Patent Publication Number: US-9417905-B2

Title: Terminating an accelerator application program in a hybrid computing environment

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The field of the invention is data processing, or, more specifically, methods, apparatus, and products for terminating an accelerator application program in a hybrid computing environment. 
     2. Description of Related Art 
     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 complicated devices. Today&#39;s computers are much more sophisticated than early systems such as the EDVAC. Computer systems typically include a combination of hardware and software components, application programs, operating systems, processors, buses, memory, input/output devices, and so on. As advances in semiconductor processing and computer architecture push the performance of the computer higher and 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. 
     Computer systems today have advanced such that some computing environments now include core components of different architectures which operate together to complete data processing tasks. Such computing environments are described in this specification as ‘hybrid’ environments, denoting that such environments include host computers and accelerators having different architectures. Although hybrid computing environments are more computationally powerful and efficient in data processing than many non-hybrid computing environments, such hybrid computing environments still present substantial challenges to the science of automated computing machinery. 
     SUMMARY OF THE INVENTION 
     Methods, apparatus, and products for terminating an accelerator application program in a hybrid computing environment are disclosed. The hybrid computing environment includes a host computer having a host computer architecture and an accelerator having an accelerator architecture. The accelerator architecture is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. The host computer and the accelerator are adapted to one another for data communications by a system level message passing module (‘SLMPM’). The accelerator is also configured to execute one or more accelerator application programs. Terminating an accelerator application program in a hybrid computing environment includes receiving, by the SLMPM from a host application executing on the host computer, a request to terminate an accelerator application program executing on the accelerator; terminating, by the SLMPM, execution of the accelerator application program; returning, by the SLMPM to the host application, a signal indicating that execution of the accelerator application program was terminated; and performing, by the SLMPM, a cleanup of the execution environment associated with the terminated accelerator application program. 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular descriptions of exemplary embodiments of the invention as illustrated in the accompanying drawings wherein like reference numbers generally represent like parts of exemplary embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  sets forth a diagram of an example hybrid computing environment useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. 
         FIG. 2  sets forth a block diagram of an exemplary hybrid computing environment useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. 
         FIG. 3  sets forth a block diagram of a further exemplary hybrid computing environment useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. 
         FIG. 4  sets forth a block diagram of a further exemplary hybrid computing environment useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. 
         FIG. 5  sets forth a flow chart illustrating an exemplary method for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. 
         FIG. 6  sets forth an exemplary method of performing, by an SLMPM, a cleanup of the execution environment associated with the terminated accelerator application program in accordance with embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Exemplary methods, apparatus, and products for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention are described with reference to the accompanying drawings, beginning with  FIG. 1 .  FIG. 1  sets forth a diagram of an example hybrid computing environment ( 100 ) useful for terminating an accelerator application according to embodiments of the present invention. A ‘hybrid computing environment,’ as the term is used in this specification, is a computing environment in that it includes computer processors operatively coupled to computer memory so as to implement data processing in the form of execution of computer program instructions stored in the memory and executed on the processors. In addition, the hybrid computing environment ( 100 ) in the example of  FIG. 1  includes at least one host computer having a host architecture that operates in cooperation with an accelerator having an accelerator architecture where the host architecture and accelerator architecture are different architectures. The host and accelerator architectures in this example are characterized by architectural registers, registers that are accessible by computer program instructions that execute on each architecture, registers such as, for example, an instruction register, a program counter, memory index registers, stack pointers, and the like. That is, the number, type, structure, and relations among the architectural registers of the two architectures are different, so different that computer program instructions compiled for execution on the host computer of a hybrid computing environment typically cannot be executed natively by any associated accelerator of the hybrid computing environment. 
     Examples of hybrid computing environments include a data processing system that in turn includes one or more host computers, each having an x86 processor, and accelerators whose architectural registers implement the PowerPC instruction set. Computer program instructions compiled for execution on the x86 processors in the host computers cannot be executed natively by the PowerPC processors in the accelerators. Readers will recognize in addition that some of the example hybrid computing environments described in this specification are based upon the Los Alamos National Laboratory (‘LANL’) supercomputer architecture developed in the LANL Roadrunner project (named for the state bird of New Mexico), the supercomputer architecture that famously first generated a ‘petaflop,’ a million billion floating point operations per second. The LANL supercomputer architecture includes many host computers with dual-core AMD Opteron processors coupled to many accelerators with IBM Cell processors, the Opteron processors and the Cell processors having different architectures. 
     The example hybrid computing environment ( 100 ) of  FIG. 1  includes a plurality of compute nodes ( 102 ), I/O nodes ( 108 ), and a service node ( 112 ). The compute nodes ( 102 ) are coupled through network ( 101 ) for data communications with one another and with the I/O nodes ( 108 ) and the service node ( 112 ). The data communications network ( 101 ) may be implemented as an Ethernet, Internet Protocol (‘IP’), PCIe, Infiniband, Fibre Channel, or other network as will occur to readers of skill in the art. 
     In the example hybrid computing environment ( 100 ) of  FIG. 1 , the compute nodes carry out principal user-level computer program execution, accepting administrative services, such as initial program loads and the like, from the service application ( 124 ) executing on the service node ( 112 ) and gaining access to data storage ( 116 ) and I/O functionality ( 118 ,  120 ) through the I/O nodes ( 108 ). In the example of  FIG. 1 , the I/O nodes ( 108 ) are connected for data communications to I/O devices ( 116 ,  118 ,  120 ) through a local area network (‘LAN’) ( 114 ) implemented using high-speed Ethernet or a data communications fabric of another fabric type as will occur to those of skill in the art. I/O devices in the example hybrid computing environment ( 100 ) of  FIG. 1  include non-volatile memory for the computing environment in the form of data storage device ( 116 ), an output device for the hybrid computing environment in the form of printer ( 118 ), and a user ( 126 ) I/O device in the form of computer terminal ( 120 ) that executes a service application interface ( 122 ) that provides to a user an interface for configuring compute nodes in the hybrid computing environment and initiating execution by the compute nodes of principal user-level computer program instructions. 
     In the example of  FIG. 1 , each compute node includes a host computer ( 110 ) having a host computer architecture and one or more accelerators ( 104 ) having an accelerator architecture. A host computer ( 110 ) is a ‘host’ in the sense that it is the host computer that carries out interface functions between a compute node and other components of the hybrid computing environment external to any particular compute node. That is, it is the host computer that executes initial boot procedures, power on self tests, basic I/O functions, accepts user-level program loads from service nodes, and so on. An accelerator ( 104 ) is an ‘accelerator’ in that each accelerator has an accelerator architecture that is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. Such accelerated computing functions include, for example, vector processing, floating point operations, and others as will occur to those of skill in the art. 
