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[dm-devel] [PATCH] dm: dm_table_unplug_all optimization

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.

Michael Anderson
andmike linux vnet ibm com

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