Performance collection compensation

A method, apparatus, system, and signal-bearing medium that, in an embodiment, determine a first number of execution cycles used by a calibration program for an event group with a collector inactive, determine a second number of execution cycles used by the calibration program for the event group with the collector active, and calculate a compensation value for the event group based on the difference between the second number and the first number of execution cycles. These determinations and calculations may be repeated for any number of event groups. When performance data is subsequently collected by the collector, collected events that belong to the event groups have their collected execution cycles adjusted based on the calculated compensation values. In various embodiments, a decision is made whether to determine the first and second numbers of execution cycles and calculate the compensation values based on whether a configuration of the computer has changed, whether resource allocation among logical partitions of the computer has changed, or whether a resource was activated under a capacity on demand plan. In this way, the performance effects of the collector may be accounted for in the collected performance data.

FIELD

This invention generally relates to computer systems and more specifically relates to performance data collection compensation in computer systems.

BACKGROUND

The development of the EDVAC computer system of 1948 is often cited as the beginning of the computer era. Since that time, computer systems have evolved into extremely sophisticated devices that may be found in many different settings. Computer systems typically include a combination of hardware (e.g., semiconductors, circuit boards, etc.) and software (e.g., computer programs). As advances in semiconductor processing and computer architecture push the performance of the computer hardware higher, more sophisticated computer software has evolved to take advantage of the higher performance of the hardware, resulting in computer systems today that are much more powerful than just a few years ago. One significant advance in computer technology is the development of parallel processing, i.e., the performance of multiple tasks in parallel.

A number of computer software and hardware technologies have been developed to facilitate increased parallel processing. From a hardware standpoint, computers increasingly rely on multiple microprocessors to provide increased workload capacity. Furthermore, some microprocessors have been developed that support the ability to execute multiple threads in parallel, effectively providing many of the same performance gains attainable through the use of multiple microprocessors. From a software standpoint, multithreaded operating systems and kernels have been developed, which permit computer programs to concurrently execute in multiple threads, so that multiple tasks can essentially be performed at the same time.

In addition, some computers implement the concept of logical partitioning, where a single physical computer is permitted to operate essentially like multiple and independent virtual computers, referred to as logical partitions, with the various resources in the physical computer (e.g., processors, memory, and input/output devices) allocated among the various logical partitions. Each logical partition executes a separate operating system, and from the perspective of users and of the software applications executing on the logical partition, operates as a fully independent computer.

Because each logical partition is essentially competing with other logical partitions for the limited resources of the computer, users are especially interested in monitoring the partitions in order to ensure that they are achieving satisfactory performance. A performance data collection tool that collects detailed performance metrics is often used for this purpose. Since the tool executes on the computer system using the same resources that the partitions may also use, the tool impacts the performance that it is measuring. In order to account for this performance impact and provide an accurate portrayal of the performance characteristics of the computer system being analyzed, the tool needs to accurately quantify the effects that the tool itself has on the collected metrics.

Current performance data collection tools use static compensation values to adjust the collected metrics in order to account for the performance impact of the tools. But, because of the dynamic nature of resource allocation between logical partitions, the environment in which the tool is executing and which the tool is measuring can change quickly and dramatically. Thus, static compensation values might not always provide the desired degree of accuracy.

Without a better technique for adjusting collected performance metrics based on the environment that exists at the time of the collection activity, users will continue to suffer from performance metrics that do not provide the desired degree of accuracy. Although the aforementioned problems have been described in the context of a logically partitioned computer system, they may occur in any environment in which the characteristics of the system being measured can change.

SUMMARY

A method, apparatus, system, and signal-bearing medium are provided that, in an embodiment, determine a first number of execution cycles used by a calibration program for an event group with a collector inactive, determine a second number of execution cycles used by the calibration program for the event group with the collector active, and calculate a compensation value for the event group based on the difference between the second number and the first number of execution cycles. These determinations and calculations may be repeated for any number of event groups. When performance data is subsequently collected by the collector, collected events that belong to the event groups have their collected execution cycles adjusted based on the calculated compensation values. In various embodiments, a decision is made whether to determine the first and second numbers of execution cycles and calculate the compensation values based on whether a configuration of the computer has changed, whether resource allocation among logical partitions of the computer has changed, or whether a resource was activated under a capacity on demand plan. In this way, the performance effects of the collector may be accounted for in the collected performance data.

