Patent Publication Number: US-8984492-B2

Title: Incremental compilation of a script code in a distributed environment

Description:
FIELD OF INVENTION 
     This invention relates to compilation of script code in general and more specifically to incremental compilation of script code at runtime in a distributed environment. 
     BACKGROUND 
     Scripting languages offer simplicity and ease of development to software developers. Script code is easy to write since scripting languages are often based on a small set of expressions and statements that are simple to learn. Software developers often use scripting languages for rapid development of applications, for example, web applications. Scripting languages may be used for server side programs or for client side programs. Examples of server side scripting languages include PHP (Personal Home Page) and examples of client side scripting languages include JAVASCRIPT. 
     Server side script code can be used for executing a user request received at a web server by dynamically generating web pages. Server side scripting is often used for implementation of interactive websites that interface with data stores for retrieving and storing information. The PHP scripting language allows embedding of script code with hypertext markup language (HTML). Client side script code is often used for performing simple computations on the client side rather than sending a request to the server for simple computations. 
     Conventional approaches to execute script code include executing the script code using an interpreter. However, an interpreter may not be able to perform several optimizations that a compiler that generates executable code can perform. Therefore, interpreting script code can be inefficient compared to running executable code obtained by compiling the script code. Furthermore, scripting languages often allow simplified syntax that makes it easier for a user to write script code. For example, scripting languages often support untyped variables that do not require a user to provide type information of the variable. As a result, a compiler processing script code may not have the required information for performing certain optimizations. For example, compiler optimizations often require knowledge of types of the variables that is not available in script code based on untyped variable. Even if the knowledge of types is available, the compilation techniques, especially just-in-time compilation techniques, do not generate optimal executable code for a particular execution instance. 
     Further, if the same script code is executing on different systems in a distributed environment, the potential knowledge of the execution pattern of the script code on different systems is not made use of to further optimize the execution of the script code in the distributed environment. Accordingly, the current distributed environments do not provide an improved efficiency in executing the script code in the distributed environment. 
     SUMMARY 
     Introduced here are methods, systems, paradigms and structures for incrementally compiling scripts at runtime to generate executable code. The incremental compilation generates executable blocks corresponding to basic blocks of a script. In a first phase, an executable block for a basic block of the script is generated for a set of types of variables of the basic block. The generated executable block is stored and executed for subsequent requests. In a second phase, a set of executable blocks whose profiling information, such as a frequency of (a) execution, (b) transition between two executable blocks, or (c) execution of a particular path, satisfies an optimization criteria is identified. The basic blocks corresponding to the identified set of executable blocks are combined, and an executable control region is generated for executing the basic blocks. The generated executable control region is stored and executed for subsequent requests. Executing the script using the executable control region is more optimal than executing using the executable blocks generated in the first phase. 
     If the script is executing on a number of systems in a distributed environment, the execution of the script on the systems may be further improvised, in a third phase, by using the profiling information of the script executing on each of the systems in the distributed environment. The profiling information generated by each of the systems is aggregated to generate an aggregated profile for the script. The executable blocks corresponding to the aggregated profile are identified, and an executable control region is generated for the portion of the script corresponding to the identified executable blocks. The generated executable control region is stored and executed for subsequent requests. The executable control region generated based on the aggregated profile is more efficient compared to the executable control region generated for each of the systems separately because the executable control region for the aggregated profile considers the execution pattern of the script in the entire distributed system rather than an individual system. 
     The profiling information may be aggregated based on various aggregation criteria. The aggregation criteria can include aggregating profiling information from systems having a particular type of processor, particular type of operating system, aggregating profiling information most frequently executed paths in the systems etc. 
     Some embodiments of the disclosed technique have other aspects, elements, features, and steps in addition to or in place of what is described above. These potential additions and replacements are described throughout the rest of the specification. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an environment in which an embodiment of the disclosed technique may operate. 
         FIG. 2  is a block diagram illustrating an architecture of an online system that generates dynamic web pages by incrementally compiling script code at runtime. 
         FIG. 3  is a flow diagram illustrating the process of compiling script code. 
         FIG. 4  illustrates the structure of executable code generated by a script compiler. 
         FIG. 5  is a block diagram illustrating incremental compilation of byte code obtained from an example script code. 
         FIG. 6  illustrates an example of combining a set of executable blocks to generate an executable control region based on a frequency of execution of the executable blocks. 
         FIG. 7  illustrates an example of combining a set of executable blocks to generate an executable control region based on a frequency of a transition between the executable blocks. 
         FIG. 8 , which includes  FIGS. 8(   a ) and  8 ( b ), illustrates an example  800  of combining a set of executable blocks to generate an executable control region based on path profiling or type profiling of the set of executable blocks. 
