VMware ESX Server 2.0Features | Documentation | Knowledge Base | Discussion ForumsVMware ESX Server uses an admission control policy to ensure that sufficient unreserved memory and swap space are available before powering on a virtual machine. Memory must be reserved for the virtual machine's guaranteed minimum size; additional overhead memory is required for virtualization. Thus the total required for each virtual machine is the specified minimum plus overhead. The overhead memory size is determined automatically; it is typically 54MB for a single virtual CPU virtual machine, and 64MB for a dual-virtual CPU SMP virtual machine. Additional overhead memory is reserved for virtual machines larger than 512MB. Note: To create SMP virtual machines with ESX Server, you must also have purchased the VMware Virtual SMP for ESX Server product. For more information on the VMware Virtual SMP for ESX Server product, contact VMware, Inc. or your authorized sales representative. Swap space must be reserved on disk for the remaining virtual machine memory — that is the difference between the maximum and minimum settings. This swap reservation is required to ensure the system is able to preserve virtual machine memory under any circumstances. In practice, only a small fraction of the swap space may actually be used. Similarly, while memory reservations are used for admission control, actual memory allocations vary dynamically, and unused reservations are not wasted. The amount of swap space configured for the system limits the maximum level of overcommitment. A default swap file size equal to the physical memory size of the computer is recommended in order to support a reasonable 2x level of memory overcommitment. You may configure larger or smaller swap files or add additional swap files. If you do not configure a swap file, memory may not be overcommitted. You may configure the swap file using the VMware Management Interface (Swap Configuration in the Options page) or from the service console using the vmkfstools command. You can create additional swap files using the vmkfstools command. You should consider adding additional swap files if you want to run additional virtual machines but you're unable to do so because of the lack of swap space. See Using vmkfstools. Virtual machines are allocated their maximum memory size unless memory is overcommitted. When memory is overcommitted, each virtual machine is allocated an amount of memory somewhere between its minimum and maximum sizes. The amount of memory granted to a virtual machine above its minimum size may vary with the current memory load. The system automatically determines allocations for each virtual machine based on two factors: the number of shares it has been given and an estimate of its recent working set size. ESX Server uses a modified proportional-share memory allocation policy. Memory shares entitle a virtual machine to a fraction of physical memory. For example, a virtual machine that has twice as many shares as another is entitled to consume twice as much memory, subject to their respective minimum and maximum constraints, provided that they are both actively using the memory they have been allocated. In general, a virtual machine with S memory shares in a system with an overall total of T shares is entitled to receive at least a fraction S/T of physical memory. However, virtual machines that are not actively using their currently allocated memory automatically have their effective number of shares reduced, by levying a tax on idle memory. This "memory tax" helps prevent virtual machines from unproductively hoarding idle memory. A virtual machine is charged more for an idle page than for a page that it is actively using. The MemIdleTax configuration option provides explicit control over the policy for reclaiming idle memory. You may use this option, together with the MemSamplePeriod configuration option, to control how the system reclaims memory. However, in most cases, changes shouldn't be necessary. For complete information on using these options, see Service Console Commands. ESX Server estimates the working set for a virtual machine automatically by monitoring memory activity over successive periods of virtual machine virtual time. Estimates are smoothed over several time periods using techniques that respond rapidly to increases in working set size and more slowly to decreases in working set size. This approach ensures that a virtual machine from which idle memory has been reclaimed is be able to ramp up quickly to its full share-based allocation once it starts using its memory more actively. You can modify the default monitoring period of 30 seconds by adjusting the MemSamplePeriod configuration option. ESX Server employs two distinct techniques for dynamically expanding or contracting the amount of memory allocated to virtual machines — a VMware-supplied vmmemctl module that is loaded into the guest operating system running in a virtual machine, and swapping pages from a virtual machine to a server swap file without any involvement by the guest operating system. The preferred mechanism is the vmmemctl driver, which cooperates with the server to reclaim those pages that are considered least valuable by the guest operating system. The vmmemctl driver uses a proprietary "ballooning" technique, that provides predictable performance which closely matches the behavior of a native system under similar memory constraints. It effectively increases or decreases memory pressure on the guest operating system, causing the guest to invoke its own native memory management algorithms. When memory is tight, the guest operating system decides which particular pages to reclaim and, if necessary, swaps them to its own virtual disk. The guest operating system must be configured with sufficient swap space. Some guest operating systems have additional limitations. See the notes in Managing Memory Resources from the Service Console for details. If necessary, you can limit the amount of memory reclaimed using vmmemctl by setting the sched.mem.maxmemctl option. This option specifies the maximum amount of memory that may be reclaimed from a virtual machine in megabytes (MB). Swapping is used to forcibly reclaim memory from a virtual machine when no vmmemctl driver is available. This may be the case if the vmmemctl driver was never installed, has been explicitly disabled, is not running (for example, while the guest operating system is booting) or is temporarily unable to reclaim memory quickly enough to satisfy current system demands. Standard demand paging techniques swap pages back in when the virtual machine needs them. The vmmemctl approach is used whenever possible for optimum performance. swapping is a reliable mechanism of last resort that the system uses to reclaim memory only when necessary. If you choose to overcommit memory with ESX Server, then you need to be sure your guest operating systems have sufficient swap space. This swap space must be greater than or equal to the difference between the virtual machine's maximum and minimum sizes. Caution: If memory is overcommitted, and the guest operating system is configured with insufficient swap space, the guest operating system in the virtual machine may fail. To prevent virtual machine failure, increase the swap size in your virtual machines:
Guest operating systems with large memory and small virtual disks (for example, a virtual machine with 3.6GB RAM and a 2 GB virtual disk) are more susceptible to this problem. Many ESX Server workloads present opportunities for sharing memory across virtual machines. For example, several virtual machines may be running instances of the same guest operating system, have the same applications or components loaded, or contain common data. In such cases, ESX Server uses a proprietary transparent page sharing technique to securely eliminate redundant copies of memory pages. With memory sharing, a workload running in virtual machines often consumes less memory than it would when running on physical machines. As a result, higher levels of overcommitment can be supported efficiently. The ESX Server approach does not require any cooperation from the guest operating system. You may use the MemShareScanVM and MemShareScanTotal configuration options to control the rate at which the system scans memory to identify opportunities for sharing memory. For more information on these options, see Service Console Commands. |
