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Date: Mon, 8 Oct 2007 09:54:33 +1000 From: David Chinner <dgc@....com> To: Fengguang Wu <wfg@...l.ustc.edu.cn> Cc: Andrew Morton <akpm@...ux-foundation.org>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, Miklos Szeredi <miklos@...redi.hu>, linux-mm@...ck.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH] remove throttle_vm_writeout() On Fri, Oct 05, 2007 at 08:30:28PM +0800, Fengguang Wu wrote: > The improvement could be: > - kswapd is now explicitly preferred to do the writeout; Careful. kswapd is much less efficient at writeout than pdflush because it does not do low->high offset writeback per address space. It just flushes the pages in LRU order and that turns writeback into a non-sequential mess. I/O sizes decrease substantially and throughput falls through the floor. So if you want kswapd to take over all the writeback, it needs to do writeback in the same manner as the background flushes. i.e. by grabbing page->mapping and flushing that in sequential order rather than just the page on the end of the LRU.... I documented the effect of kswapd taking over writeback in this paper (section 5.3): http://oss.sgi.com/projects/xfs/papers/ols2006/ols-2006-paper.pdf Cheers, Dave. -- Dave Chinner Principal Engineer SGI Australian Software Group - 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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