On Tue, Nov 13, 2018 at 08:08:39AM +1100, Dave Chinner wrote: > On Fri, Nov 09, 2018 at 07:12:39AM -0800, Christoph Hellwig wrote: > > [adding linux-mm to the CC list] > > > > On Wed, Nov 07, 2018 at 05:31:12PM +1100, Dave Chinner wrote: > > > From: Dave Chinner <dchinner@xxxxxxxxxx> > > > > > > ->writepage is only used in one place - single page writeback from > > > memory reclaim. We only allow such writeback from kswapd, not from > > > direct memory reclaim, and so it is rarely used. When it comes from > > > kswapd, it is effectively random dirty page shoot-down, which is > > > horrible for IO patterns. We will already have background writeback > > > trying to clean all the dirty pages in memory as efficiently as > > > possible, so having kswapd interrupt our well formed IO stream only > > > slows things down. So get rid of xfs_vm_writepage() completely. > > > > Interesting. IFF we can pull this off it would simplify a lot of > > things, so I'm generally in favor of it. > > Over the past few days of hammeringon this, the only thing I've > noticed is that page reclaim hangs up less, but it's also putting a > bit more pressure on the shrinkers. Filesystem intensive workloads > that drive the machine into reclaim via the page cache seem to hit > breakdown conditions slightly earlier and the impact is that the > shrinkers are run harder. Mostly I see this as the XFS buffer cache > having a much harder time keeping a working set active. > > However, while the workloads hit the working set cache, writeback > performance does seem to be slightly higher. It is, however, being > offset by the deeper lows that come from the cache being turned > over. > > So there's a bit of rebalancing to be done here as a followup, but > I've been unable to drive the system into unexepected OOM kills or > other bad behaviour as a result of removing ->writepage. FWIW I've been running this patch in my development kernels as part of exercising realtime reflink with rextsize > 1. So far I haven't seen any particularly adverse effects. --D > > Cheers, > > Dave. > -- > Dave Chinner > david@xxxxxxxxxxxxx