     Because each of the compute nodes in the example of  FIG. 1  includes a host computer and an accelerator, readers of skill in the art will recognize that each compute node represents a smaller, separate hybrid computing environment within the larger hybrid computing environment ( 100 ) of  FIG. 1 . That is, not only may the combination of the compute nodes ( 102 ) form a hybrid computing environment ( 100 ), but it is also the case that each individual compute node may also be viewed as a separate, smaller hybrid computing environment. The hybrid computing environment ( 100 ) in the example of  FIG. 1  then, may be viewed as composed of nine separate, smaller hybrid computing environments, one for each compute node, which taken together form the hybrid computing environment ( 100 ) of  FIG. 1 . Within each compute node ( 102 ) of  FIG. 1 , a host computer ( 110 ) and one or more accelerators ( 104 ) are adapted to one another for data communications by a system level message passing module (‘SLMPM’) ( 146 ) and by two or more data communications fabrics ( 106 ,  107 ) of at least two different fabric types. An SLMPM ( 146 ) is a module or library of computer program instructions that exposes an application programming interface (‘API’) to user-level applications for carrying out message-based data communications between the host computer ( 110 ) and the accelerator ( 104 ). Examples of message-based data communications libraries that may be improved for use as an SLMPM according to embodiments of the present invention include:
         the Message Passing Interface or ‘MPI,’ an industry standard interface in two versions, first presented at Supercomputing 1994, not sanctioned by any major standards body,   the Data Communication and Synchronization interface (‘DACS’) of the LANL supercomputer,   the POSIX Threads library (‘Pthreads’), an IEEE standard for distributed, multithreaded processing,   the Open Multi-Processing interface (‘OpenMP’), an industry-sanctioned specification for parallel programming, and   other libraries that will occur to those of skill in the art.       

     A data communications fabric ( 106 ,  107 ) is a configuration of data communications hardware and software that implements a data communications coupling between a host computer and an accelerator. Examples of data communications fabric types include Peripheral Component Interconnect (‘PCI’), PCI express (‘PCIe’), Ethernet, Infiniband, Fibre Channel, Small Computer System Interface (‘SCSI’), External Serial Advanced Technology Attachment (‘eSATA’), Universal Serial Bus (‘USB’), and so on as will occur to those of skill in the art. 
     The SLMPM ( 146 ) of  FIG. 1  operates generally for terminating an accelerator application program ( 204 ) in a hybrid computing environment ( 100 ). The SLMPM ( 146 ) may terminate an accelerator application program ( 204 ) by receiving, from a host application ( 166 ) executing on the host computer ( 110 ), a request ( 207 ) to terminate an accelerator application program executing on the accelerator; terminating execution of the accelerator application program by use of an instruction ( 210 ) to the terminate the application program ( 204 ); returning, to the host application ( 166 ), a signal ( 208 ) indicating that execution of the accelerator application program ( 204 ) was terminated; and performing a cleanup of the execution environment associated with the terminated accelerator application program. 
     The arrangement of compute nodes, data communications fabrics, networks, I/O devices, service nodes, I/O nodes, and so on, making up the hybrid computing environment ( 100 ) as illustrated in  FIG. 1  are for explanation only, not for limitation of the present invention. Hybrid computing environments useful for terminating an accelerator application according to embodiments of the present invention may include additional nodes, networks, devices, and architectures, not shown in  FIG. 1 , as will occur to those of skill in the art. Although the hybrid computing environment ( 100 ) in the example of  FIG. 1  includes only nine compute nodes ( 102 ), readers will note that hybrid computing environments according to embodiments of the present invention may include any number of compute nodes. The LANL supercomputer, taken as an example of a hybrid computing environment with multiple compute nodes, contains as of this writing more than 12,000 compute nodes. Networks and data communications fabrics in such hybrid computing environments may support many data communications protocols including for example TCP (Transmission Control Protocol), IP (Internet Protocol), and others as will occur to those of skill in the art. Various embodiments of the present invention may be implemented on a variety of hardware platforms in addition to those illustrated in  FIG. 1 . 
     For further explanation,  FIG. 2  sets forth a block diagram of an exemplary hybrid computing environment ( 100 ) useful for terminating an accelerator application according to embodiments of the present invention. The hybrid computing environment ( 100 ) of  FIG. 2  includes four compute nodes. Similar to the compute nodes of  FIG. 1 , each of the compute nodes in the example of  FIG. 2  may represent a small, separate hybrid computing environment which taken together make up a larger hybrid computing environment. One compute node ( 103 ) in the example of  FIG. 2  is illustrated in an expanded view to aid a more detailed explanation of such a hybrid computing environment ( 100 ). As shown in the expanded view of compute node ( 103 ), each of the compute nodes ( 102 ,  103 ) in the example of  FIG. 2  includes a host computer ( 110 ). The host computer ( 110 ) includes a computer processor ( 152 ) operatively coupled to computer memory, Random Access Memory (‘RAM’) ( 142 ), through a high speed memory bus ( 153 ). The processor ( 152 ) in each host computer ( 110 ) has a set of architectural registers ( 154 ) that defines the host computer architecture. 
     Each of the compute nodes also includes one or more accelerators ( 104 ,  105 ). Each accelerator ( 104 ,  105 ) includes a computer processor ( 148 ) operatively coupled to RAM ( 140 ) through a high speed memory bus ( 151 ). Stored in RAM ( 140 , 142 ) of the host computer and the accelerators ( 104 ,  105 ) is an operating system ( 145 ). Operating systems useful in host computers and accelerators of hybrid computing environments according to embodiments of the present invention include UNIX™, Linux™, Microsoft XP™, Microsoft Vista™, Microsoft NT™, AIX™, IBM&#39;s i5/OS™, and others as will occur to those of skill in the art. There is no requirement that the operating system in the host computers should be the same operating system used on the accelerators. 
     The processor ( 148 ) of each accelerator ( 104 ,  105 ) has a set of architectural registers ( 150 ) that defines the accelerator architecture. The architectural registers ( 150 ) of the processor ( 148 ) of each accelerator are different from the architectural registers ( 154 ) of the processor ( 152 ) in the host computer ( 110 ). With differing architectures, it would be uncommon, although possible, for a host computer and an accelerator to support the same instruction sets. As such, computer program instructions compiled for execution on the processor ( 148 ) of an accelerator ( 104 ) generally would not be expected to execute natively on the processor ( 152 ) of the host computer ( 110 ) and vice versa. Moreover, because of the typical differences in hardware architectures between host processors and accelerators, computer program instructions compiled for execution on the processor ( 152 ) of a host computer ( 110 ) generally would not be expected to execute natively on the processor ( 148 ) of an accelerator ( 104 ) even if the accelerator supported the instruction set of the host. The accelerator architecture in example of  FIG. 2  is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. That is, for the function or functions for which the accelerator is optimized, execution of those functions will proceed faster on the accelerator than if they were executed on the processor of the host computer. 
