Chapter 4. Control Group Application Examples

Procedure 4.1. I/O throughput prioritization

1. Attach the blkio subsystem to the /cgroup/blkio cgroup:

   ~]# mkdir /cgroup/blkio
   ~]# mount -t cgroup -o blkio blkio /cgroup/blkio

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2. Create a high and low priority cgroup:

   ~]# mkdir /cgroup/blkio/high_prio
   ~]# mkdir /cgroup/blkio/low_prio

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3. Acquire the PIDs of the processes that represent both virtual guests (in which the database servers are running) and move them to their specific cgroup. In our example, VM_high represents a virtual guest running a high priority database server, and VM_low represents a virtual guest running a low priority database server. For example:

   ~]# ps -eLf | grep qemu | grep VM_high | awk '{print $4}' | while read pid; do echo $pid >> /cgroup/blkio/high_prio/tasks; done
   ~]# ps -eLf | grep qemu | grep VM_low | awk '{print $4}' | while read pid; do echo $pid >> /cgroup/blkio/low_prio/tasks; done

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4. Set a ratio of 10:1 for the high_prio and low_prio cgroups. Processes in those cgroups (that is, processes running the virtual guests that have been added to those cgroups in the previous step) will immediately use only the resources made available to them.

   ~]# echo 1000 > /cgroup/blkio/high_prio/blkio.weight
   ~]# echo 100 > /cgroup/blkio/low_prio/blkio.weight

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   In our example, the low priority cgroup permits the low priority database server to use only about 10% of the I/O operations, whereas the high priority cgroup permits the high priority database server to use about 90% of the I/O operations.