Re: [PATCH 1/2] loop: Prevent that an I/O scheduler is assigned

On Mon, Aug 02, 2021 at 10:23:56PM -0700, Bart Van Assche wrote:

On 8/2/21 6:54 PM, Ming Lei wrote:

quoted

On Mon, Aug 02, 2021 at 05:01:59PM -0700, Bart Van Assche wrote:

quoted

Loop devices have a single hardware queue. Hence, the block layer function
elevator_get_default() selects the mq-deadline scheduler for loop devices.
Using the mq-deadline scheduler or any other I/O scheduler for loop devices
incurs unnecessary overhead. Make the loop driver pass the flag
BLK_MQ_F_NOSCHED to the block layer core such that no I/O scheduler can be
associated with block devices. This approach has an advantage compared to
letting udevd change the loop I/O scheduler to none, namely that
synchronize_rcu() does not get called.

It is intentional that the flag BLK_MQ_F_SHOULD_MERGE is preserved.

This patch reduces the Android boot time on my test setup with 0.5 seconds.

Can you investigate why none reduces Android boot time? Or reproduce &
understand it by a fio simulation on your setting?

Hi Ming,

The software process called apexd creates multiple loop devices while
the device is booting. Using BLK_MQ_F_NO_SCHED is faster than letting
apexd change the I/O scheduler from mq-deadline into 'none' since the
latter involves calling synchronize_rcu() once per loop device.

OK, but why does apexd switch to none during booting? Does none perform
better than deadline during booting in the Android setting?


Thanks,
Ming