Abstract:
A method and system for managing availability of a dependent thread to service a nested request is provided. A plurality of reply threads are maintained in a single thread pool. In addition, a counter is provided to track availability of a reply thread from the thread pool. A service thread that requires at least one reply thread to complete execution of a request must check the counter to determine availability of the reply thread prior to execution of the request. The process of determining availability of a reply thread prior to execution of a service thread request avoids initiating execution of a request that cannot be completed in a timely manner, or at all.

Description:
BACKGROUND OF THE INVENTION 
   1. Technical Field 
   This invention relates to managing requests in a computer system. More specifically, the invention relates to ensuring that a thread will be available to process a request prior to initiating execution of the request. 
   2. Description of the Prior Art 
   In a distributed computer system with shared persistent storage, one or more server nodes are in communication with one or more client nodes.  FIG. 1  is a block diagram ( 10 ) illustrating one example of a distributed computer system. As shown, there are two server nodes ( 12 ) and ( 14 ), three client nodes ( 16 ), ( 18 ), and ( 20 ), and a storage area network ( 5 ) that includes one or more storage devices (not shown). Each of the client nodes ( 16 ), ( 18 ), and ( 20 ) may access an object or multiple objects stored on the file data space ( 27 ) of the storage area network ( 5 ), but may not access the metadata space ( 25 ). In opening the contents of an existing file object on the storage media of the storage device in the storage area network ( 5 ), a client contacts the server node to obtain metadata and locks. Metadata supplies the client with information about a file, such as its attributes and location on the storage devices. Locks supply the client with privileges it needs to open a file and read or write data. The server node performs a look-up of metadata information for the requested file within the metadata space ( 25 ) of the storage area network ( 5 ). The server nodes ( 12 ) or ( 14 ) communicate granted lock information and file metadata to the requesting client node, including the location of the data blocks making up the file. Once the client node holds a distributed lock and knows the data block location(s), the client can access the data for the file directly from a shared storage device attached to the storage area network. 
   In the distributed computing system of  FIG. 1 , there are finite quantities of execution threads available for the servers and/or clients in executing requests. A multi-stage request, also known as a nested request, will require a first execution thread to request a second execution thread in order to complete the request. In the nested request, the first execution thread will suspend operation while it waits for a reply from the second execution thread. For example, one of the client nodes may initiate a first request for a lock that will require a response from one of the server nodes. A problem may be encountered when a first thread has been executed by one of the servers for the first request, and one of the other client nodes initiates a second request for service and no second execution thread is available for the second request. This scenario is known as a deadlock. In order to avoid a deadlock scenario the first thread must know that the second thread will be available for execution prior to the first thread committing to the execution of a request. 
   One prior art solution is to reserve a quantity of threads in a thread pool, wherein the reserved threads are exclusively for use with secondary requests in a nested request. By reserving a quantity of threads for use as secondary requests in a nested request, an operator mitigates a deadlock situation associated with a nested request and encourages availability of threads to complete the nested request. However, there are drawbacks associated with reserving a pool of threads exclusively for secondary requests of a nested request. A quantity of threads in a multithreaded processing system is statically generated. As such, there are a finite number of threads available. One drawback is designating a defined quantity of threads for secondary requests without prior knowledge as to whether a first execution thread is part of a nested request. If the reserved threads are not available for other requests, the availability of threads for all requests is reduced. Accordingly, in a system with a predefined quantity of threads, reserving a set quantity of threads for servicing secondary requests is not an efficient allocation of threads. 
   Another prior art solution, U.S. patent Publication 2002/0194377 to Doolittle et al., uses multiple pools, i.e. data structures, for holding threads, wherein different pools are reserved for different stages in a nested request. Threads are redistributed among thread pools when a potential exhaustion of a pool is discovered. Accordingly, the quantity of available threads among a plurality of thread pools is dynamically modified in order to honor all service requests. 
   However, there are limitations associated with what is disclosed in the Doolittle et al. publication. For example, management of threads in the manner taught by Doolittle et al. can be complex and expensive to operate. There is therefore a need for management of threads in a system that supports nested requests that does not require any added expense. In addition, such a solution should make all threads eligible to support any stage in a nested request. 