         FIG. 9  is a flow diagram of a method of combining executable blocks to generate an executable control region. 
         FIG. 10  is a flow diagram illustrating a method of optimizing the execution of the script in a plurality of phases of execution. 
         FIG. 11  is a distributed environment in which an embodiment of the invention may operate. 
         FIG. 12  illustrates examples of master executable control regions generated based on various aggregation criteria. 
         FIG. 13  illustrates a block diagram of an architecture of a master online system that generates a master executable control region based on an aggregated profile. 
         FIG. 14  is a flow diagram illustrating a process of creating an executable control region based on an aggregated profile in a distributed environment. 
         FIG. 15  is a block diagram of a processing system that can implement operations of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     References in this description to “an embodiment”, “one embodiment”, or the like, mean that the particular feature, function, or characteristic being described is included in at least one embodiment of the present invention. Occurrences of such phrases in this specification do not necessarily all refer to the same embodiment, nor are they necessarily mutually exclusive. 
     Disclosed here are methods, systems, paradigms and structures for incrementally compiling scripts at runtime to generate executable code. The incremental compilation generates executable code corresponding to basic blocks of a script in various phases and at various scopes. In a first phase, an executable code for a basic block of the script is generated for a set of types of variables of the basic block. The generated executable block is stored and executed for subsequent requests. In a second phase, a set of executable blocks whose profiling information, such as a frequency of (a) execution, (b) transition between two executable blocks, or (c) execution of a particular path, satisfies an optimization criteria is identified. The basic blocks corresponding to the identified set of executable blocks are combined, and an executable control region is generated for executing the basic blocks. The generated executable control region is stored and executed for subsequent requests. The executable control region is more optimal than the executable blocks generated in the first phase. 
     If the script is executing on a number of systems in a distributed environment, the execution of the script on the systems may be further improvised, in a third phase, by using, at least, the profiling information of the script executing on each of the systems in the distributed environment. The profiling information generated by each of the systems is aggregated to generate an aggregated profile for the script. The executable blocks corresponding to the aggregated profile are identified, and an executable control region is generated for the portion of the script corresponding to the identified executable blocks. The generated executable control region is stored and executed for subsequent requests. The executable control region generated based on the aggregated profile is more efficient compared to the executable control region generated for each of the systems separately because the executable control region for the aggregated profile considers the execution pattern of the script in the entire distributed system rather than an individual system. 
     The profiling information may be aggregated based on various aggregation criteria. The aggregation criteria can include aggregating profiling information from systems having a particular type of processor, particular type of operating system, aggregating profiling information most frequently executed paths in the systems etc. 
       FIG. 1  shows a system environment for allowing a client device to interact with an online system that generates dynamic web pages by compiling script code, in accordance with an embodiment of the disclosed technique.  FIG. 1  illustrates client devices  160  interacting with an online system  100  using the network  150 . The client devices  160  send requests to the online system  100  via the network  150 . The online system  100  may dynamically generate web pages in response to the request and send the generated web pages to the client device  160  in response to the request. 
       FIG. 1  and the other figures use like reference numerals to identify like elements. A letter after a reference numeral, such as “ 160   a ,” indicates that the text refers specifically to the element having that particular reference numeral. A reference numeral in the text without a following letter, such as “ 160 ,” refers to any or all of the elements in the figures bearing that reference numeral (e.g. “ 160 ” in the text refers to reference numerals “ 160   a ” and/or “ 160   b ” in the figures). 
     Embodiments of the computing environment can have multiple client devices  160  and multiple online systems  100  connected to the network  150 . Certain functionality described in one embodiment as being performed on the server side can also be performed on the client side in other embodiments if appropriate. For example, although  FIG. 1  shows the script compiler  110  running on the online system  100  for compiling server side script code, in other embodiments, the script compiler  110  may run on the client device  160  for compiling client side script code. In addition, the functionality attributed to a particular component can be performed by different or multiple components operating together. 
     The client devices  160  include one or more computing devices that can receive user input and can transmit and receive data via the network  150 . The client device  160  can execute an application, for example, a browser application  170  that allows a user of the client device  160  to interact with the online system  100 . A user may provide input using a user interface presented to the user via the browser application  170 . The interactions of the user via the browser application  170  may cause the browser application  170  to send a request for information that identifies a markup language document including server side scripting code. The markup language document is processed to obtain a transformed markup language document that is returned in response to the request. 
