An apparatus, system, and method are provided for automatically minimizing Real-Time (RT) task latency and maximizing Non-Real Time (NRT) task throughput. The apparatus may include a task manager, a determination module, and optionally a tracking module. The task manager may execute a NRT task configured to obtain exclusive access to a resource within a multi-processor environment. The determination module identifies a RT task within the multi-processor environment that shares exclusive access to the resource with the NRT task. Optionally, the tracking module selectively executes or bypasses a preemption point associated with the NRT task in response to the RT task becoming runnable. Alternatively, the NRT task selectively executes or bypasses a predefined preemption point in response to the RT task becoming runnable. In this manner, RT tasks on any processor of the multi-processor environment meet minimum latency thresholds and NRT tasks automatically provide maximum throughput in the multi-processor environment.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to multi-processor computing. Specifically, the invention relates to apparatus, systems, and methods for automatically minimizing real-time task latency and maximizing non-real time task throughput.
2. Description of the Related Art
Mainstream computer systems are currently moving from conventional single processor architectures, also known as Uniprocessor (UP), to multi-processor architectures. In particular, Symmetric Multi-Processor (SMP) architectures or environments are becoming more widely used in various fields. In an SMP environment, typically, hardware computing resources such as memory, communications bus, I/O devices, and the Operating System (OS) are shared by two or more processors. The multiple processors cooperate as peers each having an equal ability to service tasks assigned by the OS.
A multi-processor environment or SMP system can be implemented with multiple physical processors connected to a common bus. Alternatively, due to advances in processor technology a single physical processor may be used but treated as multiple logical processors by the OS and all other computer system components. One example of this technology is hyperthreading. As used herein, references to SMP system(s) and multi-processor environment(s) refers to any computer system that includes a plurality of physical and/or logical processors. Similarly, references to "processor" include both physical processors and logical processors.
SMP systems are being used both for general purpose computing such as desktop PCs and for more specialized applications such as embedded computing systems. Operating systems for the general purpose and embedded computing systems are adapting to most efficiently service both general desktop computing needs and specialized embedded computing needs. One example of this adaptation is the ability of a single SMP OS to properly manage both Non-Real Time (NRT) tasks and Real-Time (RT) tasks. As used herein, the term "task" refers to the smallest portion of a software program that the OS can switch between a waiting or blocked state and a running state in which the task executes on a particular processor. Tasks can come from a single software program or a plurality of software programs and typically include portions of the OS.
As used herein, a NRT task refers to software code that does not have explicit time constraints for when computing services are to be provided to the task. Conversely, RT tasks are tasks for which the task has a predefined maximum threshold for delay between requesting a service from the computer system and having the request fulfilled. Failure to service the RT task within the threshold can cause serious failure of the task and/or systems managed by the RT task. Furthermore, RT tasks include both hard real time tasks and soft real time tasks. Hard real time tasks require an absolute guarantee of response time below the maximum threshold. Soft real time tasks require a very high probability that the response time is below the maximum threshold, but not an absolute guarantee.
Where the threshold is set to define a NRT task or a RT task depends on the context. For a mission critical task, such as respiratory oxygen content in a hospital patient, the threshold could be measured in tens of microseconds. Such a critical task is one example of a hard real time task. For other RT tasks, soft real time tasks, the threshold could be measured in minutes, such as a weather station temperature sampling task. Another soft real time task example is a real time video or audio processing task. Failure to meet the maximum threshold for response time may result in detectable "skips" or degradation in quality, but not critical failures such as possible death.
Typically, NRT tasks are tasks that involve user interaction where long delays in response to user inputs results in a poor user experience but no loss of data, functionality, or a critical failure. However, because classifying a task depends so much on the context, NRT tasks and RT tasks are typically classified as such for the OS by the software developer. Generally, RT tasks have a very low service threshold and NRT tasks have a comparatively high service threshold.
The delay between when a task requests a service from the OS and when the service is provided is referred to as latency. Typically, the requested service is the assignment of a processor to execute the task. The service threshold defines a maximum tolerable latency for the task. As used herein, the term "latency" or "task latency" refers to the time between when the task requests a service from the OS and when the service is provided. The service may include assignment of a processor for task execution, exclusive access to a resource, and the like. Task latency typically includes other more specific latencies well known to those of skill in the art such as scheduling latency, task switching latency, and the like.
Multiple factors affect task latency. It is well known that modern OSs constantly change the task assigned to a particular processor in order to provide multitasking functionality. Consequently, the number of tasks managed by the OS can lengthen the task latency due to the increased overhead in handling each additional task. However, due to the critical nature of RT tasks, general purpose OSs have been modified to service the task latency requirements of the most demanding RT tasks in order to handle a worst-case scenario and ensure that the worst-case task latency still meets the RT task requirements.
