Extremely high context switches of i/o to NVM

[Keith Busch]

On Fri, 24 Jul 2015, Junjie Qian wrote:
> Hi List,
>
> I run experiment with NVM on NUMA, and found the context switch is extremely high.
>
> The platform is, 1. Linux 4.1-rc7 with multi-queue enabled, kernel is polling enabled (5 secs polling, but the results show little difference between polling and interrupt); 2. 4-socket NUMA machine; 3. Intel PC3700 NVM
>
> The command is sudo perf state -e context-switches nice -n -20 numactl -C 0 fio-master/fio --name=1 --bs=4k --ioengine=libaio --iodepth=1 --rw=read --numjobs=1 --filename=/dev/nvme0n1 --thread --direct=1 --group_reporting --time_based=1 --runtime=60
>
> The result is 3,567,428 context switches.
>
> Would someone give me some help on explaining this? Is this reasonable?
> Thanks!

Sounds about right with an IO depth of 1. You're going to get a context
switch per IO, right?