Hello list, Today I was trying to figure out how to get block I/O prioritization working for a certain process. The process is a streaming server that reads a big file stored in a filesystem (xfs) on top of a RAID5 configuration using 3 disks, using O_DIRECT. I'm setting up cgroups this way: $ echo 1000 > /sys/fs/cgroup/blkio/prio/blkio.weight $ echo 10 > /sys/fs/cgroup/blkio/blkio.leaf_weight meaning that all the tasks in the prio cgroup will have unconstrained access time to the disk, while all the other tasks will have their disk access time weighted by a factor. If I ignore the RAID5 setup, create a XFS filesystem on /dev/sdb2, mount it on /data and put my streaming daemon in the prio cgroup and run the daemon by streaming around 250MiB/s of data, while I launch fio with disk I/O intensive tasks. For a period of 5 minutes, the streaming deamon had to stop streaming in about 5 times to rebuffer. Now, if I consider the same scenario but using the RAID5 device and letting the daemon stream 500MiB/s of data (because the RAID has around twice the throughput of a single drive), after a period of 5 minutes the streaming daemon had to stop streaming in about 50 times! This is 10 times more than the single drive case. While streaming, I observed both blkio.sectors and blkio.io_queued for both cgroups (the root node and prio). If only the streaming daemon is run (therefore fio is stopped), the sector count in prio/blkio.sectors increases while (root)/blkio.sectors does not. This confirms the streaming daemon is correctly identified as in the prio cgroup. Then, while both the streaming daemon and fio run, observing io_queued shows that for the root cgroup there is about 50 queued request in total (in average), while for the prio cgroup there is only one ocasional delayed request from time to time. $ uname -a Linux haswell1 3.10.10 #9 SMP PREEMPT Fri Nov 29 11:38:20 CET 2013 i686 GNU/Linux Any ideas? Thanks, -- Martin Boutin -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
