Messages in this thread | | | Date | Thu, 06 Jan 2005 16:19:36 +1100 | From | Nick Piggin <> | Subject | Re: [PATCH][5/?] count writeback pages in nr_scanned |
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Andrea Arcangeli wrote: > On Wed, Jan 05, 2005 at 09:05:39PM -0800, Andrew Morton wrote: > >>Andrea Arcangeli <andrea@suse.de> wrote: >> >>>The fix is very simple and it is to call wait_on_page_writeback on one >>> of the pages under writeback. >> >>eek, no. That was causing waits of five seconds or more. Fixing this >>caused the single greatest improvement in page allocator latency in early >>2.5. We're totally at the mercy of the elevator algorithm this way. >> >>If we're to improve things in there we want to wait on _any_ eligible page >>becoming reclaimable, not on a particular page. > > > I told you one way to fix it. I didn't guarantee it was the most > efficient one. > > I sure agree waiting on any page to complete writeback is going to fix > it too. Exactly because this page was a "random" page anyway. > > Still my point is that this is a bug, and I prefer to be slow and safe > like 2.4, than fast and unreliable like 2.6. > > The slight improvement you suggested of waiting on _any_ random > PG_writeback to go away (instead of one particular one as I did in 2.4) > is going to fix the write throttling equally too as well as the 2.4 > logic, but without introducing slowdown that 2.4 had. > > It's easy to demonstrate: exactly because the page we pick is random > anyway, we can pick the first random one that has seen PG_writeback > transitioning from 1 to 0. The guarantee we get is the same in terms of > safety of the write throttling, but we also guarantee the best possible > latency this way. And the HZ/x hacks to avoid deadlocks will magically > go away too. >
This is practically what blk_congestion_wait does when the queue isn't congested though, isn't it? - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.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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