Patent Publication Number: US-9405642-B2

Title: Providing virtual machine migration reliability using an intermediary storage device

Description:
TECHNICAL FIELD 
     Embodiments of the present disclosure relate to a computer system, and more specifically, to a method for providing virtual machine migration reliability using an intermediary storage device. 
     BACKGROUND 
     A virtual machine (VM) is a portion of software that, when executed on appropriate hardware, creates an environment allowing the virtualization of a physical computer system. Virtualization permits multiplexing of an underlying host computer between different VMs. The host computer typically allocates a certain amount of its resources to each of the VMs. Each VM then can use the allocated resources to execute applications, including operating systems (referred to as guest operating systems (OS)). The software layer providing the virtualization is commonly referred to as a hypervisor and is also known as a virtual machine monitor (VMM), a kernel-based hypervisor, or a host operating system. The hypervisor usually emulates the underlying hardware of the host computer, making the use of the VM transparent to the guest operating system and the user of the computer. A VM may have a virtual processor, virtual system memory, virtual storage, and various virtual devices. Virtual machines may be migrated between a source host computing platform (“the source host”) and a destination host computing platform (“the destination host”) connected over a network, which may be a local-area network or a wide area-network that may include the Internet. 
     One method of migrating a virtual machine from a source host to a destination host over a network is to permit a running source VM to continue to run during the migration process, a technique known as live migration. Live migration typically permits a running VM to be moved between different physical machines without disconnecting a running client or application program. For a successful live migration, memory, storage, and network connectivity of the VM should be migrated from the source host to the destination host. While the VM on the source host is running, its memory can be copied to the destination host. Memory on the source VM can change during the memory copy process. If the memory of the source VM changes, the changed memory can be copied to the destination host. When a rate of change of the memory is below a threshold, the VM on the source can be suspended and the state of the VM, including memory, storage, and network connectivity, can be copied to the destination host. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is illustrated by way of example, and not by way of limitation, and will become apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which like reference characters refer to like parts throughout, and in which: 
         FIG. 1  is a block diagram that illustrates an example system architecture in which embodiments of the present disclosure may be implemented. 
         FIG. 2  is a flow diagram illustrating one embodiment of a method for live migration of a virtual machine from a source host to a destination host using an intermediary device. 
         FIG. 3  is a flow diagram illustrating another embodiment of a method for live migration of a virtual machine from a source host to a destination host using an intermediary device. 
         FIG. 4  is a flow diagram illustrating a further embodiment of a method for live migration of a virtual machine from a source host to a destination host using an intermediary device. 
         FIG. 5  illustrates a diagrammatic representation of a machine in an example form of a computer system within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. 
     
    
    
     DETAILED DESCRIPTION 
     Performing live migration of virtual machines between hosts generally includes a pre-copy memory migration stage having a warm-up phase and a stop-and-copy-phase followed by a post-copy memory migration stage. The migration process can fail at any of the above stages if, for example, a destination host or the network between the source host and the destination host encounters a problem. A failure after the source machine is suspended and has started to copy or transmit its memory state to the destination host can cause significant downtime because after a failure, the copy process is restarted. Downtime after a failure during live migration may be lengthy—especially for migration between geographically remote hosts—and is approximately proportional to the size of memory and applications running on the source virtual machine. While the VM is in a suspended state, attempts to access the VM can result in page-faults or network-faults, which can also result in loss or degradation of data. Techniques of the present disclosure address the above problems by utilizing an intermediary device. In particular, in the warm-up phase, a hypervisor copies all of the memory pages associated with the VM on the source host to the destination host while the VM is still running on the source host. Before or during the warm-up phase, the hypervisor can copy or synchronize memory pages associated with the VM to an intermediary device. If some memory pages change during the memory copy process, known as dirty pages, the dirty pages may be re-copied to the destination host and to the intermediary device until the rate of re-copied pages is more than or equal to the page dirtying rate. Once the copying is complete, the live migration process can enter the stop-and-copy phase. 
     During the stop-and-copy phase, the VM is stopped, and the remaining dirty pages are copied to the destination host. In one implementation, the remaining dirty pages can also be copied to the intermediary device. 