   SUMMARY OF THE INVENTION 
   This invention comprises a method and system for management of threads in a distributed computer system. 
   In one aspect of the invention, a method is provided for managing a computer system. Commitment of a service thread to servicing a request depending on an external reply thread is monitored. Availability of reply threads in a single thread pool is tracked. 
   In yet another aspect of the invention an article is provided in a computer-readable recordable data storage medium. Instructions are provided in communication with the medium to manage execution of a nested request. The instructions include configuration of a thread pool with a nested request wherein a service thread is dependent on a separate second execution thread to complete the request. In addition, instructions are provided to track availability of the second execution thread to complete the nested request, and to commit the service thread to the nested request based upon the availability of the second execution thread. Availability of the reply thread from the thread pool is tracked prior to commitment of the service thread to the request. 
   Other features and advantages of this invention will become apparent from the following detailed description of the presently preferred embodiment of the invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a prior art block diagram of a distributed computing system. 
       FIG. 2  is a block diagram of a thread pool. 
       FIG. 3  is a flow chart illustrating dispatch of a reply thread to a service thread according to the preferred embodiment of this invention. 
       FIG. 4  is flow chart illustrating return of a reply thread to the thread pool following completion of the task. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Overview 
   In a computer system, a plurality of threads are statically generated to satisfy requests. A nested request is a request which requires at least two non-concurrently executing threads for completion of execution of associated instructions. A first thread, known as a service thread, will require at least one second thread, known as a reply thread, for completion of the request. In an efficient system, a service thread that is part of a nested request will not begin execution until it is guaranteed that a reply thread will be available for completion of the request. 
   Technical Details 
   Threads represent a single sequence of instructions executed in parallel with other sequences. A set quantity of threads is generated when the associated computer system is configured. A data structure that maintains threads is also known as a thread pool. Requests are sent to the thread pool. Each task request to the thread pool is executed by one of the threads. If there are more requests than threads, the requests are placed in a queue in order to be serviced. 
     FIG. 2  is a block diagram of a reply thread pool ( 50 ). As shown, there are five threads ( 52 ), ( 54 ), ( 56 ), ( 58 ), and ( 60 ), and a counter ( 65 ). Although five threads are shown in  FIG. 2 , it should be noted that the thread pool ( 50 ) may include more or less than five threads. Each of the threads ( 52 ), ( 54 ), ( 56 ), ( 58 ), and ( 60 ) may be used to service any type of request. In one embodiment, the threads in thread pool ( 50 ) are dependent service threads for servicing additional dependent requests and response threads that complete servicing a request. For example, in the case of a request that requires an external reply serviced by another thread, additional threads may be requested from the reply thread pool ( 50 ). As shown in  FIG. 2 , the counter ( 65 ) is set to zero, as an indication that all of the threads in the thread pool ( 50 ) are available to service requests. As a thread is requested and granted, the counter ( 65 ) is incremented. Similarly, as a reply thread, i.e. a thread housed in the thread pool ( 50 ), completes execution of a request, the reply thread is returned to the thread pool ( 50 ), and the counter ( 65 ) is decremented. Accordingly, the thread pool includes a static quantity of threads in conjunction with a counter to track availability of a thread for servicing any request. 
     FIG. 3  is a flow chart ( 100 ) illustrating one embodiment in how the thread pool ( 50 ) may be utilized for a request that requires an external reply supplied by at least one additional thread. The first step is to conduct a test to determine if a request requires an external reply to be serviced by one or more additional threads for completion of the request ( 102 ). A negative response to the test at step ( 102 ) is an indication that the request is not a nested request and the service thread does not require a reply thread for completion of the request ( 104 ). However, a positive response to the test at step ( 102 ) is an indication that the request is a nested request and requires at least one additional reply thread to complete the request. Each thread has a bit that characterizes its ability to service a nested request, i.e. a state bit. Following a positive response to the test at step ( 102 ), a subsequent test is conducted to determine if a state bit of the requesting thread, also known as a service thread, is set for commitment to execution ( 106 ). A negative response to the test at step ( 106 ) is an indication that the service thread is capable of servicing a nested request and that it may proceed with seeking a request to access the pool of threads ( 50 ). However, a positive response to the test at step ( 106 ) is an indication that the service thread already has permission to access the pool of threads ( 108 ) and may continue to process the nested request ( 124 ). Accordingly, the first step in accessing a pool of threads for completion of a request entails determining whether such access is required. 