     In the example of  FIG. 2 , the host computer ( 110 ) and the accelerators ( 104 ,  105 ) are adapted to one another for data communications by a system level message passing module (‘SLMPM’) ( 146 ) and two data communications fabrics ( 128 ,  130 ) of at least two different fabric types. In this example, to support message-based data communications between the host computer ( 110 ) and the accelerator ( 104 ), both the host computer ( 110 ) and the accelerator ( 104 ) have an SLMPM ( 146 ) so that message-based communications can both originate and be received on both sides of any coupling for data communications. Also in the example of  FIG. 2 , the host computer ( 110 ) and the accelerators ( 104 ,  105 ) are adapted to one another for data communications by a PCIe fabric ( 130 ) through PCIe communications adapters ( 160 ) and an Ethernet fabric ( 128 ) through Ethernet communications adapters ( 161 ). The use of PCIe and Ethernet is for explanation, not for limitation of the invention. Readers of skill in the art will immediately recognize that hybrid computing environments according to embodiments of the present invention may include fabrics of other fabric types such as, for example, PCI, Infiniband, Fibre Channel, SCSI, eSATA, USB, and so on. 
     The SLMPM ( 146 ) in this example operates generally for data processing in a hybrid computing environment ( 100 ) by monitoring data communications performance for a plurality of data communications modes between the host computer ( 110 ) and the accelerators ( 104 ,  105 ), receiving a request ( 168 ) to transmit data according to a data communications mode from the host computer to an accelerator, determining whether to transmit the data according to the requested data communications mode, and if the data is not to be transmitted according to the requested data communications mode: selecting another data communications mode and transmitting the data according to the selected data communications mode. In the example of  FIG. 2 , the monitored performance is illustrated as monitored performance data ( 174 ) stored by the SLMPM ( 146 ) in RAM ( 142 ) of the host computer ( 110 ) during operation of the compute node ( 103 ). 
     A data communications mode specifies a data communications fabric type, a data communications link, and a data communications protocol ( 178 ). A data communications link ( 156 ) is data communications connection between a host computer and an accelerator. In the example of  FIG. 2 , a link ( 156 ) between the host computer ( 110 ) and the accelerator ( 104 ) may include the PCIe connection ( 138 ) or the Ethernet connection ( 131 ,  132 ) through the Ethernet network ( 106 ). A link ( 156 ) between the host computer ( 110 ) and the accelerator ( 105 ) in the example of  FIG. 2  may include the PCIe connection ( 136 ) or the Ethernet connection ( 131 ,  134 ) through the Ethernet network ( 106 ). Although only one link for each fabric type is illustrated between the host computer and the accelerator in the example of  FIG. 2 , readers of skill in the art will immediately recognize that there may any number of links for each fabric type. 
     A data communications protocol is a set of standard rules for data representation, signaling, authentication and error detection required to send information from a host computer ( 110 ) to an accelerator ( 104 ). In the example of  FIG. 2 , the SLMPM ( 146 ) may select one of several protocols ( 178 ) for data communications between the host computer ( 110 ) and the accelerator. Examples of such protocols ( 178 ) include shared memory transfers (‘SMT’) ( 180 ) executed with a send and receive operations ( 181 ), and direct memory access (‘DMA’) ( 182 ) executed with PUT and GET operations ( 183 ). 
     Shared memory transfer is a data communications protocol for passing data between a host computer and an accelerator into shared memory space ( 158 ) allocated for such a purpose such that only one instance of the data resides in memory at any time. Consider the following as an example shared memory transfer between the host computer ( 110 ) and the accelerator ( 104 ) of  FIG. 2 . An application ( 166 ) requests ( 168 ) a transmission of data ( 176 ) from the host computer ( 110 ) to the accelerator ( 104 ) in accordance with the SMT ( 180 ) protocol. Such a request ( 168 ) may include a memory address allocated for such shared memory. In this example, the shared memory segment ( 158 ) is illustrated in a memory location on the accelerator ( 104 ), but readers will recognize that shared memory segments may be located on the accelerator ( 104 ), on the host computer ( 110 ), on both the host computer and the accelerator, or even off the local compute node ( 103 ) entirely—so long as the segment is accessible as needed by the host and the accelerator. To carry out a shared memory transfer, the SLMPM ( 146 ) on the host computer ( 110 ) establishes a data communications connection with the SLMPM ( 146 ) executing on the accelerator ( 104 ) by a handshaking procedure similar to that in the TCP protocol. The SLMPM ( 146 ) then creates a message ( 170 ) that includes a header and a payload data and inserts the message into a message transmit queue for a particular link of a particular fabric. In creating the message, the SLMPM inserts, in the header of the message, an identification of the accelerator and an identification of a process executing on the accelerator. The SLMPM also inserts the memory address from the request ( 168 ) into the message, either in the header or as part of the payload data. The SLMPM also inserts the data ( 176 ) to be transmitted in the message ( 170 ) as part of the message payload data. The message is then transmitted by a communications adapter ( 160 ,  161 ) across a fabric ( 128 ,  130 ) to the SLMPM executing on the accelerator ( 104 ) where the SLMPM stores the payload data, the data ( 176 ) that was transmitted, in shared memory space ( 158 ) in RAM ( 140 ) in accordance with the memory address in the message. 
     Direct memory access (‘DMA’) is a data communications protocol for passing data between a host computer and an accelerator with reduced operational burden on the computer processor ( 152 ). A DMA transfer essentially effects a copy of a block of memory from one location to another, typically from a host computer to an accelerator or vice versa. Either or both a host computer and accelerator may include DMA engine, an aggregation of computer hardware and software for direct memory access. Direct memory access includes reading and writing to memory of accelerators and host computers with reduced operational burden on their processors. A DMA engine of an accelerator, for example, may write to or read from memory allocated for DMA purposes, while the processor of the accelerator executes computer program instructions, or otherwise continues to operate. That is, a computer processor may issue an instruction to execute a DMA transfer, but the DMA engine, not the processor, carries out the transfer. 
     In the example of  FIG. 2 , only the accelerator ( 104 ) includes a DMA engine ( 184 ) while the host computer does not. In this embodiment the processor ( 152 ) on the host computer initiates a DMA transfer of data from the host to the accelerator by sending a message according to the SMT protocol to the accelerator, instructing the accelerator to perform a remote ‘GET’ operation. The configuration illustrated in the example of  FIG. 2  in which the accelerator ( 104 ) is the only device containing a DMA engine is for explanation only, not for limitation. Readers of skill in the art will immediately recognize that in many embodiments, both a host computer and an accelerator may include a DMA engine, while in yet other embodiments only a host computer includes a DMA engine. 
