On Wed, Jul 13, 2011 at 11:39:47PM +0200, Jan Kara wrote: > On Wed 13-07-11 15:31:27, Mel Gorman wrote: > > It is preferable that no dirty pages are dispatched from the page > > reclaim path. If reclaim is encountering dirty pages, it implies that > > either reclaim is getting ahead of writeback or use-once logic has > > prioritise pages for reclaiming that are young relative to when the > > inode was dirtied. > > > > When dirty pages are encounted on the LRU, this patch marks the inodes > > I_DIRTY_RECLAIM and wakes the background flusher. When the background > > flusher runs, it moves such inodes immediately to the dispatch queue > > regardless of inode age. There is no guarantee that pages reclaim > > cares about will be cleaned first but the expectation is that the > > flusher threads will clean the page quicker than if reclaim tried to > > clean a single page. > Hmm, I was looking through your numbers but I didn't see any significant > difference this patch would make. Do you? > Marginal and well within noise. I'm very skeptical about the patch but the VM needs some way of prioritising what pages are getting written back to that pages in a particular zone can be cleaned. > I was thinking about the problem and actually doing IO from kswapd would be > a small problem if we submitted more than just a single page. Just to give > you idea - time to write a single page on plain SATA drive might be like 4 > ms. Time to write sequential 4 MB of data is like 80 ms (I just made up > these numbers but the orders should be right). It's as good as number as any for arguements sake. It's not the first time such a patch has done the rounds. The last one I did along similar lines was http://lkml.org/lkml/2010/6/8/85 although I mucked it up with respect to racing with iput. Wu posted a patch that deferred the writing of ranges to a flusher thread http://www.spinics.net/lists/xfs/msg05659.html which Dave has already commented on at http://www.spinics.net/lists/xfs/msg05665.html. The clustering size could be easily fixed but the scalability problem he pointed out is a far greater problem. > So to write 1000 times more > data you just need like 20 times longer. That's a factor of 50 in IO > efficiency. So when reclaim/kswapd submits a single page IO once every > couple of miliseconds, your IO throughput just went close to zero... > BTW: I just checked your numbers in fsmark test with vanilla kernel. You > wrote like 14500 pages from reclaim in 567 seconds. That is about one page > per 39 ms. That is going to have noticeable impact on IO throughput (not > with XFS because it plays tricks with writing more than asked but with ext2 > or ext3 you would see it I guess). > > So when kswapd sees high percentage of dirty pages at the end of LRU, it > could call something like fdatawrite_range() for the range of 4 MB > (provided the file is large enough) containing that page and IO thoughput > would not be hit that much and you will get reasonably bounded time when > the page gets cleaned... If you wanted to be clever, you could possibly be > more sophisticated in picking the file and range to write so that you get > rid of the most pages at the end of LRU but I'm not sure it's worth the CPU > cycles. Does this sound reasonable to you? > Semi-reasonable and it's along the same lines as what http://lkml.org/lkml/2010/6/8/85 tried to achieve but maybe the effort of fixing it up with respect to racing with iput() just isn't worth it. I think I'll leave it as kswapd will call writepage if the priority is high enough until a good solution for how the VM can tell the flusher to prioritise a particular page is devised. -- Mel Gorman SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>