[PATCHES]: dm lock optimization

[Mikulas Patocka <mpatocka@redhat.com>]

Hi

I placed dm lock optimization patches here

http://people.redhat.com/mpatocka/patches/kernel/dm-lock-optimization/

The patches remove cache-line bouncing from device mapper when multiple 
processors submit requests simultaneously.


I was able to measure performance improvement, although in a very specific 
setup (it is unmeasurable with normal disks).

The setup to measure it is:
- two quad-core Opterons
- use ramdisk as a block device, leave it empty
- create 11 nested dm-linear mappings on it (so that each i/o passes 
through 11 levels of dm-linear and then goes to the ramdisk)
- run fio with 8 threads, using direct-io to read 512-byte blocks
(time fio --rw=randread --size=1G --bsrange=512-512 --direct=1 
--filename=/dev/mapper/dm-test --name=job1 --name=job2 --name=job3 
--name=job4 --name=job5 --name=job6 --name=job7 --name=job8)

With this setup it can be measured that the patches help:

no patch (3.3 kernel):  68.9s
patch 1:                64.3s
patch 1,2:              52.3s
patch 1,2,3:            45.2s
patch 1,2,3,4:          41.2s
patch 1,2,3,4,5:        34.0s

Mikulas