     To implement a DMA protocol in the hybrid computing environment of  FIG. 2  some memory region is allocated for access by the DMA engine. Allocating such memory may be carried out independently from other accelerators or host computers, or may be initiated by and completed in cooperation with another accelerator or host computer. Shared memory regions, allocated according to the SMA protocol, for example, may be memory regions made available to a DMA engine. That is, the initial setup and implementation of DMA data communications in the hybrid computing environment ( 100 ) of  FIG. 2  may be carried out, at least in part, through shared memory transfers or another out-of-band data communications protocol, out-of-band with respect to a DMA engine. Allocation of memory to implement DMA transfers is relatively high in latency, but once allocated, the DMA protocol provides for high bandwidth data communications that requires less processor utilization than many other data communications protocols. 
     A direct ‘PUT’ operation is a mode of transmitting data from a DMA engine on an origin device to a DMA engine on a target device. A direct ‘PUT’ operation allows data to be transmitted and stored on the target device with little involvement from the target device&#39;s processor. To effect minimal involvement from the target device&#39;s processor in the direct ‘PUT’ operation, the origin DMA engine transfers the data to be stored on the target device along with a specific identification of a storage location on the target device. The origin DMA knows the specific storage location on the target device because the specific storage location for storing the data on the target device has been previously provided by the target DMA engine to the origin DMA engine. 
     A remote ‘GET’ operation, sometimes denominated an ‘rGET,’ is another mode of transmitting data from a DMA engine on an origin device to a DMA engine on a target device. A remote ‘GET’ operation allows data to be transmitted and stored on the target device with little involvement from the origin device&#39;s processor. To effect minimal involvement from the origin device&#39;s processor in the remote ‘GET’ operation, the origin DMA engine stores the data in an storage location accessible by the target DMA engine, notifies the target DMA engine, directly or out-of-band through a shared memory transmission, of the storage location and the size of the data ready to be transmitted, and the target DMA engine retrieves the data from storage location. 
     Monitoring data communications performance for a plurality of data communications modes may include monitoring a number of requests ( 168 ) in a message transmit request queue ( 162 - 165 ) for a data communications link ( 156 ). In the example of  FIG. 2 , each message transmit request queue ( 162 - 165 ) is associated with one particular data communications link ( 156 ). Each queue ( 162 - 165 ) includes entries for messages ( 170 ) that include data ( 176 ) to be transmitted by the communications adapters ( 160 ,  161 ) along a data communications link ( 156 ) associated with queue. 
     Monitoring data communications performance for a plurality of data communications modes may also include monitoring utilization of a shared memory space ( 158 ). In the example of  FIG. 2 , shared memory space ( 158 ) is allocated in RAM ( 140 ) of the accelerator. Utilization is the proportion of the allocated shared memory space to which data has been stored for sending to a target device and has not yet been read or received by the target device, monitored by tracking the writes and reads to and from the allocated shared memory. In the hybrid computing environment ( 100 ) of  FIG. 2 , shared memory space, any memory in fact, is limited. As such, a shared memory space ( 158 ) may be filled during execution of an application program ( 166 ) such that transmission of data from the host computer ( 110 ) to an accelerator may be slowed, or even stopped, due to space limitations in the shared memory space. 
     In some embodiments of the present invention, the hybrid computing environment ( 100 ) of  FIG. 2  may be configured to operate as a parallel computing environment in which two or more instances the host application program ( 166 ) executes on two or more host computers ( 110 ) in the parallel computing environment. In such embodiments, monitoring data communications performance across data communications modes may also include aggregating data communications performance information ( 174 ) across a plurality of instances of the host application program ( 166 ) executing on two or more host computers in a parallel computing environment. The aggregated performance information ( 174 ) may be used to calculate average communications latencies for data communications modes, average number of requests in data communications links of a particular fabric type, average shared memory utilization among the plurality of host computers and accelerators in the parallel computing environment, and so on as will occur to those of skill in the art. Any combination of such measures may be used by the SLMPM for both determining whether to transmit the data according to requested data communications mode and selecting another data communications mode for transmitting the data if the data is not to be transmitted according to the requested data communications mode. 
     The SLMPM ( 146 ) of  FIG. 2  receives, from an application program ( 166 ) on the host computer ( 110 ), a request ( 168 ) to transmit data ( 176 ) according to a data communications mode from the host computer ( 110 ) to the accelerator ( 104 ). Such data ( 176 ) may include computer program instructions compiled for execution by the accelerator ( 104 ), work piece data for an application program executing on the accelerator ( 104 ), or some combination of computer program instructions and work piece data. Receiving a request ( 168 ) to transmit data ( 176 ) according to a data communications mode may include receiving a request to transmit data by a specified fabric type, receiving a request to transmit data through a specified data communications link from the host computer to the accelerator, or receiving a request to transmit data from the host computer to the accelerator according to a protocol. 
     A request ( 168 ) to transmit data ( 176 ) according to a data communications mode may be implemented as a user-level application function call through an API to the SLMPM ( 146 ), a call that expressly specifies a data communications mode according to protocol, fabric type, and link. A request implemented as a function call may specify a protocol according to the operation of the function call itself. A dacs_put( ) function call, for example, may represent a call through an API exposed by an SLMPM implemented as a DACS library to transmit data in the default mode of a DMA ‘PUT’ operation. Such a call, from the perspective of the calling application and the programmer who wrote the calling application, represents a request to the SLMPM library to transmit data according to the default mode, known to the programmer to be default mode associated with the express API call. The called function, in this example dacs_put( ) may be coded according to embodiments of the present invention, to make its own determination whether to transmit the data according to the requested data communications mode, that is, according to the default mode of the called function. In a further example, a dacs_send( ) instruction may represent a call through an API exposed by an SLMPM implemented as a DACS library to transmit data in the default mode of an SMT ‘send’ operation, where the called function dacs_send( ) is again coded according to embodiments of the present invention to make its own determination whether to transmit the data according to the requested mode. 
     An identification of a particular accelerator in a function call may effectively specify a fabric type. Such a function call may include as a call parameter an identification of a particular accelerator. An identification of a particular accelerator by use of a PCIe ID, for example, effectively specifies a PCI fabric type. In another, similar, example, an identification of a particular accelerator by use of a media access control (‘MAC’) address of an Ethernet adapter effectively specifies the Ethernet fabric type. Instead of implementing the accelerator ID of the function call from an application executing on the host in such a way as to specify a fabric type, the function call may only include a globally unique identification of the particular accelerator as a parameter of the call, thereby specifying only a link from the host computer to the accelerator, not a fabric type. In this case, the function called may implement a default fabric type for use with a particular protocol. If the function called in the SLMPM is configured with PCIe as a default fabric type for use with the DMA protocol, for example, and the SLMPM receives a request to transmit data to the accelerator ( 104 ) according to the DMA protocol, a DMA PUT or DMA remote GET operation, the function called explicitly specifies the default fabric type for DMA, the PCIe fabric type. 
