Information Technology Reference
In-Depth Information
to operate efficiently. Memory controls should be used carefully and with
knowledge of actual RAM requirements.
Per-VE memory partitioning (RAM reservation or swap reservation) is
available for some virtualization implementations. This control provides
each VE with immediate access to all of its memory, but any reserved-but-
unused memory is wasted because no other VE can use it. Also, modifying the
reservation after the VE is running is not possible in some implementations.
Recently, virtual machine implementations have begun to include methods
that enable the hypervisor to reduce a guest's RAM consumption when the
system is under memory pressure. This feature causes the VE to begin
paging, but allows the guest to decide which memory pages it should page
out to the swap device.
A per-VE limit, also called a
memory cap
, is more flexible and less waste-
ful than a memory partition or reservation. The virtualization software
tracks the amount of memory in use by each VE. When a VE reaches its
cap, infrequently used pages of memory are copied to swap space for later
access, using the normal demand paging virtual memory system. There is a
potential drawback, however: As with dedicated memory partitions, overly
aggressive memory caps can cause unnecessary paging and decrease work-
load performance.
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Other controls have been implemented on miscellaneous resources offered
by the hypervisor or OS. One such resource is locked memory. Some operat-
ing systems offer applications the ability to lock data regions into memory
so that they cannot be paged out. This practice is widely used by database
software, which works best when it can lock a database's index into memory.
As a consequence, frequently used data is found in memory, not on relatively
slow disk drives. If the database is the only workload on the system, it can
choose an appropriate portion of memory to lock down, based on its needs.
There is no need to be concerned about unintended consequences.
On a consolidated system, the database software must still be able to lock
down that same amount of memory. At the same time, it must be prevented
from locking down so much more RAM than it needs that other workloads
suffer from insufficient memory.
Well-behaved applications will not cause problems with locked memory,
but an upper bound should be set on most VEs.
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Per-VE limits on network bandwidth usage can be used to ensure that every
VE gets access to a reasonable portion of this resource.
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