     Next, in the post-copy memory migration stage, the VM is suspended at the source host. When the VM is suspended, a minimal execution state of the source VM (CPU, registers, non-pageable memory) is transferred to the destination host. The minimal execution state of the VM can be transferred to the destination host via the intermediary device. The VM is then resumed at the destination host. 
     If a failure occurs during any migration stage or phase (e.g., pre-copy stage, warm-up phase, the stop-and-copy phase, post-copy stage) at the destination host or on the network between the source host and the destination host, instead of restarting, the VM memory can be obtained from the intermediary device. For a network failure, the destination host can obtain the VM memory from the intermediary device. For a destination host failure, another destination host can be used, which can obtain the VM memory from the intermediary device. 
     Techniques of the present disclosure that employ an intermediary device may provide a high availability solution to live migration that can reduce downtime and can be substantially faster than previous live migration methods. 
     In the following description, numerous details are set forth. It will be apparent, however, to one skilled in the art, that the present invention may be practiced without these specific details. In some instances, well-known structures and devices are shown in block diagram form, rather than in detail, in order to avoid obscuring the present invention. 
       FIG. 1  illustrates sample virtualization architecture  100  in which embodiments may operate. The virtualization architecture  100  may include a source host computer system  101   a  (the “source host  101   a ”), one or more intermediary devices  102   a ,  102   b , a first destination host computer system  101   b  (the “destination host  101   b ”) and a second destination host computer system  101   c  (the “destination host  101   c ”), each connected to a network  103 . The network  103  may be a private network (e.g., a local area network (LAN), a wide area network (WAN), intranet, etc.) or a public network (e.g., the Internet). 
     The source host  101   a  and destination host  101   b ,  101   c  may be a server, a workstation, a personal computer (PC), a mobile phone, a smart phone, a mobile computing device, a personal digital assistant (PDA), tablet, laptop computer, thin client, etc. The source host  101   a  and the destination hosts  101   b ,  101   c  can include hardware components such as one or more physical central processing units (CPUs)  130   a - 130   b , memory  135   a - 135   b , and other hardware components. 
     A source host  101   a  can include one or more virtual machines (including a virtual machine  110   a  to be migrated) and can run an operating system  105  (“host OS”) to manage its resources. Each virtual machine can run a guest operating system. In one embodiment, the source host  101   a  runs a source hypervisor  115   a  to virtualize access to the underlying source host hardware, making the use of virtual machine  110   a  transparent to the guest OS running on virtual machine  110   a  and users (e.g., a system administrator) of the source host  101   a.    
     A destination host  101   b  can include one or more virtual machines  110   b  and can run an operating system  105  (“host OS”) to manage its resources. Each virtual machine can run a guest operating system. In one embodiment, the destination host  101   b  runs a destination hypervisor  115   b  to virtualize access to the underlying destination host hardware, making the use of virtual machine(s)  110   b  transparent to the guest OS running on virtual machine  110   b  and users (e.g., a system administrator) of the source host  101   b.    
     Initially, as shown in  FIG. 1 , the virtual machine  110   a  is managed by the source hypervisor  115   a . In one embodiment, a process is provided for migrating the virtual machine  110   a  from the source host  101   a  running the source hypervisor  115   a  to the destination host  101   b  running the destination hypervisor  115   b  using at least one intermediary device  102 . 
     Intermediary device  102  can be accessible by the source host  101   a  and by the destination hosts  101   b ,  101   c . The intermediary device  102  can be part of networked storage (e.g., network attached storage (NAS), storage area network (SAN). Alternatively, the intermediary device  102  can be part of a server, a workstation, a personal computer (PC), a mobile phone, a smart phone, a mobile computing device, a personal digital assistant (PDA), tablet, laptop computer, thin client, etc. The intermediary device  102  can include hardware components such as one or more physical central processing units (CPUs), memory, and other hardware components. The intermediary device  102  can be located with the source host, within the same network as the source host, with the destination host, within the same network  150  as the destination host, or at any other location. The intermediary device  102  can be configured to perform the actions described in detail below with regard to  FIG. 4 . 
     In one embodiment, the source hypervisor  115   a  may include a source migration agent  120   a  and the destination hypervisor  115   b  may include a corresponding destination migration agent  120   b . The migration agent  120   a ,  120   b  can receive migration instructions from a host controller  125 . Migration agent  120   a ,  120   b  can also make migration decisions. Although shown as discrete components of the hypervisors  115   a - 115   b , the migration agents  120   a - 120   b  may be separate components coupled to hypervisors  115   a - 115   b.    
