This patch series provides a optimization to the dm_table_unplug_all function in device mapper (dm). A OLTP type workload on a 16core Intel box using a volume containing 48 logical drives across 3 dual port fiber adapters showed some high oprofile values. A high oprofile sampling value was seen in dm_table_unplug_all coming from high contention of the q->queue_lock. Since dm_table_unplug_all will unplug all devices in the table it can cause contention on the queue_lock with processes trying to pull requests off the queue even on devices that are not plugged. As dm_table_unplug_all is coded now blk_unplug will result in the queue_lock being taken for all checks of the queue_flags (blk_remove_plug). The change lead to a ~16 percent performance improvement on a RedHat distro kernel. It appears to be a fairly safe change as the worst case if dm missed a queue to unplug, the queue will get unplugged when the number of requests exceed (unplug_thresh (4) or unplug_delay (3 ms). It can also be the case that if post getting the queue_lock and checking for the queue to be plugged that once the lock is dropped plugging can occur. -andmike -- Michael Anderson andmike@xxxxxxxxxxxxxxxxxx -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