     The network  150  uses standard communications technologies and/or protocols. Thus, the network  150  can include links using technologies such as Ethernet, 802.11, worldwide interoperability for microwave access (WiMAX), 3G, digital subscriber line (DSL), etc. Similarly, the networking protocols used on the network  150  can include multiprotocol label switching (MPLS), the transmission control protocol/Internet protocol (TCP/IP), the User Datagram Protocol (UDP), the hypertext transport protocol (HTTP), the simple mail transfer protocol (SMTP), the file transfer protocol (FTP), etc. The data exchanged over the network  170  can be represented using technologies and/or formats including the hypertext markup language (HTML), the extensible markup language (XML), etc. In addition, all or some of links can be encrypted using conventional encryption technologies such as secure sockets layer (SSL), transport layer security (TLS), Internet Protocol security (IPsec), etc. 
     The online system  100  comprises a web server  130 , a script compiler  110  and a script code store  120 . The web server  130  is a module processing requests received by the online system  100  from client devices  160  or other external systems that interact with the online system  100 . The web server  110  may be implemented by conventional web server software, such as APACHE or INTERNET INFORMATION SERVICES. In response to a request from a client device  160 , the web server  130  may invoke other modules of the online system  100  to process the request. For example, the web server  130  may invoke modules of the online system  100  to obtain a web page in response to the request from the client device  160 . The web server  130  sends the web page to the client device  160  for presentation on the browser  170 . 
     The script code store  120  stores script code that implements portions of functionality provided by the online system  100  to client devices  160 . A script code may include a function, procedure, method, or a block of code that may be embedded within an HTML document. The script code implements functionality, for example, retrieving information stored in various databases of the online system  100 , performing computations, or interacting with other systems. 
     The script compiler  110  takes script code in source code form and generates equivalent executable code for execution by a processor of the online system  100  (in this disclosure, the term “script code” is also referred to as “script.) In an embodiment, the script compiler  110  performs incremental compilation of the script code in a lazy fashion. For example, a portion of script code is compiled if a request causes this portion of script code to execute. Once a portion of the script code is compiled, the generated executable code is available for future requests. However, if no request received by the online system  100  needs to execute a particular portion of the script code, that particular portion may not be compiled. Therefore, no executable code corresponding to a particular portion of script may exist in the online system  100  if no request from a client device needs to execute that portion of script. For example, a script may include an “if-then-else” statement that executes an “if” portion of script if a condition evaluates to true and an “else” portion of script if the condition evaluates to false. If all incoming requests evaluate the condition to a true value, these request only execute the “if” part of the script. Accordingly, executable code corresponding to the “else” part of the “if-then else” statement may never be generated, unless an incoming request results in the condition being evaluated to a false value. 
       FIG. 2  illustrates an architecture of an online system  100  that generates dynamic web pages by incrementally compiling script code at runtime, in accordance with an embodiment of the disclosed technique. The online system includes the script code store  120 , an abstract syntax tree (AST) store  250 , a byte code store  260 , an executable code store  270 , an execution engine  240 , and the script compiler  110 . The script compiler  110  further includes a parser  210 , a byte code generator  220 , and a byte code compiler  230 . Some embodiments of the online system  100  have different and/or other modules than the ones described herein, and the functions can be distributed among the modules in a different manner than is described here. For example, several modules shown in the online system  100  may be present in a client device  160  if the script code being processed is client side script code. 
     The script code store  120  stores script code, for example, script code specified as PHP, server side JAVASCRIPT, or another syntax. The script code may be input by a software developer using an editor or copied from another computer. In an embodiment, the script code is specified in a human readable text form. The parser  210  reads the script code from one or more files in the script code store  120  and builds a data structure called an AST that is stored in the AST store  250 . The AST is a hierarchical tree representation of script code. The parser  125  checks the script code for syntax errors and reports the errors to allow a user to correct the errors. 
     The byte code generator  220  traverses the AST representation of the script code and generates byte code corresponding to the script code. The byte code is stored in the byte code store  260 . The byte code includes code represented using an instruction set that is designed for efficient execution by an interpreter or for efficient compilation into executable code, for example, machine code. The byte code instructions may correspond to a virtual stack machine or a virtual register machine. The byte code compiler  230  converts byte code into executable code and stores the generated executable code in the executable code store  270 . 
     The execution engine  240  executes the instructions available in the executable store  270 . For example, the execution engine  240  may be invoked in response to a request received from a client device  160 . The execution engine  240  identifies executable code corresponding to the request received for execution. An online system  100  may compile all available byte code stored in the byte code store  260 , for example, as a batch process and store the generated executable code in the executable code store  270 . Compiling all available byte code store in advance ensures that executable code is readily available for any request that is received by the online system, so long as the corresponding script code is available in the script code store  120 . However, script code typically supports features that make it difficult to generate efficient executable code. For example, script code may support untyped variable for which the type is not available until runtime. Programmers often use untyped variables since they do not require the programmer to make early decisions regarding types of variables used. A programmer may specify a variable as untyped even if at runtime the variable only stores values of one particular type, for example, an integer value. In practice significant amount of script code is executed based on a limited set of types corresponding to the untyped variables. However, if the online system  100  compiles the byte code to executable code prior to receiving the requests at runtime, the type information may not be available for the variables. A byte code compiler  230  that compiles the byte code without making any specific assumptions about the types of the variables may generate inefficient executable code since the generated executable code accounts for all possible types that each untyped variable may take, whether or not the incoming requests use these types. 