     The host controller  125  may reside on a designated computer system (e.g., a server computer, a desktop computer, etc.) or be part of the host machines  101   a ,  101   b ,  101   c  or another machine. Host controller  125  may include a migration manager  127  that can manage virtual machine migration by issuing instructions to migration agents  120   a  and  120   b  on hypervisors  115   a ,  115   b . The migration manager  127  on host controller  125  and migration agents  120   a ,  120   b  can work together to manage migrations, including determining a destination host machine  101   b ,  101   c , identifying and coordinating one or more intermediary devices  102 , initiating and scheduling migration, detecting and managing migration failures, etc. 
     To initiate a migration, the migration agent  120   a  of the source hypervisor  115   a  can receive a request from the migration manager  127  of the host controller  125  to migrate virtual machine  110   a  from the source host  101   a  to the destination host  101   b . The host controller  125  can issue the request after a triggering event (e.g., a system administrator&#39;s migration request, system conditions, such as resource utilization by source host exceeding a threshold). Source migration agent  120   a  can begin the pre-copy warm-up phase on source host machine  101   a  by directing memory of the virtual machine  110   a  to be copied to the destination host machine  101   b . Source migration agent  120   a  can also direct memory to be copied to or synchronized with the intermediary device  102  either before or during the warm-up phase. The host controller  125  can instruct the source host  101   a  to copy its memory to the intermediary device  102 . The intermediary device  102  can also take a more active role and can request that the source host  101   a  copy its memory to the intermediary device  102 . The intermediary device  102  can transparently copy the memory by obtaining transmissions from the source host  101   a  to the destination host  101   b . The warm-up phase is complete when a significant portion of memory is copied to the destination host  101   b  and to the intermediary device  102 . 
     Once the warm-up phase is complete, source migration agent  120   a  can initiate the stop-and-copy-phase by stopping the virtual machine and copying any dirty memory pages to the destination host  101   b . Dirty memory pages refer to memory pages that were changed by the virtual machine  110   a  after the initial copying of the memory of the virtual machine  110   a  to the destination host  101   b  and the intermediary device  102 . Source migration agent  120   a  can also issue a command to copy or synchronize any dirty pages to the intermediary device  102 . Migration manager  127  can receive a notification when the stop-and-copy-phase is successful and can inform the source migration agent  120   a  to begin the post-copy memory migration stage. 
     In the post-copy memory migration stage, the source migration agent  120   a  can issue a command to suspend the virtual machine  110   a  at the source host  101   a . When the virtual machine  110   a  is suspended, the source migration agent  120   a  can transmit a minimal execution state of the source virtual machine  110   a  (e.g., CPU, registers, a state of devices accessible to virtual machine  110   a , non-pageable memory) to the destination host  101   b . The minimal execution state of the source virtual machine can be transferred directly to the destination host  101   b  or can be transferred via one or more intermediary device  102 . The minimal execution state of the source virtual machine  110   a  transmitted directly to the destination host  101   b  can include any part of the virtual machine state that host controller  125  does not cause to be stored at the intermediary device  102  (e.g., the minimal execution state, a remaining state that does not include memory). Hence, the intermediary device  102  can store the memory of virtual machine  110   a  and optionally the minimal execution state of the virtual machine  110   a.    
     If host controller  125  detects a migration failure at any phase of migration, it can determine the cause of the failure (e.g., network-based, destination host-based). If the cause of failure is the network (e.g., the network between the source host  101   a  and the destination host  101   b  is down), the destination host  101   b  can obtain the state of the virtual machine  110   a  (or any missing portion of the state) from the intermediary device  102   a . If the cause of failure is the destination host  101   b  (e.g., abnormal operation of the destination host  101   b ), host controller  125  can identify a second destination host  101   c  and initiate migration to second destination host  101   c . Second destination host  101   c  can include the same or similar components, hardware, software and features as first destination host  101   b , including destination hypervisor  115 , destination migration agent  120 , CPU  130 , memory  135 , etc. The second destination host  101   c  can obtain the state of virtual machine stored by the intermediary device  102 . If the entire state of the virtual machine  110   a  is on the intermediary device  102 , the second destination host  101   c  can obtain it from the intermediary device  102 . When the intermediary device  102  only has the memory of the virtual machine  110   a , for example, the second destination host  101   c  can obtain other elements of the state from the source host  101   a , including the minimal execution state of the virtual machine  110   a.    
