page_state(NR_WRITEBACK) <= dirty_thresh) > > > break; > > > congestion_wait(WRITE, HZ/10); > > > > > > Shouldn't the NR_FILE_DIRTY pages be considered as well? > > > > Ha, you just trod onto a piece of ugly I'd totally forgotten about ;-) > > > > The intent of throttle_vm_writeout() is to limit the total pages in > > writeout and to wait for them to go-away. > > Like this: > > vmscan fast => large NR_WRITEBACK => throttle vmscan based on it > > > Everybody hates the function, nobody managed to actually come up with > > anything better. > > btw, here is another reason to limit NR_WRITEBACK: I saw many > throttle_vm_writeout() waits if there is no wait queue to limit > NR_WRITEBACK (eg. NFS). In that case the (steadily) big NR_WRITEBACK > is _not_ caused by fast vmscan.. btw, this explanation point out why we should remove other bare congestion_wait() in reclaim path. At least, I observed the congestion_wait() in do_try_to_free_pages() decrease reclaim performance very much. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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