   Once it has been determined that the service thread requires one or more threads to complete the request, the service thread obtains an exclusive lock on the counter of the thread pool ( 110 ). The counter has a threshold value that corresponds to the maximum quantity of threads in the pool. A test is conducted to determine if the value of the counter is below the threshold limit of the counter ( 112 ). If the value of the counter has reached the threshold, this is an indication that there are no threads available from the thread pool ( 50 ) for completion of the request. Following a negative response to the test at step ( 112 ), the exclusive lock on the counter is release by the service thread ( 114 ), and a transient failure message, i.e. an abort message, is sent to the service thread that initiated the thread request and the reply thread is returned to the thread pool ( 116 ). However, if it is determined at step ( 112 ) that the value of the counter is below the threshold limit, the counter is incremented ( 118 ) and the state characteristic of the service thread is set ( 120 ) to indicate that the service thread is committed to the execution of the request. In one embodiment, a state characteristic may be a bit that identifies commitment of a thread to a request. Following step ( 120 ) the service thread releases the exclusive lock on the counter ( 122 ) and continues to process the nested request ( 124 ). Accordingly, prior to initiating execution of a request, the service thread ensures availability of at least one additional thread and sets a self identifier indicating commitment of the service thread to completion of the request. 
   Following step ( 118 ), the service thread may continue with the request, knowing that all the threads it requires have been reserved from the thread pool ( 50 ).  FIG. 4  is a flow chart ( 200 ) illustrating the process of releasing a reply thread following completion of the nested request. A test is conducted to determine if the state characteristic of the service thread is set ( 202 ). If the response to the test at step ( 202 ) is negative, this is an indication that the service thread has completed the request and does not continue to own any threads that belong to the thread pool. All reply threads that may have been requested by the service thread are returned to the thread pool ( 204 ). However, if the response to the test at step ( 202 ) is positive, this is an indication that the service thread is in the process of completing the execution of the task associated with the threads. The service thread obtains an exclusive lock on the counter of the thread pool ( 206 ). Thereafter, the counter is decremented ( 208 ), and the state characteristic of the service thread is set to indicate that the service thread is not committed to execution of a request ( 210 ). Once step ( 210 ) is complete, the service thread returns the reply thread to the thread pool ( 204 ). Accordingly, following completion of the request that requires one or more additional threads to service an external reply, all threads are returned to the thread pool together within an accounting reflecting the returned threads. 
   Advantages Over the Prior Art 
   The method and system shown herein enable efficient allocation of threads for use in a nested request while avoiding a deadlock scenario. All of the threads are maintained in a single thread pool that has a counter associated therewith to track the quantity of threads in the data structure at any one time. In addition, each service thread has an identifier to indicate whether the thread is committed to a request. Once availability of a thread from the thread pool has been confirmed, the identifier of the service thread is marked to note commitment of the thread to the associated request. Accordingly, the combination of the thread pool with the service thread identifier ensures that execution of a nested request is not initiated until it has been determined that the necessary resources are available to complete the nested request. 
   Alternative Embodiments 
   It will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without departing from the spirit and scope of the invention. In particular, a nested or non-nested request may require multiple threads from the thread pool for completion of the request(s). The counter of the thread pool may be incremented by the quantity of threads required for completion of the request(s). In addition, as more clients and/or servers are added to the distributed computer system, threads may be added to the thread pool by the additional clients and/or servers. Finally, the thread pool counter described herein is a counter of committed threads. However, the counter may be functionally replaced by a counter of uncommitted threads with suitable modifications to the operation that initialize, update, and examine the state of the counter. Accordingly, the scope of protection of this invention is limited only by the following claims and their equivalents.