     In hybrid computing environments in which only one link of each fabric type adapts a single host computer to a single accelerator, the identification of a particular accelerator in a parameter of a function call, may also effectively specify a link. In hybrid computing environments where more than one link of each fabric type adapts a host computer and an accelerator, such as two PCIe links connecting the host computer ( 110 ) to the accelerator ( 104 ), the SLMPM function called may implement a default link for the accelerator identified in the parameter of the function call for the fabric type specified by the identification of the accelerator. 
     The SLMPM ( 146 ) in the example of  FIG. 2  also determines, in dependence upon the monitored performance ( 174 ), whether to transmit the data ( 176 ) according to the requested data communications mode. Determining whether to transmit the data ( 176 ) according to the requested data communications mode may include determining whether to transmit data by a requested fabric type, whether to transmit data through a requested data communications link, or whether to transmit data according to a requested protocol. 
     In hybrid computing environments in which monitoring data communications performance across data communications modes includes monitoring a number of requests in a message transmit request queue ( 162 - 165 ) for a data communications link, determining whether to transmit the data ( 176 ) according to the requested data communications mode may be carried out by determining whether the number of requests in the message transmit request queue exceeds a predetermined threshold. In hybrid computing environments in which monitoring data communications performance for a plurality of data communications modes includes monitoring utilization of a shared memory space, determining whether to transmit the data ( 176 ) according to the requested data communications mode may be carried out by determining whether the utilization of the shared memory space exceeds a predetermined threshold. 
     If the data is not to be transmitted according to the requested data communications mode, the SLMPM ( 146 ) selects, in dependence upon the monitored performance, another data communications mode for transmitting the data and transmits the data ( 176 ) according to the selected data communications mode. Selecting another data communications mode for transmitting the data may include selecting, in dependence upon the monitored performance, another data communications fabric type by which to transmit the data, selecting a data communications link through which to transmit the data, and selecting another data communications protocol. Consider as an example, that the requested data communications mode is a DMA transmission using a PUT operation through link ( 138 ) of the PCIe fabric ( 130 ) to the accelerator ( 104 ). If the monitored data performance ( 174 ) indicates that the number of requests in transmit message request queue ( 162 ) associated with the link ( 138 ) exceeds a predetermined threshold, the SLMPM may select another fabric type, the Ethernet fabric ( 128 ), and link ( 131 ,  132 ) through which to transmit the data ( 176 ). Also consider that the monitored performance ( 176 ) indicates that current utilization of the shared memory space ( 158 ) is less than a predetermined threshold while the number of outstanding DMA transmissions in the queue ( 162 ) exceeds a predetermined threshold. In such a case, the SLMPM ( 146 ) may also select another protocol, such as a shared memory transfer, by which to transmit the data ( 174 ). 
     Selecting, by the SLMPM, another data communications mode for transmitting the data ( 172 ) may also include selecting a data communications protocol ( 178 ) in dependence upon data communications message size ( 172 ). Selecting a data communications protocol ( 178 ) in dependence upon data communications message size ( 172 ) may be carried out by determining whether a size of a message exceeds a predetermined threshold. For larger messages ( 170 ), the DMA protocol may be a preferred protocol as processor utilization in making a DMA transfer of a larger message ( 170 ) is typically less than the processor utilization in making a shared memory transfer of a message of the same size. 
     As mentioned above, the SLMPM may also transmit the data according to the selected data communications mode. Transmit the data according to the selected data communications mode may include transmitting the data by the selected data communications fabric type, transmitting the data through the selected data communications link, or transmitting the data according to the selected protocol. The SLMPM ( 146 ) may effect a transmission of the data according to the selected data communications mode by instructing, through a device driver, the communications adapter for the data communications fabric type of the selected data communications mode to transmit the message ( 170 ) according to a protocol of the selected data communications mode, where the message includes in a message header, an identification of the accelerator, and in the message payload, the data ( 176 ) to be transmitted. 
     The SLMPM ( 146 ) in the example of  FIG. 2  operates generally for terminating an accelerator application program ( 204 ) in a hybrid computing environment ( 100 ) of  FIG. 2 . An ‘accelerator’ application program ( 204 ) is a module of computer program instructions executed on an accelerator that performs one or more tasks, typically but not necessarily, for the benefit of a host application program ( 166 ) that requests execution of the accelerator application program. A ‘host’ application program, by contrast, is an application that runs on a host computer and may request execution of one or more accelerator application programs for its own benefit. An accelerator typically processes the accelerator application program in an accelerated fashion, that is, the accelerator carries out one or more functions of the accelerator application program faster than the functions would normally be processed by a host computer due to the fact that the accelerator&#39;s architecture is optimized, with respect to the host computer architecture, for speed of execution of the one or more functions. Accelerator application programs executed in accordance with embodiments of the present invention may be standalone modules of computer program instructions or modules of computer program instructions that would otherwise be executed at the behest of a host application program. The accelerator application program ( 204 ) of  FIG. 2 , for example, may be a standalone module of computer program instructions configured for execution on the accelerator ( 104 ) or a module of computer program instructions that is executed at the behest and for the benefit of the host application program ( 166 ) executing on the host computer ( 110 ). 
     As mentioned above, the SLMPM ( 146 ) in the example of  FIG. 2  operates generally for terminating an accelerator application ( 204 ) in a hybrid computing environment ( 100 ) in accordance with embodiments of the present invention. The SLMPM ( 146 ) may terminate an accelerator application ( 204 ) by receiving, from a host application ( 166 ) executing on the host computer ( 110 ), a request ( 207 ) to terminate an accelerator application program ( 204 ) executing on the accelerator ( 104 ). The example SLMPM ( 146 ) of  FIG. 2  may receive a request ( 207 ) to terminate an accelerator application program ( 204 ) from a host application ( 166 ) by receiving from the host application ( 166 ) through an application program interface exposed by the SLMPM ( 146 ), a function call to an SLMPM function with parameters that uniquely identify the accelerator application program to be terminated. The following function call, written in pseudocode for clarity of explanation, is an example of a function call with parameters that uniquely identify the accelerator application program to be terminated: dacs_de_kill (‘ACC_ID_1’, ‘PID_101’, DACS_KILL_TYPE_ASYNC). This example function call is ‘pseudocode,’ so called because it is an explanation expressed in the form of code as opposed to an actual working model of computer code. This pseudocode is an example of a call to a DACS library function called ‘de_kill’ which initiates termination of a particular accelerator application program ( 204 ) executing on an accelerator ( 104 ). The parameters of the function include an identity of the accelerator ( 104 ), ‘ACC_ID_1,’ a process identifier (‘ID’) of the accelerator application program ( 204 ), ‘PID_101,’ and a type of the function, in this example an asynchronous type. In the example hybrid computing environment ( 100 ) of  FIG. 2 , the SLMPM ( 146 ) may terminate execution of the accelerator application program ( 204 ) without terminating the host application ( 110 ) and without terminating other accelerator application programs (not shown) executing on the accelerator. In this way, a single unique accelerator application may be targeted and terminated without affecting operation of other applications. 
