device: make sure we don't generate bogus PropertiesChanged signals

[msekletar]

No description provided.

[rmetrich]

Example of test scenario

1. Set up a system with 200 mount points on a "data" volume group

# vgcreate data /dev/sdb
# for i in $(seq 1 200); do lvcreate -n lv$i -L 10M data; mkfs.ext4 /dev/data/lv$i; echo "/dev/mapper/data-lv$i /data/lv$i ext4 defaults 0 0" >> /etc/fstab; mkdir -p /data/lv$i; done
# systemctl daemon-reload
# mount -a

2. Monitor systemd's "sendmsg" while performing 100 remounts of /data/lv1

In a terminal:

# timeout 1m strace -ttTvyy -o /tmp/systemd.strace -s 1024 -e sendmsg -p 1

In another terminal:

# for i in $(seq 1 100); do mount -o remount /data/lv1; sleep 0.1; done

3. Count the number of PropertiesChanged messages sent

# grep -c PropertiesChanged /tmp/systemd.strace

Actual results:

40600

With the patch:

200

[keszybz]

Looks OK, but the first patch seems unnecessary. Could you check if you still get the same improvement without the first patch?

[keszybz]

(I set the flag because I'm 80% sure that the first patch is unnecessary.)

[msekletar]

@keszybz I've dropped the unnecessary patch. PTAL.