On Thu, 6 Feb 2020 16:08:53 -0800 Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > On Fri, Feb 07, 2020 at 10:19:28AM +1100, Dave Chinner wrote: > > But detecting an abundance dirty pages/inodes on the LRU doesn't > > really solve the problem of determining if and/or how long we should > > wait for IO before we try to free more objects. There is no problem > > with having lots of dirty pages/inodes on the LRU as long as the IO > > subsystem keeps up with the rate at which reclaim is asking them to > > be written back via async mechanisms (bdi writeback, metadata > > writeback, etc). > > > > The problem comes when we cannot make efficient progress cleaning > > pages/inodes on the LRU because the IO subsystem is overloaded and > > cannot clean pages/inodes any faster. At this point, we have to wait > > for the IO subsystem to make progress and without feedback from the > > IO subsystem, we have no idea how fast that progress is made. Hence > > we have no idea how long we need to wait before trying to reclaim > > again. i.e. the answer can be different depending on hardware > > behaviour, not just the current instantaneous reclaim and IO state. > > > > That's the fundamental problem we need to solve, and realistically > > it can only be done with some level of feedback from the IO > > subsystem. > > That triggered a memory for me. Jeremy Kerr presented a paper at LCA2006 > on a different model where the device driver pulls dirty things from the VM > rather than having the VM push dirty things to the device driver. It was > prototyped in K42 rather than Linux, but the idea might be useful. > > http://jk.ozlabs.org/projects/k42/ > http://jk.ozlabs.org/projects/k42/device-driven-IO-lca06.pdf Fun. Device drivers says "I have spare bandwidth so send me some stuff". But if device drivers could do that, we wouldn't have broken congestion in the first place ;)