It is to be noted, however, that the appended drawings illustrate only example embodiments of the invention, and are therefore not considered limiting of its scope, for the invention may admit to other equally effective embodiments.

DETAILED DESCRIPTION

In an embodiment, a performance data collection tool determines compensation values in response to changes in the computer system that may have an effect on the compensation values. These changes include, but are not limited to, processor frequency adjustments, partition migration actions, processor configuration changes, resource allocation changes, resource activations and deactivations under an on demand capacity plan, and memory configuration changes. A user interface may also force the performance collection tools to regenerate compensation values. This user interface may be useful, e.g., in situations where the user wants the compensation values for a particular performance data collection action to be matched as close as possible with the current state of the computing system environment, even if the performance data collection tool has not automatically determined that an adjustment is necessary.

Components in the computer system that cause changes to the current operating environment communicate to the performance data collection tool via an indicator, or other communication means, that such a change in the system has occurred. This indicator is tested, e.g., by the collection tool when a collection action is initiated, such as by a user or other application. If the indicator is set, the tool initiates a sequence of collections using calibration programs with known performance characteristics. The calibration programs are first run without a performance metric collector active and then run with the performance metric collector active. The collection tool compares the execution cycles for the calibration programs and calculates compensation values for groups of events based on the difference between the execution cycles. The collection tool then uses these compensation values on the user requested collection and on subsequent collections. These updated compensation values allow the collection tool to accurately report performance metrics taking into account the effects that the collection tool has on the metrics.

Referring to the Drawings, wherein like numbers denote like parts throughout the several views,FIG. 1depicts a high-level block diagram representation of a client computer system100connected via a network130to a server computer system132, according to an embodiment of the present invention. The designations “client” and “server” are used for convenience only, and, in an embodiment, a computer that operates as a client to one computer may operate as server to another-computer, and vice versa. In an embodiment, the hardware components of the computer system100may be implemented by an IBM eServer iSeries or pSeries computer system. However, those skilled in the art will appreciate that the mechanisms and apparatus of embodiments of the present invention apply equally to any appropriate computing system.

The major components of the computer system100include one or more processors101, a main memory102, a terminal interface111, a storage interface112, an I/O (Input/Output) device interface113, and communications/network interfaces114, all of which are coupled for inter-component communication via a memory bus103, an I/O bus104, and an I/O bus interface unit105.

The computer system100contains one or more general-purpose programmable central processing units (CPUs)101A,101B,101C, and101D, herein generically referred to as the processor101. In an embodiment, the computer system100contains multiple processors typical of a relatively large system; however, in another embodiment the computer system100may alternatively be a single CPU system. Each processor101executes instructions stored in the main memory102and may include one or more levels of on-board cache.

The main memory102is a random-access semiconductor memory for storing data and programs. In another embodiment, the main memory102represents the entire virtual memory of the computer system100, and may also include the virtual memory of other computer systems coupled to the computer system100or connected via the network130. The main memory102is conceptually a single monolithic entity, but in other embodiments the main memory102is a more complex arrangement, such as a hierarchy of caches and other memory devices. For example, memory may exist in multiple levels of caches, and these caches may be further divided by function, so that one cache holds instructions while another holds non-instruction data, which is used by the processor or processors. Memory may be further distributed and associated with different CPUs or sets of CPUs, as is known in any of various so-called non-uniform memory access (NUMA) computer architectures.

The memory102is illustrated as containing the primary software components and resources utilized in implementing a logically partitioned computing environment on the computer100, including a plurality of logical partitions134managed by a partition manager or hypervisor136. The memory102further includes a capacity on demand manager135, which allows customers to effectively rent or lease resources, e.g., processors, memory, or other resources, on an as-needed or on-demand basis. More particularly, customers may temporarily enable standby resources that are initially dormant or unused within their machine. Where desired, the standby resources are not included in the up-front, baseline cost of the machine. As such, for a relatively smaller initial capital investment, a customer may activate and deactivate standby or on-demand resources as needed for a fee, which provides the customer with customized access and optimized usage.