In certain cases, real-time specific OSs (RTOSs) have been developed. Unfortunately, the RTOSs favor the RT tasks over the NRT tasks. Consequently, a NRT task may experience poor responsiveness on an RTOS system. Often, if the RTOS support NRT tasks, the NRT task is so delayed in responding to user inputs that the user notices a delay in response to a user-initiated action. The responsiveness of a NRT task in an OS is referred to herein as task throughput. Task throughput represents how quickly a task is able to complete a command and provide a response to the user. Task throughput also includes the number of units of work a task can complete in a given time period.
If NRT tasks and RT tasks are run together on the same computer system, the optimizations for servicing RT tasks adversely affect NRT task throughput. In certain cases, the NRT task throughput is affected regardless of whether any RT tasks are running on the system. Similarly, conventional optimizations to improve NRT throughput can adversely affect RT task latency. Consequently, the industry has experienced a trade-off in OSs attempting to service both RT tasks and NRT tasks. Until the present invention, the industry has been unable to satisfactorily minimize RT task latency and maximize NRT task throughput automatically.
Typically, NRT task throughput is sacrificed in favor of RT task latency. Currently, the OS scheduling algorithm is optimized such that RT tasks, if present, are assigned a processor ahead of NRT tasks. Consequently, in a typical Uniprocessor (UP) system, NRT tasks are generally preempted if a RT task becomes runnable. Preemption means the task currently executing on the processor is halted before the task has reached a natural termination point. The task is forced to wait while a higher priority task, such as a RT task, is assigned a processor. However, as noted above, under this approach, as the number of RT tasks increases the NRT task throughput decreases. Examples of these scheduling optimizations include a Priority Level Scheduler (PLS) and Multi-queue Scheduler (MQS).
The problem of optimizing the OS to minimize RT task latency and maximize NRT throughput is even more difficult in an SMP system. In a UP system, RT tasks can simply be tracked and provided priority over runnable NRT tasks. However, in SMP systems, there is currently no efficient way to determine whether a RT task exists on a processor other than the processor executing a NRT task.
In addition, in an SMP system the concurrent nature of multiple processors (each executing a different task) sharing resources such as data structures, memory, devices, and the like requires that access to the shared resources be controlled. The access to the resources is controlled such that only one processor and its currently executing task, are permitted to access the resource at a given time. This process of controlling access is referred to as serialization.
Serialization is particularly desirable to preserve data integrity when multiple tasks/processors can modify data in a shared resource. Preempting a task while writing to a shared resource can corrupt the data. Consequently, serialization should provide exclusive access for the task requesting the shared resource and exclude preemption. If one task has access to the resource, all other tasks are excluded from con accessing the resource until the one task has finished. Exclusive access is provided atomically meaning a task executes a single command to obtain access and is either successful or not, there is no opportunity to preempt the task while requesting the exclusive access.
Generally, serialization of SMP tasks to shared resources is controlled by locks. If a task desires exclusive access to a resource, the task requests the lock. If the lock is not held by any other task, the lock is atomically provided to the requesting task. If the lock is held by another task, the requesting task often enters a loop in which the task continually requests the lock until the lock becomes free. Once a task holds a lock, the task modifies or otherwise uses the shared resource in some manner and then releases the lock. Typically, a lock is implemented using a boolean value, False if the lock is available and True if the lock is being held.
FIG. 1?illustrates a conventional multi-processor environment?100?with NRT tasks102?and RT tasks?104?that share exclusive access to a common resource?106. The environment?100?includes a memory?108, a plurality of processors?110, also referred to as Central Processing Units (CPUs)?110, and a communications bus112. The memory?108, CPUs?110, and communications bus?112?are well known. The CPUs?110?are identified by subscripts 1, 2, . . . n. Those of skill in the art will recognize various different hardware configurations for a multi-processor environment?100.
The memory?108?includes a set of executable code that includes a multi-processor operating system?114?such as an SMP OS?114?and a data section116. The SMP OS?114?includes a task manager?118?also referred to as a scheduler?118?and a runqueue?120?associated with each CPU?110. The runqueue?120?include subscripts 1, 2, . . . n corresponding to the associated CPU?110. The data section?116?includes task-specific data as well as data structures shared by the tasks. Certain exclusive access data structures are controlled by locks?122. Different locks?122?are represented using alphabetic identifiers and an arrow?124?to the associated resource?106, such as a data structure.
By way of example, in the current multi-processor environment?100, CPU1?110?is executing a NRT task?102?that has acquired the lock A?122?a?for resource?106. At substantially the same time, a RT task?104?on CPUn?110?has become runnable and begins to run on CPUn. Furthermore, one of the first instructions executed by RT task?104?is to acquire lock A?122?b.
