Need fix performance issue during spawn with powervm driver

[ChangBo Guo(gcb)]

nova-compute is single threaded , and powervm driver will caculate image file's checksum ,then ftp the image to the IVM, maybe need compress and decompress the image file . In this situation , nova-compute can't process any other work .

The following is suggestion from libvirt developer :
"We have code similar to this in libvirt, and people complain about it all the time. The big problem is that if an image is large, nova-compute is going to lock up calculating this checksum. Because nova-compute is single threaded, you wont process any other work while performing this calculation.
I wonder if you should add some time.sleep(0) calls? This would act as a yield of the thread for eventlet."

[ChangBo Guo(gcb) (glongwave)]

I open this bug for tracking the performence issue mentioned in code review https://review.openstack.org/#/c/27534/

[ChangBo Guo(gcb) (glongwave)]

We need double check all the operations with powervm driver to find potential performance issue.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/36196

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/36196
Committed: http://github.com/openstack/nova/commit/c4dcc659419a371e0d7316b831c5ce21a0c3c947
Submitter: Jenkins
Branch: master

commit c4dcc659419a371e0d7316b831c5ce21a0c3c947
Author: Chang Bo Guo <email address hidden>
Date: Tue Jul 9 01:01:07 2013 -0700

    Fix blocking issue when powervm calculate checksum

    In powervm driver, it takes long time to calculate file checksum
    before and after transferring image file. Because nova-compute is
    a single thread, it won't process any other work at the same time.
    Here give other threads chance to run by using time.sleep(0), that
    acts as a yield of the thread for eventlet.

    Fixes bug #1181895

    Change-Id: I79fbd9d7d4fb7791ca599b136cf9945dec8996e2