Although the partitions134, the capacity on demand manager135, and the hypervisor136are illustrated as being contained within the memory102in the computer system100, in other embodiments some or all of them may be on different computer systems, e.g., the server132, and may be accessed remotely, e.g., via the network130. Further, the computer system100may use virtual addressing mechanisms that allow the programs of the computer system100to behave as if they only have access to a large, single storage entity instead of access to multiple, smaller storage entities. Thus, while the partitions134, the capacity on demand manager135, and the hypervisor136are illustrated as residing in the memory102, these elements are not necessarily all completely contained in the same storage device at the same time.

Each of the logical partitions134utilizes an operating system142, which controls the primary operations of the logical partition134in the same manner as the operating system of a non-partitioned computer. For example, each operating system142may be implemented using the i5OS operating system available from International Business Machines Corporation, but in other embodiments the operating system142may be Linux, AIX, UNIX, Microsoft Windows, or any appropriate operating system. Also, some or all of the operating systems142may be the same or different from each other. Any number of logical partitions134may be supported as is well known in the art, and the number of the logical partitions134resident at any time in the computer100may change dynamically as partitions are added or removed from the computer100.

Each of the logical partition134executes in a separate, or independent, memory space, and thus each logical partition acts much the same as an independent, non-partitioned computer from the perspective of each application144that executes in each such logical partition. As such, user applications typically do not require any special configuration for use in a partitioned environment. Given the nature of logical partitions134as separate virtual computers, it may be desirable to support inter-partition communication to permit the logical partitions to communicate with one another as if the logical partitions were on separate physical machines. As such, in some implementations it may be desirable to support an unillustrated virtual local area network (LAN) adapter associated with the hypervisor136to permit the logical partitions134to communicate with one another via a networking protocol such as the Ethernet protocol. In another embodiment, the virtual network adapter may bridge to a physical adapter, such as the network interface adapter114. Other manners of supporting communication between partitions may also be supported consistent with embodiments of the invention.

Although the hypervisor136is illustrated as being within the memory102, in other embodiments, all or a portion of the hypervisor102may be implemented in firmware or hardware. The hypervisor136may perform both low-level partition management functions, such as page table management and may also perform higher-level partition management functions, such as creating and deleting partitions, concurrent I/O maintenance, allocating processors, memory and other hardware or software resources to the various partitions134.

The hypervisor136statically and/or dynamically allocates to each logical partition134a portion of the available resources in computer100. For example, each logical partition134may be allocated one or more of the processors101and/or one or more hardware threads, as well as a portion of the available memory space. The logical partitions134can share specific software and/or hardware resources such as the processors101, such that a given resource may be utilized by more than one logical partition. In the alternative, software and hardware resources can be allocated to only one logical partition134at a time. Additional resources, e.g., mass storage, backup storage, user input, network connections, and the I/O adapters therefor, are typically allocated to one or more of the logical partitions134. Resources may be allocated in a number of manners, e.g., on a bus-by-bus basis, or on a resource-by-resource basis, with multiple logical partitions sharing resources on the same bus. Some resources may even be allocated to multiple logical partitions at a time. The resources identified herein are examples only, and any appropriate resource capable of being allocated may be used.

The capacity on demand manager135activates resources (previously described above) that are present at the computer system100, but dormant or not used, so that the resource may be used and allocated to one or more of the partitions134. In another embodiment, the partitions134are not present or not used, and the hypervisor136activates a resource or resources, so that they are available for use by the entire computer system100. In another embodiment, the capacity on demand manager135may activate resources for a network of connected computers, e.g., the network130.

In an embodiment, at least one of the applications144is a performance data collection tool, as further described below with reference toFIG. 2. The application144includes instructions capable of executing on the processor101or statements capable of being interpreted by instructions executing on the processor101to perform the functions as further described below with reference toFIGS. 3,4,5, and6. In another embodiment, the application144may be implemented in microcode or firmware. In another embodiment, the application144may be implemented in hardware via logic gates and/or other appropriate hardware techniques. Each of the applications144illustrated inFIG. 1may be the same or some or all of them may be different from each other. Further, each of the partitions134may include multiple of the applications144.

The memory bus103provides a data communication path for transferring data among the processor101, the main memory102, and the I/O bus interface unit105. The I/O bus interface unit105is further coupled to the system I/O bus104for transferring data to and from the various I/O units. The I/O bus interface unit105communicates with multiple I/O interface units111,112,113, and114, which are also known as I/O processors (IOPs) or I/O adapters (IOAs), through the system I/O bus104. The system I/O bus104may be, e.g., an industry standard PCI bus, or any other appropriate bus technology.