This presents a problem. The task latency will be increased because the NRT task?102?is holding lock A?122?a?which the RT task?104?needs and the NRT task102?is not preemptable. Furthermore, depending on which type of lock?122?NRT task?102?is holding interrupts could be disabled. This means that the RT task?104must wait for the NRT task?102?to release the lock before the RT task?104?can perform its work. In addition, if interrupts are enabled, an interrupt arrive while the NRT task?102?holds the lock. The interrupt may be long-running such that the interrupt also delays the release of the lock by NRT task?102. The delays caused by the NRT task?102?and/or interrupts are generally unacceptable and the RT task latency maximum threshold is consequently exceeded.
In a conventional multi-processor environment?100, the solution is to defer to the needs of the RT task?104. One proposed solution is to include multiple preemption points?126?in the code of the NRT task?102. Alternatively, if the NRT task?102?is executing object code in the kernel of the OS, the preemption points126?are in the kernel. The preemption point?126?is executed indiscriminately. There is currently no way for a NRT task?102?to avoid the preemption point?126and its associated delay.
A preemption point?126?is a predefined point in the object code when a developer has determined that the NRT task?102?can voluntarily give up ownership of the CPU1?110. Generally, as part of executing the preemption point?1126?the NRT task?102?will also release any locks?122?being held. Typically, the preemption points?126?are in the kernel object code and the NRT task?102?is forced to give up the CPU1?110.
Preemption points?126?ensure that the most time added to the RT task latency is the time between preemption points?126. RT tasks?104?are not delayed by a NRT task?102?holding a lock?122?too long. Preemption points?126?also introduce overhead as the NRT task?102?performs steps to preserve its state, release any locks?122, sleep, and then resume operations after the preemption point?126.
Unfortunately, indiscriminate execution of preemption points?126?incurs this overhead delay even if there are no RT tasks?104?in the environment?100. The overhead delays caused by mandatory preemption points?126?unnecessarily reduce the NRT task throughput.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method for automatically minimizing RT task latency and maximizing NRT task throughput in a multi-processor environment. Beneficially, such an apparatus, system, and method would conditionally execute preemption points in response to the presence or absence of a runnable RT task in the multi-processor environment. In addition, the apparatus, system, and method would automatically and optimally handle both RT tasks and NRT tasks, incur minimal processing overhead, and prevent shared resource contention between NRT tasks and RT tasks.
SUMMARY OF THE INVENTION
The present invention has been developed in response to the present state of the art, and in particular, in response to the problems and needs in the art that have not yet been met for automatically minimizing real-time (RT) task latency and maximizing non-real time (NRT) task throughput in a multi-processor environment. Accordingly, the present invention has been developed to provide an apparatus, system, and method for automatically minimizing real-time (RT) task latency and maximizing non-real time (NRT) task throughput in a multi-processor environment that overcomes many or all of the above-discussed shortcomings in the art.
An apparatus according to the present invention includes a task manager, and a determination module. The task manager executes a NRT task in a multi-processor environment. The NRT task is configured to obtain exclusive access to a resource. The determination module is configured to identify a RT task within the multi-processor environment. The RT task may be executing on any processor and shares exclusive access to the resource with the NRT task. In one embodiment, the NRT task may be configured to selectively execute a preemption point in response to the RT task becoming runnable. The NRT task may use various techniques to determine whether the RT task is runnable, discussed in more detail below. If the RT task is not runnable, the NRT task may bypass the preemption point. The preemption point may be part of the NRT task or an operating system executing the NRT task.
Preferably, execution of the preemption point causes the NRT task to release any exclusive access the NRT task has to the resource such that the RT task may obtain exclusive access to the resource. In one embodiment, a resource manager allocates the resource to the RT task in response to execution of the preemption point for the NRT task.
In certain embodiments, the NRT task recognizes a runnable RT task by way of a tracking module. The tracking module may increment or decrement a global indicator accessible to tasks in the multi-processor environment. The global indicator is incremented in response to an identified runnable RT task and decremented in response to a blocked RT task. The NRT task executes the preemption point in response to the global indicator being nonzero and bypasses the preemption point in response to the global indicator being zero. Alternatively, the tracking module may be configured to selectively execute or bypass the preemption point for the NRT task.
As used herein, the term "indicator" means any structure, data, or value capable of conveying information by virtue of its existence or nonexistence or the presence or absence of a value for the indicator including a quantity or count. Examples of indicators include counters, boolean variables, flags, bits in a bitmap, and the like. For clarity in the embodiments described below, reference is made hereinafter to the term "counter" as one example of an "indicator" within the scope of the present invention. Those of skill in the art will recognize that any indicators, not just those embodied as counters, may be used with the present invention. The term "counter" is used as one example of an "indicator" regardless of the adjectives such as global or processor-specific used with the term.