     In the hybrid computing environment ( 100 ) of  FIG. 2 , the host application ( 166 ) may send, asynchronously with respect to operation of the host application ( 166 ), the request to terminate the accelerator application program ( 204 ) responsive to determining that the accelerator application program is nonresponsive. That is, the host application ( 166 ) may determine, from lack of communications, a failed ping, or other method, that the accelerator application program ( 204 ) is nonresponsive and as such decide to terminate the accelerator application program ( 204 ). 
     The SLMPM ( 146 ), responsive to receiving the request ( 207 ) to terminate the accelerator application program ( 204 ), may terminate execution of the accelerator application program ( 204 ). The SLMPM ( 146 ) may terminate execution of the accelerator application program ( 204 ) by sending a POSIX-style ‘ SIGTERM’ signal followed by a ‘SIGKILL’ signal to the accelerator application program ( 204 ), depicted as an instruction to terminate ( 210 ) the accelerator application ( 204 ) in the example of  FIG. 2 . SIGTERM, on POSIX-compliant platforms, is a default signal sent to a process, to cause the termination of a process, but unlike the SIGKILL signal, it can be caught and interpreted (or ignored) by the process. SIGTERM is akin to asking a process to terminate nicely, allowing cleanup and closure of files. For this reason, on many Unix systems during shutdown, init issues SIGTERM to all processes that are not essential to powering off, waits a few seconds, and then issues SIGKILL to forcibly terminate any such processes that remain. SIGKILL, on POSIX-compliant platforms, is a signal sent to a process to cause it to terminate immediately. The SIGKILL signal cannot be caught by a processor or ignored by the process. 
     The SLMPM ( 146 ) may returning, to the host application ( 166 ), a signal indicating that execution of the accelerator application program was terminated. The SLMPM ( 146 ) may return a signal indicating that execution of the accelerator application program was terminated by sending to the host application ( 166 ) a POSIX-style, SIGCHLD message. On POSIX-compliant platforms, SIGCHLD is the signal sent to a process when a child process terminates. In Unix, a process can have children created by fork or similar system calls. In a hybrid computing environment, the accelerator application program executed at the behest of a host application is an analog of a child process created by a form or other system call. When the child terminates a SIGCHLD signal is sent to the parent. 
     The SLMPM ( 146 ) may also perform a cleanup of the execution environment associated with the terminated accelerator application program. Performing a cleanup of the execution environment associated with the terminated accelerator application program ( 204 ) may include performing a device cleanup including deregistering a process identifier of the terminated accelerator application program ( 204 ) with the accelerator. Performing a cleanup of the execution environment associated with the terminated accelerator application program ( 204 ) may also include performing a connection cleanup including removing mappings of a process identifier of the terminated accelerator application program ( 204 ) to process identifiers of one or more other accelerator application programs. Performing a cleanup of the execution environment associated with the terminated accelerator application program ( 204 ) may also include causing undelivered messages associated with the terminated accelerator application program ( 204 ) to indicate an error. Performing a cleanup of the execution environment associated with the terminated accelerator application program may also include administering message queues associated with the terminated accelerator application program ( 204 ), administering execution environment objects shared by the terminated accelerator application program ( 204 ), and invalidating a process identifier of the terminated accelerator application program for access through the SLMPM ( 146 ). 
     Upon receiving the signal ( 208 ) indicating that execution of the accelerator application program ( 204 ) was terminated, the host application ( 166 ) may reap the process by executing a dacs_de_test or dacs_de_wait function call. In reaping the process, additional cleanup of shared memory regions or shared objects may be performed by the host application ( 166 ) or the SLMPM ( 146 ). 
     For further explanation,  FIG. 3  sets forth a block diagram of a further exemplary hybrid computing environment ( 100 ) useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. The hybrid computing environment of  FIG. 3  is similar the hybrid computing environment of  FIG. 2 , including as it does, four compute nodes ( 102 ,  103 ), each of which includes a host computer ( 110 ) having a host computer architecture and an accelerator ( 104 ) having an accelerator architecture where the accelerator architecture is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. The host computer ( 110 ) and the accelerator ( 104 ) are adapted to one another for data communications by a system level message passing module ( 146 ) and two or more data communications fabrics ( 128 ,  130 ) of at least two different fabric types. In the example of  FIG. 3 , the host computer ( 110 ) is adapted to accelerator ( 104 ) by an Ethernet fabric ( 128 ) and a PCIe fabric ( 130 ). 
     The host computer ( 110 ) as illustrated in the expanded view of the compute node ( 103 ) includes an x86 processor. An x86 processor is a processor whose architecture is based upon the architectural register set of the Intel x86 series of microprocessors, the 386, the 486, the 586 or Pentium™, and so on. Examples of x86 processors include the Advanced Micro Devices (‘AMD’) Opteron™, the AMD Phenom™, the AMD Athlon XP™, the AMD Athlon 64™, Intel Nehalam™, Intel Pentium 4, Intel Core 2 Duo, Intel Atom, and so on as will occur to those of skill in the art. The x86 processor ( 152 ) in the example of Figure illustrates a set of a typical architectural registers ( 154 ) found in many x86 processors including, for example, an accumulator register (‘AX’), a base register (‘BX’), a counter register (‘CX’), a data register (‘DX’), a source index register for string operations (‘SI’), a destination index for string operations (‘DI’), a stack pointer (‘SP’), a stack base pointer for holding the address of the current stack frame (‘BP’), and an instruction pointer that holds the current instruction address (‘IP’). 
     The accelerator ( 104 ) in the example of  FIG. 3  is illustrated as a Cell Broadband Engine (‘CBE’) having a Cell Broadband Engine Architecture (‘CBEA’). A CBEA is a microprocessor architecture jointly developed by Sony Computer Entertainment, Toshiba, and IBM, an alliance known as “STI.” Microprocessors implemented according to the CBEA are sometimes referred to as ‘Cell’ processors or simply as CBEs. The CBEA combines a general-purpose POWER architecture core, a Power Processing Element (‘PPE’) ( 148 ), of modest performance with streamlined co-processing elements, called Synergistic Processing Elements (‘SPEs’) ( 308 ) which greatly accelerate multimedia and vector processing applications, as well as many other forms of dedicated computation. The CBE architecture emphasizes efficiency/watt, prioritizes bandwidth over latency, and favors peak computational throughput over simplicity of program code. 
