Red Hat Enterprise Linux (RHEL) 6 and Red Hat Enterprise Virtualization (RHEV) 2.2 offer some options for memory overcommit, but these features can interfere with one another in some scenarios, according to IBM engineers.
Red Hat and IBM formally announced a partnership to develop RHEL KVM and RHEV together at last week’s Red Hat Summit in Boston. Also at the show, Joel Schopp, a Linux software engineer for IBM, presented an open-source utility, Memory Overcommit Manager (MOM), meant to reduce the interference between the memory overcommit features in RHEL and RHEV.
MOM focuses on balancing two key features, Kernel Same-page Merging (KSM) and memory ballooning. KSM compresses like memory pages, and ballooning allows KVM to reclaim unused memory from a guest. KSM can limit the effectiveness of ballooning, because there’s currently no way to communicate from a guest whether a memory page is shared, Schopp said. And ballooning can also hurt the performance of VMs if not used properly, he added.
MOM is an IBM open source project available for download and trial on the Web, but it’s not approved for any major Linux distributions yet. It’s a daemon written in Python that can turn knobs in the Red Hat operating system as performance data comes in.
“It seems to behave pretty well from our experience, but … the tool is still in the development process, and your mileage may vary,” Schopp cautioned.