In another embodiment, the tracking module may increment or decrement a processor-specific indicator accessible to tasks on a particular processor for each processor in a multi-processor environment. If a RT task becomes runnable or blocked on a particular processor, the processor-specific indicator, is respectively incremented or decremented. In addition, a global indicator is maintained which is incremented if any processor-specific indicator is incremented to one and decremented if any processor specific indicator is decremented to zero. The processor-specific indicator for a processor is also decremented if a RT task is moved to another processor. The preemption point of a NRT task executing on a processor is executed if the processor-specific indicator or the global indicator is non-zero. The preemption point of a NRT task executing on a processor is bypassed if the processor-specific indicator and the global indicator are zero.
In yet another embodiment, the tracking module may reference a global data structure such as a runnable RT task queue. The presence of a task in the runnable RT task queue indicates that a runnable RT task exists in the multi-processor environment. Accordingly, the tracking module will execute the preemption point. If no tasks are in the runnable RT task queue, the tracking module will bypass a preemption point for the NRT task. In another embodiment, the runnable RT task queue may be part of a generic run queue. Alternatively, the tracking module references a plurality of runnable RT task queues, each associated with a specific processor. Again the presence or absence of a task on any of the plurality of runnable RT task queues is used to selectively execute or bypass the preemption point for the NRT task.
In certain embodiments, the NRT and/or tracking module may determine whether runnable RT tasks are present in a multi-processor environment by referencing the shared resource. In one embodiment, the RT task sets an indicator associated with the resource. The indicator signals a NRT task to execute the preemption point because a RT task is using or will use the resource very soon.
A method of the present invention is also presented for automatically minimizing real-time (RT) task latency and maximizing non-real time (NRT) task throughput in a multi-processor environment. In one embodiment, the method includes executing a NRT task in a multi-processor environment, the NRT task configured to obtain exclusive access to a resource. The method identifies a RT task within the multi-processor environment, the RT task sharing exclusive access to the resource with the NRT task. Lastly, the method selectively executes a preemption point for the NRT task in response to the RT task becoming runnable.
The present invention also includes embodiments arranged as a system, alternative apparatus, additional method steps, and machine-readable instructions that comprise substantially the same functionality as the components and steps described above in relation to the apparatus and method. The present invention provides a solution for minimizing RT task latency and maximizing NRT throughput that is automatic/autonomic, incurs minimal processing overhead, and prevents shared resource contention between NRT tasks and RT tasks in a multi-processor environment. The features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.
DETAILED DESCRIPTION
FIG. 2?illustrates a logical block diagram of an apparatus?200?for minimizing RT task latency and maximizing NRT throughput in a multi-processor environment?100?(See?FIG. 1). The apparatus?200?includes a task manager?202, a determination module?204, and optionally a resource manager?206.
The task manager?202?reviews a list of runnable tasks?208?and determines which task to provide to a processor?110?(SeeFIG. 1) to allow execution of the task. Those of skill in the art will recognize that various algorithms exist for determining which task to assign to an available CPU?110. All such algorithms are considered within the scope of this invention. Once CPU?110?assignments have been made the task manager?202?preempts the currently running tasks and switches the assigned processors to the newly selected tasks. Preferably, the task manager?202?is configured to execute a NRT task?102(See?FIG. 1). The NRT task?102?is configured to obtain exclusive access to a shared resource?106, such as R1. As described above, this is typically managed by holding a lock associated with the shared resource?106?(i.e., R1).
The determination module?204?identifies a RT task?104. The determination module?204?is specifically configured to determine if a RT task?104?shares or can potentially share exclusive access to the same resource as the NRT task?102. Those of skill in the art will recognize that the determination module?204?may strictly (liberal approach) or loosely (conservative approach) determine whether a RT task?104?shares exclusive access to the same resource as the NRT task102.
In one embodiment, the determination module?204?may review the locks?122?currently held by the NRT task?102. This may be done, for example, by referencing characteristics of the NRT task?102. Next, based on the characteristics of the RT task104, the determination module?204?may determine that there is a high likelihood that the NRT task?102?and the RT task?104will contend for the same locks?122. For example, if the RT task?104?is a kernel task, then the lock contention likelihood may be sufficiently high to identify the RT task?104?for consideration at a preemption point?126?for the NRT task?102.
Alternatively, if the NRT task?102?holds kernel locks?122, the determination module?204?may simply presume that the RT task may also want kernel locks?122. Consequently, the determination module?204?may identify the RT task?104?for consideration at a preemption point?126?for the NRT task?102. In yet another alternative embodiment, the determination module?204?may search for any RT task?104?on any CPU?110. If a RT task?104?is found, that RT task?104?is identified by the determination module?204.