The I/O interface units support communication with a variety of storage and I/O devices. For example, the terminal interface unit111supports the attachment of one or more user terminals121,122,123, and124. The storage interface unit112supports the attachment of one or more direct access storage devices (DASD)125,126, and127(which are typically rotating magnetic disk drive storage devices, although they could alternatively be other devices, including arrays of disk drives configured to appear as a single large storage device to a host). The contents of the main memory102may be stored to and retrieved from the direct access storage devices125,126, and127.

The I/O and other device interface113provides an interface to any of various other input/output devices or devices of other types. Two such devices, the printer128and the fax machine129, are shown in the exemplary embodiment ofFIG. 1, but in other embodiment many other such devices may exist, which may be of differing types. The network interface114provides one or more communications paths from the computer system100to other digital devices and computer systems; such paths may include, e.g., one or more networks130.

Although the memory bus103is shown inFIG. 1as a relatively simple, single bus structure providing a direct communication path among the processors101, the main memory102, and the I/O bus interface105, in fact the memory bus103may comprise multiple different buses or communication paths, which may be arranged in any of various forms, such as point-to-point links in hierarchical, star or web configurations, multiple hierarchical buses, parallel and redundant paths, or any other appropriate type of configuration. Furthermore, while the I/O bus interface105and the I/O bus104are shown as single respective units, the computer system100may in fact contain multiple I/O bus interface units105and/or multiple I/O buses104. While multiple I/O interface units are shown, which separate the system I/O bus104from various communications paths running to the various I/O devices, in other embodiments some or all of the I/O devices are connected directly to one or more system I/O buses.

The computer system100depicted inFIG. 1has multiple attached terminals121,122,123, and124, such as might be typical of a multi-user “mainframe” computer system. Typically, in such a case the actual number of attached devices is greater than those shown inFIG. 1, although the present invention is not limited to systems of any particular size. The computer system100may alternatively be a single-user system, typically containing only a single user display and keyboard input, or might be a server or similar device which has little or no direct user interface, but receives requests from other computer systems (clients). In other embodiments, the computer system100may be implemented as a personal computer, portable computer, laptop or notebook computer, PDA (Personal Digital Assistant), tablet computer, pocket computer, telephone, pager, automobile, teleconferencing system, appliance, or any other appropriate type of electronic device.

The network130may be any suitable network or combination of networks and may support any appropriate protocol suitable for communication of data and/or code to/from the computer system100and the server132. In various embodiments, the network130may represent a storage device or a combination of storage devices, either connected directly or indirectly to the computer system100. In an embodiment, the network130may support Infiniband. In another embodiment, the network130may support wireless communications. In another embodiment, the network130may support hard-wired communications, such as a telephone line or cable. In another embodiment, the network130may support the Ethernet IEEE (Institute of Electrical and Electronics Engineers) 802.3x specification. In another embodiment, the network130may be the Internet and may support IP (Internet Protocol).

In another embodiment, the network130may be a local area network (LAN) or a wide area network (WAN). In another embodiment, the network130may be a hotspot service provider network. In another embodiment, the network130may be an intranet. In another embodiment, the network130may be a GPRS (General Packet Radio Service) network. In another embodiment, the network130may be a FRS (Family Radio Service) network. In another embodiment, the network130may be any appropriate cellular data network or cell-based radio network technology. In another embodiment, the network130may be an IEEE 802.11B wireless network. In still another embodiment, the network130may be any suitable network or combination of networks. Although one network130is shown, in other embodiments any number (including zero) of networks (of the same or different types) may be present.

FIG. 1is intended to depict the representative major components of the computer system100, the network130, and the server132at a high level; individual components may have greater complexity than represented inFIG. 1; components other than or in addition to those shown inFIG. 1may be present; and the number, type, and configuration of such components may vary. Several particular examples of such additional complexity or additional variations are disclosed herein; it being understood that these are by way of example only and are not necessarily the only such variations.

The various software components illustrated inFIG. 1and implementing various embodiments of the invention may be implemented in a number of manners, including using various computer software applications, routines, components, programs, objects, modules, data structures, etc., referred to hereinafter as “computer programs,” or simply “programs.” The computer programs typically comprise one or more instructions that are resident at various times in various memory and storage devices in the computer system100, and that, when read and executed by one or more processors101in the computer system100, cause the computer system100to perform the steps necessary to execute steps or elements comprising the various aspects of an embodiment of the invention.

