> Il giorno 5 mag 2021, alle ore 18:20, Jan Kara <jack@xxxxxxx> ha scritto: > > Hi Paolo! > > I have two processes doing direct IO writes like: > > dd if=/dev/zero of=/mnt/file$i bs=128k oflag=direct count=4000M > > Now each of these processes belongs to a different cgroup and it has > different bfq.weight. I was looking into why these processes do not split > bandwidth according to BFQ weights. Or actually the bandwidth is split > accordingly initially but eventually degrades into 50/50 split. After some > debugging I've found out that due to luck, one of the processes is decided > to be a waker of the other process and at that point we loose isolation > between the two cgroups. This pretty reliably happens sometime during the > run of these two processes on my test VM. So can we tweak the waker logic > to reduce the chances for false positives? Essentially when there are only > two processes doing heavy IO against the device, the logic in > bfq_check_waker() is such that they are very likely to eventually become > wakers of one another. AFAICT the only condition that needs to get > fulfilled is that they need to submit IO within 4 ms of the completion of > IO of the other process 3 times. > Hi Jan! as I happened to tell you moths ago, I feared some likely cover case to show up eventually. Actually, I was even more pessimistic than how reality proved to be :) I'm sorry for my delay, but I've had to think about this issue for a while. Being too strict would easily run out journald as a waker for processes belonging to a different group. So, what do you think of this proposal: add the extra filter that a waker must belong to the same group of the woken, or, at most, to the root group? Thanks, Paolo > Honza > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