With the RT task?104?identified, the NRT task?102?is preferably configured to execute a preemption point?126?if the RT task is runnable. In one embodiment, the RT task?104?may quickly reference a characteristic of the RT task?104?to determine whether the RT task?104?is runnable. As mentioned above, execution of the preemption point?126?preferably releases any locks?122?held and frees a CPU?110?for use by another task, such as the RT task?104.
A task is runnable if the task is fully prepared to use time allotted to it for execution on a CPU?110. If a task is waiting for some kind of input, feedback, or condition such as passage of time, the task is said to be blocked. Additional techniques to determine whether the RT task?104?is runnable or blocked are described below.
Blocked RT tasks, although present, can not execute on a CPU?110?and thus can not benefit from a NRT task?102?executing a preemption point?126. Consequently, if a RT task?104?is blocked, the NRT task?102?bypasses the preemption point?126?and continues to hold any locks?122?previously held. Holding the locks?122?allows the NRT task?102?to get more work done. Consequently, the NRT task?102?throughput is increased.
By checking the runnable characteristic of an identified RT task?104?on any CPU?110?in the multi-processor environment, the NRT task?102?can conditionally execute or bypass the preemption point?126. Consequently, if runnable RT tasks?104?are on any of the CPUs?110, the RT task?104?is quickly serviced because the preemption point?126?is executed. The worst-case latency is the time between preemption points?126?plus a minor delay to check for the runnable state of the identified RT task104. However, if no runnable RT tasks?104?are in the multi-processor environment, the NRT task?102?is permitted to continue execution free from preemption and overhead involved in releasing shared locks. In this manner, RT task latency is minimized and NRT task throughput is maximized automatically.
Optionally, the apparatus?200?includes a resource manager?206. The resource manager?206?manages access to shared resources between tasks. In particular, the resource manager?206?may manage locks?122?for shared resources. Consequently, in one embodiment, once the NRT task?102?executes the preemption point?126, the resource manager?206may allocate the locks?122?or other shared resources (released by the NRT task?102) to the identified RT task?104. Typically, locks?122?are transferred by the RT task?104?constantly asking for the lock?122, "Spinning" for the lock. Where types of locks other than spinlocks are shared, the resource manager?206?may facilitate transfer of exclusive access to the shared resource to the RT task?104.
FIG. 3?illustrates an apparatus?300?according to one embodiment for minimizing RT task latency and maximizing NRT throughput in a multi-processor environment?100?(See?FIG. 1). Reference will now be made directly to?FIG. 3?and indirectly to?FIGS. 1 and 2. The apparatus?300?communicates via the bus?112?to a plurality of CPUs 1-n?110. The apparatus?300includes a task manager?202, a determination module?204, and a resource module?206?which are substantially the same as those described in relation to?FIG. 2.
In addition, the apparatus?300?includes a tracking module?302. The tracking module?302, in certain embodiments, may selectively execute the preemption point?126?for the NRT task?102. If the RT task?104?is runnable, the tracking module?302executes the preemption point?126. If the RT task is blocked for any reason, the tracking module?302?bypasses the preemption point?126.
In certain embodiments, placing the logic to conditionally determine whether to execute a preemption point?126?with the tracking module?302?may be desirable. For example, rather than having each NRT task?102?evaluate the condition, a central set of code may evaluate the condition for all NRT tasks?102, as needed.
In one embodiment, the tracking module?302?may be implemented as part of a scheduler or task manager for an OS such as Linux. The preemption points?126?may be within kernel code. The kernel code may simply make a function call to the tracking module?302?to evaluate whether or not to execute the preemption point?126.
The tracking module?302?illustrated implements one of many techniques that may be used to evaluate whether execution of the preemption point?126?is appropriate. In one embodiment, the tracking module?302?maintains a global counter (GC)?304. Preferably, the GC?304?is a positive integer counter accessible to each task in the multi-processor environment.
The tracking module?302?keeps track of changes in the states of tasks executing on any of the CPUs?110. In one embodiment, the tracking module?302?may monitor the types of tasks on a global runqueue or on cpu-specific runqueues120. Those of skill in the art will readily recognize that how this monitoring is performed depends in part on the implementation of the runqueues.
In one embodiment, a NRT global runqueue having only NRT task?102?members and a RT global runqueue having only RT task?104?members may exist. In such an embodiment, the tracking module?302?may simply monitor the RT global runqueue for any members, provided such a check can be done efficiently. Alternatively, if there are separate cpu-specific runqueues120?for NRT tasks?102?and RT tasks?104, the tracking module?302?may monitor cpu-specific RT runqueues for any members. Typically, the NRT tasks?102?and RT tasks?104?are maintained in the same runqueue, consequently in one embodiment, the tasks?102/104?in the runqueue are reviewed to determine what type of task is present.