Moreover, while embodiments of the invention have and hereinafter will be described in the context of fully-functioning computer systems, the various embodiments of the invention are capable of being distributed as a program product in a variety of forms, and the invention applies equally regardless of the particular type of signal-bearing medium used to actually carry out the distribution. The programs defining the functions of this embodiment may be delivered to the computer system100via a variety of signal-bearing media, which include, but are not limited to:

(1) information permanently stored on a non-rewriteable storage medium, e.g., a read-only memory device attached to or within a computer system, such as a CD-ROM, DVD-R, or DVD+R;

(2) alterable information stored on a rewriteable storage medium, e.g., a hard disk drive (e.g., the DASD125,126, or127), CD-RW, DVD-RW, DVD+RW, DVD-RAM, or diskette; or (3) information conveyed by a communications medium, such as through a computer or a telephone network, e.g., the network130, including wireless communications.

Such signal-bearing media, when carrying machine-readable instructions that direct the functions of the present invention, represent embodiments of the present invention.

Embodiments of the present invention may also be delivered as part of a service engagement with a client corporation, nonprofit organization, government entity, internal organizational structure, or the like. Aspects of these embodiments may include configuring a computer system to perform, and deploying software systems and web services that implement, some or all of the methods described herein. Aspects of these embodiments may also include analyzing the client company, creating recommendations responsive to the analysis, generating software to implement portions of the recommendations, integrating the software into existing processes and infrastructure, metering use of the methods and systems described herein, allocating expenses to users, and billing users for their use of these methods and systems.

In addition, various programs described hereinafter may be identified based upon the application for which they are implemented in a specific embodiment of the invention. But, any particular program nomenclature that follows is used merely for convenience, and thus embodiments of the invention should not be limited to use solely in any specific application identified and/or implied by such nomenclature.

The exemplary environments illustrated inFIG. 1are not intended to limit the present invention. Indeed, other alternative hardware and/or software environments may be used without departing from the scope of the invention.

FIG. 2depicts a block diagram of an example performance data collection tool144-1, according to an embodiment of the invention. The performance data collection tool144-1is an example of the application144, as previously described above with reference toFIG. 1. The performance data collection tool144-1includes a compensator205, a collector210, calibration programs215, event data220, and compensation data225.

The calibration programs215are a predefined set of applications with known performance characteristics that exercise functions in the computer100, the network130, and/or the server132in order to cause events that the collector210may detect (if the collector210is active) and record in the event data220. The compensator205adjusts the event data220using the compensation data225.

The event data220includes records230and235, but in other embodiments any number of records with any type of data may be present. The records230and235are created by the collector210in response to events caused by the software and/or hardware of the computer system100, the network130, and/or the server132. Each of the records230and235includes an event field240, an execution cycles field245, and optional other data250. The event field240indicates a type of the event encountered, performed, generated by the hardware and/or software of the computer system100, the network130, and/or the server132, as detected by the collector210. The execution cycles245indicates the number of processor cycles used, e.g., by the processor101while encountering, performing, generating, or processing the event240. The other data250may include any other type of data associated with the event240.

The compensation data225includes records255,260, and265, but in other embodiments any number of records with any type of data may be present. Each of the records255,260, and265includes an event group identifier270and a compensation value275. The event group identifier270indicates a group to which the events described in the event240belong. For example, the task switch-in event of record230and the task switch-out event of record235both belong to the task switching group of record255. The compensation value field275indicates a number by which the execution cycles245needs to be adjusted in order to accurately reflect the number of processor cycles used by events that belong to the associated event group identifier270. The compensator205determines the compensation values275as further described below with reference toFIGS. 5 and 6. Thus, all events240associated with a particular event group270share the same compensation value275.

FIG. 3depicts a flowchart of example processing for responding to a configuration change, a partition change, or a resource allocation change, according to an embodiment of the invention. Control begins at block300. Control then continues to block305where a user, the capacity on demand manager135, the hypervisor136, or any other appropriate functions causes a configuration change to the computer100, a change to one of the partitions134, a change to the resources allocated among the partitions134, or a change to the resources activated at the computer100. Control then continues to block310where the performance data collection tool144-1receives an indication of the change that previously occurred at block305. Control then continues to block315where the performance data collection tool144-1sets an indicator that calibration is required. Control then continues to block399where the logic ofFIG. 3returns.