     Embodiments of the byte code compiler  230  compile byte code to executable code based on information available at runtime. For example, the byte code compiler  230  may utilize type information of variables obtained during an execution of the script code to generate executable code optimized for these specific types. Accordingly, executable code required for executing a request may or may not be available in the executable code store  270  at runtime. If executable code corresponding to the request is not available in the executable code store  270 , the execution engine  240  identifies byte code corresponding to the request from the byte code store  230 . The execution engine  240  invokes the byte code compiler  230  to compile the byte code corresponding to the request to generate executable code. The execution engine  240  provides type information of variables obtained during the current execution of the script code to the byte code compiler  230 . Accordingly, the byte code compiler  230  generates efficient executable code based on the type information of variables available. The execution engine  240  executes the generated executable code. In some embodiments, executable code may be generated directly from script code without requiring byte code generation. 
     If future executions of the script code provide variables of the same type as the first request, the executable code can be reused for the future requests. However, if a subsequent execution provides a different combination of types of variables compared to the first execution, the execution engine  240  invokes the byte code compiler  230  to generate executable code corresponding to the new combination of types corresponding to the variables. Accordingly, the executable code store  270  may store different executable codes for the same byte code program, each executable code corresponding to a different combination of variable types. The byte code compiler  230  may never generate executable code corresponding to type combinations that are never received in executions of the script code. 
     In an embodiment, the byte code compiler  230  compiles a basic block of byte code at a time. A basic block of code has one entry point, i.e., no instruction within the basic block other than the entry point can be reached from anywhere in the script code as a destination of a jump instruction. The entry point is typically the first instruction of the basic block. A basic block may have one or more exit point, i.e., typically the last instruction of the basic block causes the program control to start executing another basic block. The last instruction may evaluate certain condition and determine the next basic block for execution based on the result of the evaluation of the condition. For example, the last instruction may evaluate a binary condition and transfer program control to one basic block if the condition evaluates to true otherwise transfer program control to another basic block (if condition evaluates to false). Similarly, the last instruction of the basic block may transfer control to different basic blocks based on the value of a particular variable. For example, if the variable value is 1, program control is transferred to basic block B1, if the variable value is 2, program control is transferred to basic block B2, if the variable value is 3, program control is transferred to basic block B3, and so on. The simple structure of a basic block makes it easy for the byte code compiler  230  to optimize and compile a basic block. 
     The executable code of the script includes executable blocks (also referred as “executable basic blocks”) of the script and/or executable control regions of the script. An executable block corresponds to a basic block of the script (which is described in further detail with reference to  FIGS. 4 and 5 ), whereas an executable control region of the script includes instructions for executing a set of basic blocks. An executable control generator  235  generates an executable control region for a set of basic blocks based on various criteria (which is described in further detail with reference to  FIGS. 6-10 ). 
       FIG. 3  is a flow diagram illustrating the process of compiling script code, in accordance with one embodiment of the disclosed technique. The process illustrated in  FIG. 3  may be executed by the script compiler  110  as a batch process that compiles script code stored in the script code store  120 . For example, software developers may periodically provide new scripts implementing certain features of the online system  100 . The received script code may be compiled as a batch process by the online system  100 . Alternatively, software developers may update existing script code, thereby requiring recompilation of the updated script code. The script compiler  110  may repeat the steps shown in  FIG. 3  for all the script code that need to be recompiled. 
     The script compiler  110  identifies  310  a script for compilation. The script compiler  110  invokes the parser  210  for parsing the script. The parser  210  parses  320  the script code to generate an AST representation of the script code. The parser  210  stores the AST representation in the AST store  250 . In an embodiment, modules of the script compiler  110  perform various operations using the AST representation, for example, static analysis, type inference, and optimizations. As a result, the script compiler  110  may annotate the AST and/or transform the AST. The script compiler  110  stores the annotated ASTs or the transformed ASTs in the AST store  250 . Each step performed by the script compiler  110  typically use the latest version of the AST from the AST store  250  and generates a subsequent version of the AST. The byte code compiler  230  generates 330 byte code from the AST representation of the script code and stores  340  the generated byte code in the byte code store  260 . 
     The byte code compiler  230  incrementally compiles the byte code stored in the byte code store at runtime to generate corresponding executable code. The byte code compiler  230  performs the incremental compilation responsive to executions of the script code, for example, executions caused by requests received from client devices  160 . 
