DEFINITIONS AND MOTIVATIONS in Software Implementation barcode code 128 in Software DEFINITIONS AND MOTIVATIONS

1.1 DEFINITIONS AND MOTIVATIONS generate, create barcode standards 128 none in software projects Overview of GS1 General Specification provides predictable perfo Code 128 Code Set B for None rmance and avoids performance overhead. A workload assigned to a set of CPUs will always have access to its assigned CPUs, and will never be required to wait until another VE completes its time slice. A resource manager can reduce wasted capacity by reassigning idle CPUs.

The amount of waste will be determined by two factors: (1) recon guration latency the time it takes to shift a CPU from one partition to another and (2) resource granularity the unconsumed partition of, at most, a single CPU. This model of CPU control is shown in Figure 1.4.

. Figure 1.4 CPU Partitioning A software scheduler such Code 128 Code Set A for None as FSS may allow the administrator to enforce minimum response times either directly or via VE prioritization. Early implementations included software schedulers for VM/XA on mainframes and BSD UNIX on VAX 11/780s in the 1980s. This approach is often the best general-purpose solution.

It is very exible, in that the minimum amount of processing power assigned to each VE can be changed while the VE is running. Moreover, a software scheduler does not force workloads to wait while unused CPU cycles are wasted. System administrators can use an FSS to enforce the assignment of a particular minimum portion of compute capacity to a speci c workload.

A quantity of shares a unitless value is assigned to each workload, as depicted in Figure 1.5. The scheduler sums the shares assigned to all of the current workloads, and divides each workload s share quantity by the sum to obtain the intended minimum portion.

. 1 . Introduction to Virtualization Web: 100 shares Database: 200 shares App 1: 200 shares App 2: 250 shares Figure 1.5 Using FSS to Ensure Minimum CPU Portions Insuf cient memory can cau se more signi cant performance problems than insuf cient CPU capacity. If a workload needs 10% more CPU time than it is currently getting, it will run 10% more slowly than expected. By comparison, if a program needs 10% more RAM than it is currently getting, it will cause excessive paging.

Such paging to the swap disk can decrease workload performance by an order of magnitude or more. Excessive memory use by one VE may starve other VEs of memory. If multiple VEs begin paging, the detrimental effects on performance can be further exacerbated by various factors:.

A shared I/O channel can be a bottleneck. If VEs share swap space, f ragmentation of the swap space can cause excessive head-seeking within the swap area. If each VE has a separate swap area but all of these areas are present on one disk drive, the drive head will continuously seek between the two swap areas..

If paging cannot be avoide code 128a for None d, swap areas should be spread across multiple drives or, if possible, placed on low-latency devices such as solid-state drives (SSDs). However, it is usually dif cult to justify the extra cost of those devices. Instead, you should try to avoid paging by con guring suf cient RAM for each VE.

Memory controls can be used to prevent one VE from using up so much RAM that another VE does not have suf cient memory. The appropriate use of memory controls should be a general practice for consolidated systems. Inappropriate use of memory controls can cause poor performance if applications are granted use of less RAM than the working set they need.

1.1 DEFINITIONS AND MOTIVATIONS to operate ef ciently. Mem code128b for None ory 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-butunused 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 calle d a memory cap, is more exible and less wasteful 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 workload performance. Other controls have been implemented on miscellaneous resources offered by the hypervisor or OS. One such resource is locked memory.

Some operating 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 insuf cient memory. Well-behaved applications will not cause problems with locked memory, but an upper bound should be set on most VEs..

Per-VE limits on network b Software ANSI/AIM Code 128 andwidth usage can be used to ensure that every VE gets access to a reasonable portion of this resource..
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