Regardless of how runnable RT tasks?104?are detected, if a RT task?104?becomes runnable (typically determined by its presence in a runqueue), the tracking module?302?increments the GC?304. If a RT task?104?becomes blocked or terminates, the tracking module?302?decrements the GC?304. Furthermore, as described in more detail below, detection of RT tasks?104on specific CPUs?110?may cause cpu-specific counters to be adjusted.
Preferably, the tasks have access to the GC?304?such that the NRT task?102?may quickly read the GC?304?to determine whether or not to execute the preemption point?126. Alternatively, the tracking module?302?references the GC?304?to determine whether to execute the preemption point?126. For example, the NRT task?102?may query the tracking module?302to determine whether to execute the preemption point?126.
Regardless of whether the NRT task?102?or the tracking module?302?makes the decision regarding executing the preemption point?126, in one embodiment, if the GC?304?is not zero the preemption point?126?is executed. A nonzero GC?304?indicates that somewhere in the multi-processor environment a RT task?104?is ready to run and either clearly does share exclusive access to a resource?106?with the NRT task?102?or potentially shares exclusive access to the resource?106. If the GC?304?is zero, the preemption point?126?is bypassed. A zero GC?304?indicates that there are currently no runnable RT tasks?104?in the multi-processor environment.
Those of skill in the art will recognize that tracking RT tasks?104?that are runnable and blocked as their state changes using a global counter is but one of many ways the pertinent information regarding the state of RT tasks?104?in the multi-processor environment can be obtained. The scope of the present invention is not limited to the few exemplary embodiments described herein for identifying RT tasks?104?among a plurality of CPUs or for determining whether a RT task?104?in the environment is runnable or blocked. Other techniques are also contemplated such as the tracking module?302?scanning all tasks in the environment as needed to determine whether a RT task?104?is runnable and present that may require exclusive access to resources?106?shared with the NRT task.
FIGS. 4A and 4B?illustrate alternative embodiments for the tracking module?302?described in relation to?FIG. 3. In?FIG. 4A, the tracking module?402?maintains the GC?304?and performs substantially the same functionality as the tracking module?302in?FIG. 3. One difference is the level of granularity for the counters.
In addition to a GC?304, the tracking module?402?may maintain processor-specific counters?404. The processor-specific counters?404?are designated by subscripts 1, 2, . . . n and correspond to the plurality of CPUs?110?1, 2, 3, . . . n as illustrated in?FIG. 3. The processor-specific counters?404?may reside in a central location such as in the tracking module402. Alternatively, the processor-specific counters?404?may reside in the cache of the associated processor?110.
In the embodiment of?FIG. 4?a, RT tasks?104?are tracked at the individual CPU level. For a first CPU/processor?110, if a runnable RT task?104?is identified on the first processor?110, the processor-specific counter?404?is incremented. Similarly, if a runnable RT task?104?is blocked on the first processor?110?or is moved to a second processor?110, the processor-specific counter?404?for the first processor?110?is decremented.
In one embodiment, the GC?304?is still incremented and decremented such that the status of RT tasks?104?on the processors110?is tracked. In other words, the local processor-specific counter?404?identifies how many RT tasks?104?are actually runnable on that processor?110. While, the GC?304?maintains whether any processor?110?still includes at least one RT task104. Consequently, the tracking module?402?increments the GC?304?if a processor-specific counter?404?is incremented to one. The tracking module?402?decrements the GC?304?if a processor-specific counter?404?is decremented to zero.
The tracking module?402?may be configured to check both the local processor-specific counter?404?and the GC?304. Alternatively, the NRT task?102?itself may include logic to perform this check. If the processor-specific counter?404?for the processor?110?that the NRT task?102?is executing on is nonzero, the preemption point?126?is executed. In addition, if the GC304?is nonzero, the preemption point?126?is executed. If the processor-specific counter?404?is zero and the GC?304?is nonzero, there is a RT task?104?on a different processor?110. Consequently, the preemption point?126?should be executed to enable the RT task?104?to be serviced and obtain any needed locks?122.
Similarly, if the processor-specific counter?404?for the processor?110?that the NRT task?102?is executing on is zero and the GC?304?is zero, the preemption point?126?is bypassed. If the processor-specific counter?404?is zero and the GC?304?is zero, there are no runnable RT tasks?104?in the multi-processor environment.
Advantageously, using a GC?304?and processor-specific counters?404?may reduce cache thrashing between processors110. However, this implementation may not be as efficient when multiple RT tasks?104?are on each processor?110.