     In at least some embodiments, the execution engine  240  requests compilation of one basic block of byte code at a time. More specifically, the execution engine  240  requests compilation of one basic block for a particular combination of types of the variables as required for execution of an incoming request. In at least some other embodiments, the execution engine  240  may request compilation of a group of basic blocks to generate more optimal executable code. Additionally or alternatively, the execution engine  240  may request the compilation of one basic block during a first phase of execution of the script, and request compilation of group of basic blocks during a second phase of execution. 
       FIG. 4  is a diagram illustrating the structure of executable code generated by a script compiler, in accordance with one embodiment of the disclosed technique. The generated executable basic block  410  includes a portion of guard code  430 , a basic block body  440 , and one or more exit pointers  450 . The executable basic block  410  is generated in response to receiving a request from the client device  160 . Accordingly, the executable basic block  410  is optimized for the types of variables as provided by the incoming request. 
     The following example illustrates how executable code is generated for a given basic block. Assume that a basic block includes two untyped variables varA and varB. Further assume that for a particular execution it is determined that both variables varA and varB are integers. Accordingly, the byte code compiler  230  compiles the basic block to generate the basic block body  440  assuming the variables varA and varB are integers. The guard code  430  includes instructions that check a particular condition before executing the basic block body  440 . In the above example, the generated guard code  430  verifies that the types of variables varA and varB are integers. If the types of both variables are integers, the guard code  430  continues execution of the basic block body  440 . 
     The last instruction of an executable basic block  410   a  that is executed typically causes the program control to begin execution of another executable basic block  410   b . Accordingly, the last instruction of the executable basic block  410  may include an exit pointer  450  that specifies the address of an executable basic block  410   b  for execution after the execution of the executable basic block  410 . The last instruction of an executable basic block  410   a  that is executed may transfer control to different executable basic blocks  410  depending on certain criteria. For example, the last instruction in a basic block may correspond to an “if” condition that executes one basic block if the condition is evaluated to true and another basic block if the condition is evaluated to false. Therefore, the last instruction of the executable basic block  410  may include one or more exit pointers  450   a ,  455   a , and so on. 
     The exit pointer  450   a  points to another executable basic block  410   b . If a particular executable block that needs to be executed subsequent to the execution of the executable basic block  410   a  has not been compiled so as to generate a corresponding executable basic block, the corresponding exit pointer  455   a  transfers control to the byte code compiler  420 . The byte code compiler  420  may be provided with information describing the subsequent basic block that needs to be compiled. The address of the subsequent basic block may be communicated to the byte code compiler  420  using function-calling conventions of the native machine in which the system is hosted. In this embodiment, the byte code compiler  420  obtains the address of the byte code corresponding to the subsequent basic block to be compiled from the top of the stack. Once the byte code compiler  420  generates an executable basic block  410  corresponding to the subsequent basic block, the pointer  455   a  is changed to point to the generated executable basic block instead of the byte code compiler  420 . 
     In at least some embodiments, the byte code compiler  230  generates different executable basic blocks for different combinations of type of the variables of a basic block. That is, the byte code compiler  230  generates one executable basic block for variable types integer, another executable basic block where both the variables are float, another executable basic block where one variable is integer and another is float and so on. 
     Further, in at least some other embodiments, the executable basic blocks may be generated based on criterion other than type of variables in the basic block. The guard code would have instructions accordingly to verify the criteria based on which the executable block is created before the basic block body of the executable block is executed. 
       FIG. 5  illustrates incremental compilation of byte code obtained from an example script code, in accordance with one embodiment of the disclosed technique.  FIG. 5  shows an example script code  500  executed in response to a request from client device  160 . The example script code  500  includes a portion of byte code  510   a  followed by an if-then-else statement, followed by another portion of byte code  510   d . The if-then-else statement includes a condition  510   e , a portion of byte code  510   b  that is executed if condition  510   e  evaluates to true, and a portion of byte code  510   c  that is executed if the condition  510   e  evaluates to false. 
     Assume that a request is received from a client  160  that includes values of variables that result in the condition  410   e  evaluating to true. The resulting executable code generated by the byte code compiler  230  includes the executable code  550   a  shown in  FIG. 5 . The portion of script code  510   a  combined with the condition  510   e  corresponds to executable code  520   a . The executable code  520  includes a guard code  430  in the beginning to verify whether the types of the variables correspond to a specific combination. The end of the executable code  520   a  includes instructions evaluating the condition  510   e . If the condition  410   e  evaluates to true, the program control is transferred according to exit pointer  530   a  otherwise the program control is transferred according to exit pointer  540   a.    
