I have attended the “ Performance Management in the Virtual Data Center: Virtual Memory
Management “ Mark’s B. Friedman presentation and learned a lot. I would like to
share here a few most informative (by my opinion) citations from Mark’s paper
about
-
Memory over-commitment,
-
Memory State performance counter,
-
Ballooning, Swapping and
-
Memory Reservations.
Introduction.
“…This paper explores the strategies that VMware ESX employs to manage
machine memory, focusing on the ones that are designed to support aggressive
consolidation of virtual machine guests on server hardware..”
Memory over-commitment
“…Allowing applications to collectively commit more virtual memory
pages than are actually present in physical memory, but biasing the contents of
physical memory based on current usage patterns, permits operating systems that
support virtual memory addressing to utilize physical memory resources very
effectively…”
Memory State performance
counter
“…VMware’s current level of physical memory contention is
encapsulated in a performance counter called Memory State. This Memory State
variable is set based on the amount of Free memory available. Memory state
transitions trigger the reclamation actions reported in Table 1:
|
State
|
Value
|
Free Memory Threshold
|
Reclamation Action
|
|
High
|
0
|
> 6%
|
None
|
|
Soft
|
1
|
< 6%
|
Ballooning
|
|
Hard
|
2
|
< 4%
|
Swapping to Disk or Pages compressed
|
|
Low
|
3
|
<2 span="">
|
Blocks execution of active VMs > target allocations
|
..”
Ballooning
“…ballooning occurs when the VMware Host recognizes that
there is a shortage of machine memory and must be replenished using page
replacement. Since VMware has only limited knowledge of current page access
patterns, it is not in a position to implement an optimal LRU page replacement
strategy. Ballooning attempts to shift responsibility for page replacement to
the guest machine OS, which presumably can implement a more optimal page
replacement strategy than the VMware hypervisor.
… Using ballooning,
VMware reduces the amount of physical memory available for internal use within
the guest machine.
In Windows, when VMware’s vmmemsty.sys balloon driver
inflates, it allocates physical memory pages and pins them in physical memory
until explicitly released. To determine how effective ballooning works to
relieve a shortage of machine memory condition, it is useful to drill into the
guest machine performance counters and look for signs of increased demand
paging and other indicators of memory contention….
…ballooning successfully transforms the external
contention for machine memory that the VMware Host detects into contention for
physical memory that the Windows guest machine needs to manage internally...”
Swapping
“… VMware has recourse to steal physical memory pages
granted to a guest OS at random, which VMware terms swapping, to relieve a
serious shortage of machine memory. When free machine memory drops below a 4%
threshold, swapping is triggered..”
Memory Reservations.
“In VMware, customers do have the ability to prioritize guest machines
so that all tenants sharing an over-committed virtualization Host machine are
not penalized equally when there is a resource shortage. The most effective way
to protect a critical guest machine from being subjected to ballooning and
swapping due to a co-resident guest is to set up a machine memory Reservation.
A machine memory Reservation establishes a floor guaranteeing that a certain
amount of machine memory is always granted to the guest. With a Reservation
value set, VMware will not subject a guest machine to ballooning or swapping
that will result in the machine memory granted to the guest falling below that
minimum…”