     To minimize downtime during live-migration, host controller  125  can oversee and direct the live-migration processing by issuing migration instructions to the migration agents  120  on hosts  101   a ,  101   b  and  101   c , and to intermediary device  102 . Example migration instructions to the migration agents  120  include instructions to destination migration agent  120   b  that identify the intermediary device  102  as having a state of the virtual machine  110   a . The instructions to a migration agent on second destination host  101   c  can include instructions to retrieve the state of the virtual machine  110   a  from the intermediary device  102 . Example migration instructions to intermediary device  102  can include an instruction to transmit the state of the virtual machine  110   a  to the destination host machine  101   c.    
     In alternative implementation, iintermediary device  102  can have a more active role in the migration process. For example, intermediary device  102  can receive an error message that indicates a migration failure. In response to the error message, intermediary device  102  can determine a second destination host machine  101   c  and transmit the state of the virtual machine  110   a  to the second destination host machine  101   c . Yet alternatively, migration agents  120  can have a more active role in the migration process. For example, migration agents  120  can issue or relay migration instructions to intermediary device  102 . 
     The “source” and “destination” designations for the, hosts, hypervisors and migration agents are provided for reference purposes in illustrating an exemplary implementation of the migration process according to embodiments. It will be further appreciated that depending on the particulars of a given migration event, a hypervisor may at one time serve as the source hypervisor, while at another time the hypervisor may serve as the destination hypervisor. 
       FIG. 2  is a flow diagram illustrating of one embodiment of a method  200  for live migration of a virtual machine from a source host to a destination host using an intermediary device. Although the method  200  is described in terms of live migration of a virtual machine (e.g.,  110 ) residing on the source host  101   a  to a destination host (e.g.,  101   b ,  101   c ) using an intermediary device (e.g.,  102 ), the method  200  of  FIG. 2  may be applied to migration of a virtual machine between any two hosts ( 101   a - 101   n ) over the network  103  or residing on the same host machine (e.g., source host  101   a ). Method  200  may be performed by processing logic (e.g., in computer system  500  of  FIG. 5 ) that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), firmware, or a combination thereof. In one embodiment, method  200  is performed primarily by the migration manager  127  on host controller  125  of  FIG. 1 . 
     Referring to  FIG. 2 , in one embodiment, method  200  begins when, at block  202 , a migration manager can send a request to a source host machine to migrate a virtual machine to a first destination host machine. The host controller can issue the request after a triggering event (e.g., a system administrator&#39;s migration request, system conditions, such as resource utilization by source host above a threshold). At block  204 , the migration manager can detect that the migration of the virtual machine from the source host machine to the first destination host machine has failed. The migration manager can detect that the failure was caused by the network or by the first destination host machine, as described herein. 
     At block  206 , the migration manager can identify an intermediary storage device that has a stored state of the virtual machine. The stored state can include the memory of the virtual machine. For failures with the first destination host machine, the migration manager can identify a second destination host at block  208 . For network-based failures, the migration manager can identify a second destination host at block  208 , or can issue instructions to retry migration to the first destination host machine (not shown). 
     In implementations utilizing a second destination host, at block  210  the migration manager can cause the second destination host machine to obtain the state of the virtual machine from the intermediary storage device to migrate the virtual machine to the second destination host machine. The migration manager can instruct the second destination host machine to retrieve the state of the virtual machine from the intermediary device. Alternatively, the migration manager can instruct the intermediary device to transmit the state of the virtual machine to the second destination host machine. 
     Next, at block  212  the migration manager can send a request to the source host machine to migrate a remaining state of the virtual machine to the second destination host machine. The remaining state can include: CPU, registers, a state of devices accessible to virtual machine  110 , non-pageable memory, etc. The second destination host machine can then start executing the virtual machine using the received memory of the vrtual machine and received remaining state of the virtual machine  110   a , thus completing the live migration of the virtual machine. 