     Since the current request received from the client  160  results in the condition  410   e  evaluating to true, the executable basic block  520   b  corresponding to portion of script code  510   b  is also generated. The script code  500  shows that after execution of script code  510   b , the script code  510   d  is executed. Accordingly, the executable basic block  520   d  corresponding to the script code  510   d  is also generated. For the execution of the current request, the script code  510   c  is never executed since it corresponds to the “else” portion of the if-the-else statement that is not executed when the condition  510   e  evaluates to true. Accordingly, the end of executable basic block  520   a  includes an exit pointer  540   a  pointing to the byte code compiler  230  with information identifying byte code corresponding to script code  510   c.    
     If several subsequent requests all include variables with types matching those corresponding to the previous request and result in condition  510   e  evaluating to true, the executable code  550   a  can be executed to process these requests. A new set of executable code  550  may be generated if a request is received that requires execution of script code  500  with a new combination of types of variables, different from those corresponding to executable code  550 . However, if all requests received from the client device  160  continue providing the same combination of variable types and always result in the condition  510   e  evaluating to true, the executable code  550  continues to process the requests and no new executable code needs to be generated. 
     If at any stage, an execution of the script code is performed that provides the previous combination of variable types that cause the condition  510   e  to evaluate to false, the exit pointer  540   a  causes the byte code compiler  420  to be invoked causing an executable basic block to be generated corresponding to the script code  510   c . The script compiler  110  changes the exit pointer  540   a  to point to the generated executable basic block instead of the byte code compiler  420 . Since the execution of the script code  510   c  is followed by the execution of the script code  510   d , the exit pointer at the end of the executable basic block is configured to point to the executable block  520   d  corresponding to script code  510   d . The executable code  550   a  which now includes executable block for script code  510   c  can process requests that result in the condition  510   e  evaluating to true as well as false without having to invoke the byte code compiler  420 . Furthermore, the executable basic block for script code  510   c  is not generated unless an execution that causes the condition  510   e  to evaluate to false is received. Accordingly, the script compiler  110  generates executable code in a lazy fashion, the generation performed only if a request requires certain portion of script code to be executed. As a result, the script compiler  110  does not generate dead code, i.e., code that is never executed. 
     In at least some embodiments, the execution of the script can be further optimized by generating a single executable control region for a set of basic blocks of the script. The executable control region having instructions for executing a set of basic blocks can be generated by combining the executable basic blocks generated for each of the set of basic blocks. The execution engine  240  executes the executable control regions in subsequent requests to execute the executable blocks. The time taken to generate or load one executable control region per multiple basic blocks is lesser compared to the time taken to generate or load one executable block per each of the multiple basic blocks. Accordingly, the efficiency of execution of the script is improved by executing one executable control region instead of executable blocks. However, to combine appropriate executable blocks, the byte code compiler  230  has to have knowledge or context of a set of executable blocks. Since the executable blocks are generated on a need basis, the execution engine  240  may not have the knowledge or context of a group of executable blocks. Accordingly, the execution engine  240  may have to wait until necessary knowledge for further optimization is obtained. 
     In at least some embodiments, the execution of the script is optimized in two different phases of execution. In a first phase, the execution of the script is optimized by generating the executable blocks for the basic blocks of the script as described in  FIGS. 4 and 5 . In the second phase, the byte code compiler  230  further optimizes the execution of the script by combining a set of executable blocks (generated in the first phase) whose profiling information satisfies an optimization criterion, and generating an executable control region for the combined set of executable blocks. The executable control region, which is more optimal than the executable blocks generated in the first phase, is stored and executed in response to subsequent requests. 
     In at least some embodiments, the execution engine  240  collects the profiling information of the executable blocks while the script is executing in the first phase. The executable blocks are configured to have attributes that provide the profiling information. The profiling information can include, but is not limited to, (a) a number of times a particular executable block has executed, (b) a number of times a transition has occurred between a first executable block and a second executable block, (c) a number of times a particular path of execution is encountered by a set of executable blocks, etc. 
     The optimization criterion includes at least one of (a) a frequency of execution of a particular executable block exceeds a first predefined threshold, (b) a frequency of a particular transition between executable blocks exceeds a second predefined threshold, or (c) a frequency of a particular path of execution encountered by a set of executable blocks exceeds a third predefined threshold. All the thresholds are configurable by a user. 
       FIGS. 6-8  provide examples of generating an executable control region for a script based on various optimization criteria. The executable blocks C1-C7 shown in  FIGS. 6-8  may be similar to executable basic blocks  520   a ,  520   b  or  520   d , and may be generated as per the techniques of  FIGS. 4 and 5 . 
