On Sat, May 18, 2019 at 08:39:54PM +0200, Paolo Valente wrote: > I've addressed these issues in my last batch of improvements for > BFQ, which landed in the upcoming 5.2. If you give it a try, and > still see the problem, then I'll be glad to reproduce it, and > hopefully fix it for you. Hi Paolo, I'm curious if you could give a quick summary about what you changed in BFQ? I was considering adding support so that if userspace calls fsync(2) or fdatasync(2), to attach the process's CSS to the transaction, and then charge all of the journal metadata writes the process's CSS. If there are multiple fsync's batched into the transaction, the first process which forced the early transaction commit would get charged the entire journal write. OTOH, journal writes are sequential I/O, so the amount of disk time for writing the journal is going to be relatively small, and especially, the fact that work from other cgroups is going to be minimal, especially if hadn't issued an fsync(). In the case where you have three cgroups all issuing fsync(2) and they all landed in the same jbd2 transaction thanks to commit batching, in the ideal world we would split up the disk time usage equally across those three cgroups. But it's probably not worth doing that... That being said, we probably do need some BFQ support, since in the case where we have multiple processes doing buffered writes w/o fsync, we do charnge the data=ordered writeback to each block cgroup. Worse, the commit can't complete until the all of the data integrity writebacks have completed. And if there are N cgroups with dirty inodes, and slice_idle set to 8ms, there is going to be 8*N ms worth of idle time tacked onto the commit time. If we charge the journal I/O to the cgroup, and there's only one process doing the dd if=/dev/zero of=/root/test.img bs=512 count=10000 oflags=dsync then we don't need to worry about this failure mode, since both the journal I/O and the data writeback will be hitting the same cgroup. But that's arguably an artificial use case, and much more commonly there will be multiple cgroups all trying to at least some file system I/O. - Ted