     In alternative implementations in which the migration manager issues instructions to retry migration to the first destination host machine, the migration manager can instruct the first destination host to remove any previously-received memory or state of the virtual machine  110   a , and to obtain the memory from the intermediary device  102 . The migration manager can then send a request to the source host machine to migrate the remaining state of the virtual machine (if it is not stored on the intermediary device) to the first destination host machine. 
       FIG. 3  is a flow diagram illustrating of an embodiment of obtaining a state of a virtual machine. Although the method  300  is described in terms of live migration of a virtual machine (e.g.,  110 ) residing on the source host  101   a  to a destination host (e.g.,  101   b ,  101   c ) using an intermediary device (e.g.,  102 ), the method  300  of  FIG. 3  may be applied to migration of a virtual machine between any two hosts ( 101   a - 101   n ) over the network  103  or residing on the same host machine (e.g., source host  101   a ). Method  300  may be performed by processing logic (e.g., in computer system  500  of  FIG. 5 ) that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), firmware, or a combination thereof. In one embodiment, method  300  is performed primarily by the destination hypervisor (e.g.,  115   b ,  115   c ) residing on a destination host (e.g.,  101   b ,  101   c ) of  FIG. 1 . 
     Referring to  FIG. 3 , in one embodiment, method  300  begins when, at block  305 , the hypervisor at the destination host machine can obtain a state of a virtual machine from an intermediary device. The state of the virtual machine can include memory of the virtual machine. At block  310 , the hypervisor can obtain a remaining state of the virtual machine, as described herein. At block  315 , the hypervisor can start the execution of the virtual machine on the destination host using the received state of the virtual machine and the remaining state of the virtual machine. 
       FIG. 4  is a flow diagram illustrating of an embodiment of virtual machine migration using an intermediary device. Although the method  400  is described in terms of live migration of a virtual machine (e.g.,  110 ) residing on the source host  101   a  to a destination host (e.g.,  101   b ,  101   c ) using an intermediary device (e.g.,  102 ), the method  400  of  FIG. 4  may be applied to migration of a virtual machine between any two hosts ( 101   a - 101   n ) over the network  103  or residing on the same host machine (e.g., source host  101   a ). Method  400  may be performed by processing logic (e.g., in computer system  500  of  FIG. 5 ) that may comprise hardware (e.g., circuitry, dedicated logic, programmable logic, microcode, etc.), software (such as instructions run on a processing device), firmware, or a combination thereof. In one embodiment, method  400  is performed primarily by intermediary device  102  of  FIG. 1 . 
     Referring to  FIG. 4 , in one embodiment, method  400  begins when, at block  402 , the intermediary device receives a state of a virtual machine from a source host machine. The state of the virtual machine can include memory of the virtual machine. In one implementation, the intermediary device can store or cache the state of the virtual machine at block  404 . In another implementation, the intermediary device can relay the state of the virtual machine without storing it. At block  406 , intermediary device can transmit the state of the virtual machine to a destination host machine in response to an instruction from a host controller, a message from a hypervisor, or from an error message (e.g., a migration failure message). 
       FIG. 5  illustrates a diagrammatic representation of a machine in the exemplary form of a computer system  500  within which a set of instructions, for causing the machine to perform any one or more of the methodologies discussed herein, may be executed. In alternative embodiments, the machine may be connected (e.g., networked) to other machines in a local area network (LAN), an intranet, an extranet, or the Internet. The machine may operate in the capacity of a server or a client machine in a client-server network environment, or as a peer machine in a peer-to-peer (or distributed) network environment. The machine may be a personal computer (PC), a tablet PC, a set-top box (STB), a personal digital assistant (PDA), a cellular telephone, a web appliance, a server, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. Further, while only a single machine is illustrated, the term “machine” shall also be taken to include any collection of machines that individually or jointly execute a set (or multiple sets) of instructions to perform any one or more of the methodologies discussed herein. 
     The example computer system  500  includes a processing device  502 , a main memory  504  (e.g., read-only memory (ROM), flash memory, dynamic random access memory (DRAM) (such as synchronous DRAM (SDRAM) or DRAM (RDRAM), etc.), a static memory  506  (e.g., flash memory, static random access memory (SRAM), etc.), and a data storage device  518 , which communicate with each other via a bus  530 . 