Referring now to?FIG. 4B, the information that a runnable RT task?104?con exists and is sharing exclusive access to a shared resource may be provided in yet another embodiment.?FIG. 4B?illustrates a shared resource?406. The shared resource?406may comprise a lock?122, token, a header for another shared data structure, a particular buffer associated with a shared device, or the like.
The shared resource?406?includes an indicator?408. The indicator?408?signals a NRT task?102?that obtains exclusive use of the resource?406?to execute preemption points?126. In one embodiment, only a RT task?104?can set the indicator?408. Consequently, the indicator?408?may comprise a simple flag that is set and cleared by RT tasks?104.
Preferably, the RT task?104?obtains exclusive access to the resource?406. Before becoming blocked, the RT task?104?sets the indicator to True. The RT task?104?then blocks and the resource?406?is released. Later, when a NRT task?102?obtains exclusive access to the resource?406, the NRT task?102?may simply reference the indicator?408?to determine whether to execute the preemption point?126. If the indicator?408?is True, the resource?406?is reserved and the preemption point?126?is executed. If the indicator?408?is False, the resource?406?is not reserved. The NRT task?102?bypasses the preemption point126?and can retain exclusive use of the resource?406.
FIG. 5?illustrates a system?500?for minimizing RT task latency and maximizing NRT throughput in a multi-processor environment. Reference is now made directly to?FIG. 5?and indirectly to?FIG. 3. The system?500?includes a memory?108, plurality of CPUs?110, and bus?112?that collectively function and perform substantially the same as like hardware described in relation to?FIG. 1. The memory?108?includes an SMP OS?114, a data segment?116, and a runqueue (RQ)?120?associated with each CPU?110. The data segment?116?and RQs?120?are substantially similar to the like numbered components described above in relation to?FIG. 1.
In contrast to?FIG. 1, the SMP OS?114?includes a running queue?502?and a task manager?504?configured for minimizing RT task latency and maximizing NRT throughput. The running queue?502?tracks what tasks are running on which CPUs?110. The task manager?504?manages execution of both NRT tasks?102?and RT tasks?104?(See?FIG. 1).
In one embodiment, the task manager?504?includes a determination module?204?and a resource manager?206?that function in substantially the same manner as similarly numbered components described above in relation to?FIG. 2. In one embodiment, the task manager?504?comprises a scheduler for the SMP OS. The task manager?504?may be initiated directly or in response to an interrupt such as a clock interrupt. Once the task manager?504?is executed, it communicates with the determination module?204. The determination module?204?attempts to identify any RT tasks?104?on the system?500. If one is found, the determination module signals the tracking module?506.
The tracking module?506?determines whether the identified RT task?104?is runnable. As described in detail above, this may be accomplished using various techniques. In system?500, another technique is illustrated. The SMP OS?114?may include a separate Real-Time Run Queue (RTRQ)?508?for each CPU?110. Alternatively, a consolidated Real-Time Run Queue may store all runnable RT tasks?104?executing on any CPU?110?in the system?500.
The RTRQ?508?may be used to optimize scheduling where large numbers of tasks are processed such that task switching overhead is impacting task latency. The RTRQ?508?may store just RT tasks?104?that are runnable. Runnable NRT tasks?102may continue to be stored on the RQs?120. With such a configuration, information about the type of task and its runnable status is implicit in the data structure RTRQ?508.
Consequently, once a RT task?104?is identified by the determination module?204, the tracking module?506?may quickly scan the RTRQs?508?for the system?500. If a RT task?104?is present on one of the RTRQs?508, the tracking module?506?may immediately stop scanning and communicate to a NRT task?102?to execute the preemption point?126. The task manager?504may then switch tasks to the runnable RT task?104?once the preemption point?126?is executed. If a scan of the RTRQs?508finds no tasks, the tracking module?506?similarly signals a bypass of a preemption point?126?for the NRT task?102.
In this manner, the tracking module?506?may reference a plurality of data structures (the RTRQs?508) very quickly to determine whether or not a preemption point?126?should be executed. Alternatively, in embodiments having a single RTRQ508?storing all runnable RT tasks, the tracking module?506?determines whether or not to execute the preemption point?126?by referencing a single data structure (the single RTRQ?508). Preferably, the tracking module?506?stops scanning once a first runnable RT task?104?is found.
FIG. 6A?illustrates a schematic flow chart diagram illustrating certain steps for one embodiment of a method?600?for minimizing RT task latency and maximizing NRT throughput in a multi-processor environment. The method?600?begins once a NRT task?102?is runnable on a SMP system?500?(See?FIG. 5). First, the task manager?202?(See?FIG. 2) executes?602?the NRT task?102?on the SMP system?500. Next, the determination module?204?attempts to identify a RT task?104?on one of the CPUs?110. In one embodiment, the determination module?204?references a queue or a task directory. The RT task?104typically shares exclusive access to a resource with the NRT task?102.
