Subject: [PATCH 0/3] cfq-iosched: Fair cross-group preemption
Date: Tuesday 22nd March 2011 01:10:42 UTC (over 5 years ago)
This patchset introduces fair cross-group preemption. Right now, we don't have strict isolation between processes in different cgroups. For example: currently an RT ioprio thread in one group can preempt a BE ioprio thread in another group. We would like to have an application specify the relative priorities of its threads, but still allow strict isolation between applications. With this patch series, we can configure an entire cgroup as needing low-latency service. Then that group will be able to preempt another group. To prevent a runaway application from starving other applications, we allow this preemption only until it has exceeded its fair share (as specified by its relative weight). So a rate-limited, but latency sensative application (like streaming audio or video) can get front-of-the-line service without fear of it hogging a whole disk's IO. Note that this series is targeted to shallow-queue devices (e.g. a single spinning disk). Without hardware support, it is much more difficult to provide low-latency on a device with a deep request queue. The following bash script demonstrates the feature: cd /dev/cgroup mkdir c1 c2 ( echo '8:16 900' > c1/blkio.weight_device echo '8:16 2' > c1/blkio.class echo $BASHPID > c1/tasks dd if=/dev/sdb of=/dev/null bs=1M iflag=direct ) & ( echo '8:16 100' > c2/blkio.weight_device echo '8:16 1' > c2/blkio.class echo $BASHPID > c2/tasks while : ; do dd if=/dev/sdb of=/dev/null bs=1M count=1 iflag=direct; sleep .1 ; done ) & sleep 10 kill %1 %2 grep 8:16 c?/blkio* # end Since the c2 reader is "well behaved" and only reads 10MB/s, it will be able to consistenly preempt c1, and therefore see low latency for its requests.