     Processing device  502  represents one or more general-purpose processing devices such as a microprocessor, central processing unit, or the like. More particularly, the processing device may be complex instruction set computing (CISC) microprocessor, reduced instruction set computer (RISC) microprocessor, very long instruction word (VLIW) microprocessor, or processor implementing other instruction sets, or processors implementing a combination of instruction sets. Processing device  502  may also be one or more special-purpose processing devices such as an application specific integrated circuit (ASIC), a field programmable gate array (FPGA), a digital signal processor (DSP), network processor, or the like. Processing device  502  can be configured to execute the migration manager  127  and the migration agent  120  for performing the operations and steps discussed herein. 
     Computer system  500  may further include a network interface device  508 . Computer system  500  also may include a video display unit  510  (e.g., a liquid crystal display (LCD) or a cathode ray tube (CRT)), an alphanumeric input device  512  (e.g., a keyboard), a cursor control device  514  (e.g., a mouse), and a signal generation device  516  (e.g., a speaker). 
     Data storage device  518  may include a machine-readable storage medium (or more specifically a computer-readable storage medium)  520  having one or more sets of instructions (e.g., migration manager  127 , migration agent  120 ) embodying any one or more of the methodologies of functions described herein. The migration manager  127  and the migration agent  120  may also reside, completely or at least partially, within main memory  504  and/or within processing device  502  during execution thereof by computer system  500 ; main memory  504  and processing device  502  also constituting machine-readable storage media. The migration manager  127  and the migration agent  120  may further be transmitted or received over a network  526  via network interface device  508 . 
     Machine-readable storage medium  520  may also be used to store the device queue manager logic persistently. While machine-readable storage medium  520  is shown in an exemplary embodiment to be a single medium, the term “machine-readable storage medium” should be taken to include a single medium or multiple media (e.g., a centralized or distributed database, and/or associated caches and servers) that store the one or more sets of instructions. The term “machine-readable storage medium” shall also be taken to include any medium that is capable of storing or encoding a set of instruction for execution by the machine and that causes the machine to perform any one or more of the methodologies of the present disclosure. The term “machine-readable storage medium” shall accordingly be taken to include, but not be limited to, solid-state memories, and optical and magnetic media. 
     The components and other features described herein can be implemented as discrete hardware components or integrated in the functionality of hardware components such as ASICs, FPGAs, DSPs or similar devices. In addition, these components can be implemented as firmware or functional circuitry within hardware devices. Further, these components can be implemented in any combination of hardware devices and software components. 
     Some portions of the detailed descriptions are presented in terms of algorithms and symbolic representations of operations on data bits within a computer memory. These algorithmic descriptions and representations are the means used by those skilled in the data processing arts to most effectively convey the substance of their work to others skilled in the art. An algorithm is here, and generally, conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like. 
     It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, as apparent from the above discussion, it is appreciated that throughout the description, discussions utilizing terms such as “enabling”, “transmitting”, “requesting”, “identifying”, “querying”, “retrieving”, “forwarding”, “determining”, “passing”, “processing”, “causing”, “instructing”, “disabling”, or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical (electronic) quantities within the computer system&#39;s registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices. 
     Embodiments also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes or it may comprise a general purpose computer selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a computer readable storage medium, such as, but not limited to, any type of disk including floppy disks, optical disks, CD-ROMs and magnetic-optical disks, read-only memories (ROMs), random access memories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, flash memory devices including universal serial bus (USB) storage devices (e.g., USB key devices) or any type of media suitable for storing electronic instructions, each of which may be coupled to a computer system bus. 
     The algorithms and displays presented herein are not inherently related to any particular computer or other apparatus. Various general purpose systems may be used with programs in accordance with the teachings herein or it may prove convenient to construct more specialized apparatus to perform the required method steps. The required structure for a variety of these systems will be apparent from the description above. In addition, the present invention is not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reading and understanding the above description. Although the present invention has been described with reference to specific exemplary embodiments, it will be recognized that the invention is not limited to the embodiments described, but can be practiced with modification and alteration within the spirit and scope of the appended claims. Accordingly, the specification and drawings are to be regarded in an illustrative sense rather than a restrictive sense. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.