     The accelerator ( 104 ) of  FIG. 3 , implemented as a CBE, includes a main processor ( 148 ) that in this example is a Power Processing Element (‘PPE’), eight fully-functional co-processors called SPEs ( 308 ), and a high-bandwidth circular data bus connecting the PPE and the SPEs, called the Element Interconnect Bus (‘EIB’) ( 312 ). The PPE ( 148 ) is a POWER architecture processor with a two-way multithreaded core acting as a controller for the eight SPEs ( 308 ). The term “POWER architecture” here refers to IBM&#39;s different generations of processor architectures, a broad term including all products based on POWER, PowerPC and Cell architectures. The architectural registers ( 150 ) of the PPE ( 148 ) of the CBE accelerator ( 104 ) therefore are different from those of the x86 processor ( 152 ) of the host computer ( 110 ). The PPE ( 148 ) of  FIG. 3  includes an example set of architectural registers ( 150 ) of the POWER architecture, including 32 general purpose registers (‘GPRs’), 32 floating point registers (‘FPRs’), a fixed-point exception register (‘XER’), a count register (‘CTR’), a Condition register (‘CR’), an instruction address register (‘IAR’), a link register (‘LR’), and a processor version register (‘PVR’). 
     The SPEs ( 308 ) handle most of the computational workload of the CBE ( 104 ). While the SPEs are optimized for vectorized floating point code execution, the SPEs also may execute operating systems, such as, for example, a lightweight, modified version of Linux with the operating system stored in local memory ( 141 ) on the SPE. Each SPE ( 308 ) in the example of  FIG. 3  includes a Synergistic Processing Unit (‘SPU’) ( 302 ), and a Memory Flow Controller (‘MFC’) ( 310 ). An SPU ( 302 ) is a Reduced Instruction Set Computing (‘RISC’) processor with 128-bit single instruction, multiple data (‘SIMD’) organization for single and double precision instructions. In some implementations, an SPU may contain a 256 KB embedded Static RAM ( 141 ) for instructions and data, called local storage which is visible to the PPE ( 148 ) and can be addressed directly by software. Each SPU ( 302 ) can support up to 4 Gigabyte (‘GB’) of local store memory. The local store does not operate like a conventional CPU cache because the local store is neither transparent to software nor does it contain hardware structures that predict which data to load. The SPUs ( 302 ) also implement architectural registers ( 306 ) different from those of the PPE which include a 128-bit, 128-entry register file ( 307 ). An SPU ( 302 ) can operate on 16 8-bit integers, 8 16-bit integers, 4 32-bit integers, or 4 single precision floating-point numbers in a single clock cycle, as well as execute a memory operation. 
     The MFC ( 310 ) integrates the SPUs ( 302 ) in the CBE ( 104 ). The MFC ( 310 ) provides an SPU with data transfer and synchronization capabilities, and implements the SPU interface to the EIB ( 312 ) which serves as the transportation hub for the CBE ( 104 ). The MFC ( 310 ) also implements the communication interface between the SPE ( 308 ) and PPE ( 148 ), and serves as a data transfer engine that performs bulk data transfers between the local storage ( 141 ) of an SPU ( 302 ) and CBE system memory, RAM ( 140 ), through DMA. By offloading data transfer from the SPUs ( 302 ) onto dedicated data transfer engines, data processing and data transfer proceeds in parallel, supporting advanced programming methods such as software pipelining and double buffering. Providing the ability to perform high performance data transfer asynchronously and in parallel with data processing on the PPE ( 148 ) and SPEs ( 302 ), the MFC ( 310 ) eliminates the need to explicitly interleave data processing and transfer at the application level. 
     The SLMPM ( 146 ) in the example of  FIG. 3  processes data in the hybrid computing environment ( 100 ) by monitoring data communications performance for a plurality of data communications modes between the host computer ( 110 ) and the accelerator ( 104 ); receiving, from an application program ( 166 ) on the host computer ( 110 ), a request to transmit data according to a data communications mode from the host computer ( 110 ) to the accelerator ( 104 ); determining, in dependence upon the monitored performance, whether to transmit the data according to the requested data communications mode; and if the data is not to be transmitted according to the requested data communications mode: selecting, in dependence upon the monitored performance, another data communications mode for transmitting the data and transmitting the data according to the selected data communications mode. 
     The SLMPM ( 146 ) of  FIG. 3  operates generally for terminating an accelerator application program in a hybrid computing environment ( 100 ). The SLMPM ( 146 ) may terminate an accelerator application program by receiving, from a host application executing on the host computer ( 110 ), a request to terminate an accelerator application program executing on the accelerator; terminating execution of the accelerator application program; returning, to the host application, a signal indicating that execution of the accelerator application program was terminated; and performing a cleanup of the execution environment associated with the terminated accelerator application program. 
     For further explanation,  FIG. 4  sets forth a block diagram of a further exemplary hybrid computing environment ( 100 ) useful for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. The hybrid computing environment of  FIG. 4  is similar the hybrid computing environment of  FIG. 2 , including as it does, four compute nodes ( 102 ,  103 ), each of which includes a host computer ( 110 ) having a host computer architecture and one or more accelerators ( 104 ) each having an accelerator architecture where the accelerator architecture is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. The host computer ( 110 ) and the accelerator ( 104 ) in the example of  FIG. 4  are adapted to one another for data communications by a system level message passing module ( 146 ) and two or more data communications fabrics ( 128 ,  130 ) of at least two different fabric types. In the example of  FIG. 4 , the host computer ( 110 ) is adapted to accelerator ( 104 ) by an Ethernet fabric ( 128 ) and a PCIe fabric ( 130 ). 
       FIG. 4  illustrates an example of a hybrid computing environment similar to that implemented in the LANL supercomputer. The host computer ( 110 ), as illustrated by the expanded view of the compute node ( 103 ), implemented in the LANL supercomputer includes two AMD Opteron processors ( 155 ), each of which is a dual-core processor. Each of the cores ( 152 ) of the host computer ( 110 ) is illustrated in the example of  FIG. 4  as a single, fully functional x86 processor core with each core having its own set of architectural registers ( 154 ). Each of the processor cores ( 152 ) in the example of  FIG. 4  is operatively coupled to RAM ( 142 ) where an instance of an application program ( 166 ), an instance of the SLMPM ( 146 ), and an operating system ( 145 ) is stored. In the example of the LANL supercomputer, the SLMPM ( 146 ) is the Data Communication and Synchronization (‘DACS’) library improved according to embodiments of the present invention. 
     Each x86 processor core ( 152 ) in the example of  FIG. 4  is adapted through an Ethernet ( 128 ) and PCIe ( 130 ) fabric to a separate accelerator ( 104 ) implemented as a CBE as described above with respect to  FIG. 3 . Each core ( 152 ) of each AMD Opteron processor ( 155 ) in the host computer ( 110 ) in this example is connected to at least one CBE. Although in this example the ratio of cores of the Opteron processors to CBEs ( 104 ) is one-to-one, readers of skill in the art will recognize that other example embodiments may implement different ratios of processor cores to accelerators such as, for example, one-to-two, one-to-three, and so on. 
