On Mon, Jan 28, 2019 at 06:41:29PM +0100, Andrea Righi wrote: > Hi Vivek, > > sorry for the late reply. > > On Mon, Jan 21, 2019 at 04:47:15PM -0500, Vivek Goyal wrote: > > On Sat, Jan 19, 2019 at 11:08:27AM +0100, Andrea Righi wrote: > > > > [..] > > > Alright, let's skip the root cgroup for now. I think the point here is > > > if we want to provide sync() isolation among cgroups or not. > > > > > > According to the manpage: > > > > > > sync() causes all pending modifications to filesystem > > > metadata and cached file data to be > > > written to the underlying filesystems. > > > > > > And: > > > According to the standard specification (e.g., POSIX.1-2001), > > > sync() schedules the writes, but > > > may return before the actual writing is done. However Linux > > > waits for I/O completions, and > > > thus sync() or syncfs() provide the same guarantees as fsync > > > called on every file in the sys‐ > > > tem or filesystem respectively. > > > > > > Excluding the root cgroup, do you think a sync() issued inside a > > > specific cgroup should wait for I/O completions only for the writes that > > > have been generated by that cgroup? > > > > Can we account I/O towards the cgroup which issued "sync" only if write > > rate of sync cgroup is higher than cgroup to which page belongs to. Will > > that solve problem, assuming its doable? > > Maybe this would mitigate the problem, in part, but it doesn't solve it. > > The thing is, if a dirty page belongs to a slow cgroup and a fast cgroup > issues "sync", the fast cgroup needs to wait a lot, because writeback is > happening at the speed of the slow cgroup.
Hi Andrea, But that's true only for I/O which has already been submitted to block layer, right? Any new I/O yet to be submitted could still be attributed to faster cgroup requesting sync. Until and unless cgroups limits are absurdly low, it should not take very long for already submitted I/O to finish. If yes, then in practice, it might not be a big problem? Vivek > > Ideally in this case we should bump up the writeback speed, maybe even > temporarily inherit the write rate of the sync cgroup, similarly to a > priority-inversion locking scenario, but I think it's not doable at the > moment without applying big changes. > > Or we could isolate the sync domain, meaning that a cgroup issuing a > sync will only wait for the syncing of the pages that belong to that > sync cgroup. But probably also this method requires big changes... > > -Andrea