FIG. 4depicts a flowchart of example processing for responding to a request for calibration, according to an embodiment of the invention. Control begins at block400. Control then continues to block405where the user requests calibration via a user interface. Control then continues to block410where the performance data collection tool144-1receives an indication of the calibration request, previously requested at block405, and sets an indicator that calibration is required. Control then continues to block499where the logic ofFIG. 4returns.

FIG. 5depicts a flowchart of example processing for responding to a request for data collection, according to an embodiment of the invention. Control begins at block505where the performance data collection tool144-1starts and the user or any appropriate application or function requests data collection. Control then continues to block510where the performance data collection tool144-1determines whether calibration is required (e.g., as previously described above with reference toFIGS. 3 and 4). If the determination at block510is true, then calibration is required, so control continues to block515where the performance data collection tool144-1determines whether a group270exists in a record in the compensation data225that is unprocessed by the logic ofFIG. 5.

If the determination at block515is true, then an unprocessed group does exist in the compensation data225, so control continues to block520where the compensator205starts a calibration program215and passes the current event group identifier270to the calibration program215without the collector210being active. Control then continues to block525where the calibration program215executes and generates functions in the operating system142, the application144, or any software and/or hardware component of the computer100, the network130, and/or or the server132that generate events associated with the passed group270. In another embodiment each group has its own associated calibration program215. Control then continues to block530where the compensator205determines the number of execution cycles used by the calibration program215for the group270. Control then continues to block535where the compensation values275are calculated when a collector210is active, as further described below with reference toFIG. 6. Control then returns to block515, as previously described above.

If the determination at block515is false, then all groups270in the compensation data225have been processed by the logic ofFIG. 5, so control continues to block545where the performance data collection tool144-1starts the collector210. Control then continues to block550where (in response to an event caused by the operating system142, the application144, or any software and/or hardware component of the computer100, the network130, and/or or the server132) the collector210creates a new record, such as the record230or235, associated with the event240in the event data220, determines the execution cycles consumed by the event240, determines the event group270to which the event240belongs, and adjusts the execution cycles245in the newly created record in the event data220based on the compensation value275associated with the event group270to which the event240belongs. For example, the collector210subtracts the compensation value275for the group from the detected execution cycles and stores the result in the execution cycles245. Control then continues to block599where the logic ofFIG. 5returns.

If the determination at block510is false, then calibration is not required, so control continues to block545, as previously described above.

FIG. 6depicts a flowchart of example processing for calculating compensation values when the collector210is active, according to an embodiment of the invention. Control begins at block600. Control then continues to block605where the compensator205starts the collector210and passes the current event group identifier270. The compensator205collects data relevant to the current event group identifier270. The collector210also creates associated records in the event data220.

Control then continues to block610with the compensator205invokes the calibration program215and passes the current event group identifier270. The calibration program215performs functions to cause events in that group. Control then continues to block615where the compensator205determines the number of execution cycles of the processor101used by the calibration program215for the current group270with the collector210active. Control then continues to block620where the compensator205calculates the compensation value to be the execution cycles with the collector210active minus the execution cycles without the collector active210divided by the number of events that occurred in the group. The compensator205further stores the calculated compensation value in the compensation value field275. Control then continues to block699where the logic ofFIG. 6returns.

In the previous detailed description of exemplary embodiments of the invention, reference was made to the accompanying drawings (where like numbers represent like elements), which form a part hereof, and in which is shown by way of illustration specific exemplary embodiments in which the invention may be practiced. These embodiments were described in sufficient detail to enable those skilled in the art to practice the invention, but other embodiments may be utilized and logical, mechanical, electrical, and other changes may be made without departing from the scope of the present invention. Different instances of the word “embodiment” as used within this specification do not necessarily refer to the same embodiment, but they may. The previous detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present invention is defined only by the appended claims.

In the previous description, numerous specific details were set forth to provide a thorough understanding of embodiments of the invention. But, the invention may be practiced without these specific details. In other instances, well-known circuits, structures, and techniques have not been shown in detail in order not to obscure the invention.