     Each instance of the SLMPM ( 146 ) executing on each x86 processor core ( 152 ) in the example of  FIG. 4  processes data in the hybrid computing environment ( 100 ) by monitoring data communications performance across data communications modes between the host computer ( 110 ) and the accelerator ( 104 ) connected to the processor core ( 152 ); receiving, from the instance of the application program ( 166 ) executing on the processor core ( 152 ) of the host computer ( 110 ), a request to transmit data according to a data communications mode from the host computer ( 110 ) to the accelerator ( 104 ) connected to the processor core ( 152 ); determining, in dependence upon the monitored performance, whether to transmit the data according to the requested data communications mode; and if the data is not to be transmitted according to the requested data communications mode: selecting, in dependence upon the monitored performance, another data communications mode for transmitting the data and transmitting the data according to the selected data communications mode. 
     Each instance of the SLMPM ( 146 ) of  FIG. 4  may operate generally for terminating an accelerator application program in a hybrid computing environment ( 100 ). An SLMPM ( 146 ) may terminate an accelerator application program by receiving, from a host application executing on the host computer ( 110 ), a request to terminate an accelerator application program executing on the accelerator; terminating execution of the accelerator application program; returning, to the host application, a signal indicating that execution of the accelerator application program was terminated; and performing a cleanup of the execution environment associated with the terminated accelerator application program. 
     For further explanation,  FIG. 5  sets forth a flow chart illustrating an exemplary method for terminating an accelerator application program in a hybrid computing environment according to embodiments of the present invention. The method of  FIG. 5  is carried out in a hybrid computing environment similar to the hybrid computing environments ( 100 ) depicted in  FIGS. 1-4 . The hybrid computing environment ( 100 ) includes a host computer ( 110 ) having a host computer architecture and an accelerator ( 104 ) having an accelerator architecture. The accelerator architecture is optimized, with respect to the host computer architecture, for speed of execution of a particular class of computing functions. The host computer ( 110 ) and the accelerator ( 104 ) are adapted to one another for data communications by a system level message passing module (‘SLMPM’) ( 146 ). The accelerator ( 104 ) is also configured to execute one or more accelerator application programs. 
     The method of  FIG. 5  includes sending ( 516 ), asynchronously by the host application to the SLMPM ( 146 ), a request ( 207 ) to terminate the accelerator application program executing on the accelerator responsive to determining that the accelerator application program is nonresponsive. In the method of  FIG. 5 , the request ( 207 ) to terminate the accelerator application program ( 204 ) includes an accelerator identifier ( 520 ), uniquely identifying the accelerator ( 104 ) on which the accelerator application program ( 104 ) to terminate is presently executing, and a process identifier ( 522 ) uniquely identifying the accelerator application ( 204 ). 
     The method of  FIG. 5  includes receiving ( 502 ), by the SLMPM ( 146 ) from the host application ( 166 ) executing on the host computer ( 110 ), the request ( 207 ) to terminate an accelerator application program ( 204 ) executing on the accelerator ( 104 ). In the example of  FIG. 5 , receiving ( 502 ) a request ( 207 ) to terminate accelerator application program ( 204 ) executing on the accelerator ( 104  includes receiving ( 512 ) the request ( 207 ) to terminate the accelerator application program through an Application Programming Interface (‘API’) ( 518 ) exposed to the host application ( 166 ) by the SLMPM ( 146 ). 
     The method of  FIG. 5  also includes terminating ( 504 ), by the SLMPM ( 146 ), execution of the accelerator application program ( 204 ). IN the method of  FIG. 5 , the SLMPM ( 146 ) terminates execution of the accelerator application program ( 204 ) by sending to the accelerator application program ( 204 ) a POSIX-style SIGKILL signal. In the method of  FIG. 5 , terminating ( 504 ), by the SLMPM ( 146 ), execution of the accelerator application program ( 204 ) also includes terminating ( 514 ) execution of the accelerator application program without terminating the host application and without terminating other accelerator application programs executing on the accelerator. 
     The method of  FIG. 5  also includes returning ( 506 ), by the SLMPM ( 146 ) to the host application ( 166 ), a signal ( 208 ) indicating that execution of the accelerator application program ( 204 ) was terminated. Such a signal ( 208 ) may include a Posix-style SIGCHLD signal. 
     The method of  FIG. 5  also includes performing ( 508 ), by the SLMPM ( 146 ), a cleanup of the execution environment associated with the terminated accelerator application program. For further explanation of the method of  FIG. 5 ,  FIG. 6  sets forth an exemplary method of performing ( 508 ), by the SLMPM ( 146 ), a cleanup of the execution environment associated with the terminated accelerator application program in accordance with embodiments of the present invention. 
     The method of  FIG. 6  includes performing ( 602 ) a device cleanup including deregistering a process identifier of the terminated accelerator application program with the accelerator. The method of  FIG. 6  also includes, performing ( 604 ) a connection cleanup. Performing ( 604 ) a connection cleanup includes removing ( 606 ) mappings of a process identifier of the terminated accelerator application program to process identifiers of one or more other accelerator application programs. The method of  FIG. 6  also includes causing ( 608 ) undelivered messages associated with the terminated accelerator application program to indicate an error. The method of  FIG. 6  also includes administering ( 610 ) message queues associated with the terminated accelerator application program, administering ( 612 ) execution environment objects shared by the terminated accelerator application program, and invalidating ( 614 ) a process identifier of the terminated accelerator application program for access through the SLMPM. 
     Exemplary embodiments of the present invention are described largely in the context of a fully functional computer system for terminating an accelerator application program in a hybrid computing environment. Readers of skill in the art will recognize, however, that the present invention also may be embodied in a computer program product disposed on signal bearing media for use with any suitable data processing system. Such signal bearing media may be transmission media or recordable media for machine-readable information, including magnetic media, optical media, or other suitable media. Examples of recordable media include magnetic disks in hard drives or diskettes, compact disks for optical drives, magnetic tape, and others as will occur to those of skill in the art. Examples of transmission media include telephone networks for voice communications and digital data communications networks such as, for example, Ethernets™ and networks that communicate with the Internet Protocol and the World Wide Web as well as wireless transmission media such as, for example, networks implemented according to the IEEE 802.11 family of specifications. Persons skilled in the art will immediately recognize that any computer system having suitable programming means will be capable of executing the steps of the method of the invention as embodied in a program product. Persons skilled in the art will recognize immediately that, although some of the exemplary embodiments described in this specification are oriented to software installed and executing on computer hardware, nevertheless, alternative embodiments implemented as firmware or as hardware are well within the scope of the present invention. 
     It will be understood from the foregoing description that modifications and changes may be made in various embodiments of the present invention without departing from its true spirit. The descriptions in this specification are for purposes of illustration only and are not to be construed in a limiting sense. The scope of the present invention is limited only by the language of the following claims.