qemu-img attempts to launch multiple threads by default *and*
attempts to have multiple memory allocation arenas to operate
from. While multithreading can be good for performance, this
pattern and the memory footprint for process launch and
dependencies can turn the memory footprint for a cirros image
conversion (16MB) into 1.2GB of memory being asked for by the
qemu-img tool.
In order to limit this impact, as the default number of arenas
is governed by the number of CPUs times the number 8, it seems
reasonable to lower this to a more reasonable number which
also helps keep our possible memory footprint from being exceeded.
NOTE: This change is largely different than the original change
as an intermediate change converted write_image.sh to python.
As it is unlikely for us to backport the intermediate change,
it is logical for us to just modify the original script.
Otherwise the release note is ultimately what is backported for
release note tooling continutity.
Change-Id: I71a28ec59ec31c691205eb34d9fcab63a2ccb682
Story: 2008928
Task: 42528
(cherry picked from commit 9e4c7052a2)
(cherry picked from commit 9c20cca362)
ee8118423b