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Date: Tue, 6 Apr 2010 11:06:19 +0900 (JST) From: KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com> To: Wu Fengguang <fengguang.wu@...el.com> Cc: kosaki.motohiro@...fujitsu.com, "Li, Shaohua" <shaohua.li@...el.com>, "linux-mm@...ck.org" <linux-mm@...ck.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>, Rik van Riel <riel@...hat.com> Subject: Re: [PATCH]vmscan: handle underflow for get_scan_ratio > On Tue, Apr 06, 2010 at 09:25:36AM +0800, Li, Shaohua wrote: > > On Sun, Apr 04, 2010 at 10:19:06PM +0800, KOSAKI Motohiro wrote: > > > > On Fri, Apr 02, 2010 at 05:14:38PM +0800, KOSAKI Motohiro wrote: > > > > > > > > This patch makes a lot of sense than previous. however I think <1% anon ratio > > > > > > > > shouldn't happen anyway because file lru doesn't have reclaimable pages. > > > > > > > > <1% seems no good reclaim rate. > > > > > > > > > > > > > > Oops, the above mention is wrong. sorry. only 1 page is still too big. > > > > > > > because under streaming io workload, the number of scanning anon pages should > > > > > > > be zero. this is very strong requirement. if not, backup operation will makes > > > > > > > a lot of swapping out. > > > > > > Sounds there is no big impact for the workload which you mentioned with the patch. > > > > > > please see below descriptions. > > > > > > I updated the description of the patch as fengguang suggested. > > > > > > > > > > Umm.. sorry, no. > > > > > > > > > > "one fix but introduce another one bug" is not good deal. instead, > > > > > I'll revert the guilty commit at first as akpm mentioned. > > > > Even we revert the commit, the patch still has its benefit, as it increases > > > > calculation precision, right? > > > > > > no, you shouldn't ignore the regression case. > > > I don't think this is serious. In my calculation, there is only 1 page swapped out > > for 6G anonmous memory. 1 page should haven't any performance impact. > > 1 anon page scanned for every N file pages scanned? > > Is N a _huge_ enough ratio so that the anon list will be very light scanned? > > Rik: here is a little background. The problem is, the VM are couteniously discarding no longer used file cache. if we are scan extra anon 1 page, we will observe tons swap usage after few days. please don't only think benchmark. > Under streaming IO, the current get_scan_ratio() will get a percent[0] > that is (much) less than 1, so underflows to 0. > > It has the bad effect of completely disabling the scan of anon list, > which leads to OOM in Shaohua's test case. OTOH, it also has the good > side effect of keeping anon pages in memory and totally prevent swap > IO. > > Shaohua's patch improves the computation precision by computing nr[] > directly in get_scan_ratio(). This is good in general, however will > enable light scan of the anon list on streaming IO. In such case, percent[0] should be big. I think underflowing is not point. His test case is merely streaming io copy, why can't we drop tmpfs cached page? his /proc/meminfo describe his machine didn't have droppable file cache. so, big percent[1] value seems makes no sense. no? I'm not sure we need either below detection. I need more investigate. 1) detect no discardable file cache 2) detect streaming io on tmpfs (as regular file) -- 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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