       FIG. 6  illustrates an example  600  of combining a set of executable blocks based on a frequency of execution of the set of executable blocks, according to an embodiment of the disclosed technique. Each of the executable blocks C1-C4 has a counter that has a count of the number of times the executable block has executed over a predefined period of time. The predefined period of time can include, for example, a certain duration since the executable block was created, a certain duration since the executable block executed last, etc. The execution engine  240  identifies the executable blocks that have executed more than a predefined threshold number of times. Assume that the executable blocks C3 and C4 have executed more than the predefined threshold number of times. The executable blocks C3 and C4 are combined to a single executable control region CR  605 . The executable control region CR  605  is stored, and executed in subsequent requests to execute the executable blocks C3 and/or C4. The executable control region  605  includes instructions for executing the basic blocks corresponding to the executable blocks C3 and C4. In at least some embodiments, a structure of the executable control region CR  605  is different from the structure of executable blocks C3 or C4. 
     In at least some embodiments, combining the executable blocks C3 and C4 to generate executable control region CR  605  includes recompiling the basic blocks of the script (or the byte code corresponding to the basic blocks) corresponding to the executable blocks C3 and C4, for example, by the executable control region generator  235 , to generate the executable control region CR  605 . In at least some other embodiments, executable control region CR  605  may be generated by recompiling the basic blocks of the script (or the byte code corresponding to the basic blocks) corresponding to the executable blocks C3 and C4 using the byte code compiler  230 . In at least some other embodiments, the executable control region  605  may also be generated by combining the executable blocks C3 and C4 directly. 
       FIG. 7  illustrates an example  700  of combining a set of executable blocks to generate an executable control region based on a frequency of a transition between executable blocks, according to an embodiment of the disclosed technique.  FIG. 7  shows executable blocks C1-C5 and the transitions e1-e5 between the executable blocks. A transition can be defined as a flow of execution from one executable block to another. For example, a transition (flow of execution) from executable block C1 to C5 is indicated by edge e1. The transitions that can occur between the executable blocks depends on the script code (as described in  FIG. 5 , for example). The execution engine  240  maintains a counter for each of the edges e1-e5 that provides a count of the number of times a particular transition has occurred over a predefined period of time during the execution of the script. In at least some embodiments, the set of executable blocks between which a frequency of transition exceeds a predefined threshold is identified, and combined to generate an executable control region. 
     For example, if a frequency of transition e1 exceeds a predefined threshold, executable blocks C1 and C5 are combined to generate an executable control region CR1  705 . The executable control region CR1  705  is stored, and executed in future requests to execute the executable blocks C1 and C5. In another example, if frequency of transitions e1 and e5 exceed a predefined threshold, executable blocks C1, C4 and C5 may be combined to generate an executable control region CR2  710 . 
       FIG. 8 , which includes  FIGS. 8(   a ) and  8 ( b ), illustrates an example  800  of combining a set of executable blocks based on path profiling or type profiling to generate an executable control region, according to an embodiment of the disclosed technique. Both figures show a set of executable blocks C1-C7  805  that are generated for executing a portion of a script, and transitions e1-e8 between the executable blocks.  FIG. 8(   a ) illustrates combining executable blocks based on path profiling. The execution engine  240  maintains information regarding a path of execution of the script and the executable blocks along the path of execution. The execution engine  240  also has a path counter that provides a count of number of times a particular path of execution has occurred during the execution of the script. A path whose frequency of execution exceeds a predefined threshold is identified and the executable blocks along that path are combined into an executable control region. In at least some other embodiments, a most frequently executing path is identified and executable blocks along the most frequently executing path are combined to generate an executable control region. 
     For example, as shown in  FIG. 8(   a ), if a path of execution such as C1→C2→C4→C5→C7 has occurred more than a predefined threshold number of times in a predefined period of time, then executable blocks C1, C2, C4, C5 and C7 are combined into one executable control region CR1  810 . 
     In at least some embodiments, the executable blocks C1-C7 may be combined into one or more executable control regions based on the types of variables in the basic blocks to which the set of executable blocks correspond.  FIG. 8(   b ) illustrates combining executable blocks based on type profiling. As described, for example, in  FIGS. 4 and 5 , different executable blocks may be generated to handle different combinations of types of the variables in the corresponding basic block of the script. The guard code of an executable block verifies, during the execution, if the type of the variables in the basic block to which the executable block corresponds is of a particular type. If the variables are of a particular type, the executable block is executed, else the control is transferred to another executable block which handles the particular type. Accordingly, based on the type of variables, the execution engine  240  determines the possible paths of execution of the script. The execution engine  240  may then ask the byte code compiler  230  to generate an executable control region for each of, or some of the possible paths. In at least some embodiments, the executable control region may be generated for a path corresponding to the most frequently appearing variable type. 