The determination module?204?determines?604?if a RT task?104?in the SMP system?500?has become runnable. Preferably, this determination?604?may include whether the RT task?104?shares exclusive access to a resource?106?with the NRT task102. Alternatively, sharing of exclusive access to a resource?106?between NRT tasks?102?and RT tasks?104?may be presumed.
If a RT task?104?has become runnable, the determination module?204?records?606?that a runnable RT task?104?was identified. As discussed above, recording this information may be accomplished a variety of ways. In one embodiment, the determination module?204?communicates with a tracking module?302. The tracking module?302?monitors the RT task?104until the RT task?104?becomes runnable. Preferably, the tracking module?302?performs a monitoring check each time a scheduler/task manager?202?for the SMP OS?114?executes on a CPU?110. Preferably, the tracking module?302/402increments a global counter?304?and/or processor-specific counters?404?to indicate identification of one or more runnable RT tasks?104.
In addition, the determination module?204?may also determine?608?whether one or more RT tasks?104?being managed by the SMP OS?114?have become blocked. As described above, a blocked RT task?104?does not benefit from NRT task preemption points.
If the RT task?104?has become blocked, the determination module?204?records?610?that one less runnable RT task?104?is now present. The determination module?204?may communicate with a tracking module?304/402?which decrements a global counter?304?and/or processor-specific counters?404?to indicate the presence of one less runnable RT task?104.
In certain embodiments, the determination module?204?may determine?608?whether a RT task?104?has been moved from one CPU?110?to another. If so, the determination module?204?records?610?the migration. The determination module?204?may communicate with a tracking module?304/402?which decrements a global counter?304?and adjusts processor-specific counters?404, as described above, to indicate that one runnable RT task?104?has migrated between CPUs?110.
Finally, the steps?602-610?end when the determination module?204?and task manager?202?have finished scheduling operations for the SMP OS?114. Subsequent steps in the method?600?may be completed by other modules. Specifically, logic in each NRT task?102?may selectively determine whether to execute a preemption point. Alternatively, such logic may reside centrally for example in a tracking module?302.
Referring now to?FIG. 6B, additional steps in the method?600?may be initiated once a NRT task?102?comes to a preemption point?126. Preferably, these steps are completed by the NRT tasks?102. Alternatively, a tracking module?302/402communicates with the NRT task?102?to indicate whether the NRT task?102?should execute the preemption point?126.
In one embodiment, once a NRT task?102?reaches a preemption point?126, a determination?612?is made whether there are runnable RT tasks?104. In one embodiment, the determination?612?is made by quickly reading the global counter?304?and/or processor-specific counters?404. Those of skill in the art will readily recognize that the counters?304,404?that are consulted may vary depending on the implementation.
If the counters?304,404?indicate the presence of a RT task?104, in one embodiment, the NRT task?102?executes?614?the preemption point?126?for the NRT task?102. Preferably, this means that the shared resource?106?is released. Next, the resource manager?206?allocates?616?the shared resource?106?to the RT task?104?and the method?600?ends. Typically, once the method?600?ends, the method is re-initiated the next time the task manager?202, determination module?204, and tracking module?204?are executed. Consequently, step?602?(See?FIG. 6A) may not execute a new NRT task?102?but continue executing existing NRT tasks?102.
If the counters?304,404?indicate no RT tasks?104, the NRT task?102?bypasses?618?the preemption point?126. Alternatively, a tracking module?302?may indicate to a NRT task?102?to bypass?618?the next preemption point?126. In this manner, preemption points?126?are conditionally implemented. Consequently, if no runnable RT tasks?104?exist, the NRT task?102?can attain higher throughput.
Advantageously, the present invention in various embodiments provides for minimizing RT task latency and maximizing NRT task throughput in a multi-processor environment. The present invention incurs a minimum overhead to check for runnable RT tasks that share exclusive access to resources with existing executing NRT tasks. The present invention provides conditional preemption points for NRT tasks?102. Consequently, if runnable RT tasks?104?exist a CPU?110?and/or exclusive resource?106?is provided for the RT task?104?to meet its RT task latency thresholds. In addition, if no runnable RT tasks?104exist, NRT tasks?102?are not unnecessarily interrupted by preemption points?126. Accordingly, the NRT tasks?102?can achieve higher throughputs and responsiveness automatically. The present invention automatically accommodates RT tasks104?and NRT tasks?102.
SRC=https://www.google.com.hk/patents/US7721291
Apparatus, system, and method for automatically minimizing real-time task latency and maximizing non-real time task throughput