     Consider, for example, a type of a set of variables in the basic block to which the executable block C1 corresponds can be one of an “integer,” or a “float.” The executable blocks C1 and C4 which have multiple possible transitions decide the path of execution based on the type. Assume that, if the type is “integer,” executable block C1 transfers the control to executable block C2 and executable block C4 transfers the control to executable block C5. On the other hand, assume that, if the type is “float,” executable block C1 transfers the control to executable block C3 and executable block C4 transfers the control to executable block C6. 
     Based on the above described execution pattern, the execution engine  240  determines that the path of execution for the variable type “integer” is C1→C2→C4→C5→C7. The execution engine  240  also infers from the above execution path that another path such as C1→C2→C4→C6→C7 cannot be a valid path if the type of the set of variables is “integer” because the path C4→C6 is only executed if the type of the variables is “float” (which is learnt from the guard code in executable block C4). Accordingly, an executable control region may be created for specific paths based on the type of the set of variables. For example, an executable control region CR-I  815  can be created for the path of execution for variable type “integer” by combining executable blocks C1, C2, C4, C5 and C7. Similarly, an executable control region CR-F  820  can be created for variable type “float” by combining executable blocks C1, C3, C4, C6 and C7. In subsequent requests to execute a set of executable blocks, the executable control region created for that specific type is loaded and executed. This improves the execution speed of the script since the time consumed to (a) load one executable control region is lesser compared to loading a number of executable blocks, and (b) perform the guard code checks in a number of executable blocks is eliminated. 
     Though the above embodiments describe combining the executable blocks based on a frequency of execution of an executable block, a frequency of a transition between executable blocks, a frequency of execution of a particular path, the optimization criteria for combining the executable blocks is not limited to the above described embodiments. Once the execution engine  240  has the necessary profiling information, the executable blocks may be combined in a number of ways to achieve optimization for different scenarios of execution of the script. 
       FIG. 9  is a flow diagram of a method  900  of combining executable blocks into an executable control region, according to an embodiment of the disclosed technique. The method  900  may be executed in a system such as online system  100 . At step  905 , the execution engine  240  generates profiling information for a plurality of executable blocks of a script. Each of the executable blocks correspond to one of a plurality of basic blocks of the script, and has instructions to execute the basic block of the script. At step  910 , the execution engine  240  identifies a set of executable blocks whose profiling information satisfies the optimization criterion. At step  915 , the byte code compiler  230 /executable control region generator  235  combines the basic blocks of the script corresponding to the identified executable blocks to form a region of the script. At step  920 , the byte code compiler  230 /executable control region generator  235  generates an executable control region for the region of the script. The executable control region includes instructions to execute the region of the script. 
       FIG. 10  is a flow diagram illustrating a method  1000  of optimizing the execution of the script in a plurality of phases of execution, according to an embodiment of the disclosed technique. The method  1000  may be executed in a system such as online system  100 . The first phase of execution begins from step  1005  where the execution engine  240  receives a request to execute a script. At step  1010 , the byte code compiler  230  generates a plurality of executable blocks for a portion of the script. Each of the executable blocks corresponds to one of a plurality of basic blocks of the portion of the script. At determination step  1015 , the execution engine  240  determines whether a trigger condition for starting a second phase of execution is satisfied. 
     In at least some embodiments, the trigger condition for starting a second phase of execution includes at least one of (a) a number of the executable blocks generated for the script exceeds a first predefined threshold, (b) a rate at which the executable blocks are generated is below a second predefined threshold, (c) a duration for which the script has executed exceeds a third predefined threshold, (d) a number of times a particular executable block has executed exceeds a fourth predefined threshold, or (e) a number of times any of the executable blocks has executed exceeds a fifth predefined threshold. 
     Responsive to a determination that the trigger condition for the second phase of execution is not satisfied, the method  1000  returns to step  1010 . On the other hand, responsive to a determination that the trigger condition for starting the second phase of execution of is satisfied, at step  1020 , the execution engine  240  identifies the executable blocks whose profiling information satisfy the optimization criteria. At step  1025 , the byte code compiler  230 /executable control region generator  235  combines the basic blocks of the script corresponding to the identified executable blocks to form a region of the script. At step  1030 , the byte code compiler  230 /executable control region generator  235  generates an executable control region for the region of the script. The executable control region includes instructions for executing the region of the script. The executable control region is stored, and used for executing in response to subsequent requests for executing the script. In at least some embodiments, the executable control region is generated by recompiling the basic blocks from the region of the script. 
     In at least some embodiments, the execution engine  240  can generate the profiling information in “burst profiling” mode. In the burst profiling mode, the profiling information is generated in a number of short bursts. The frequency of the bursts and a duration of a burst may be configured by the users. Burst profiling helps in minimizing the computing resources consumed for generating the profiling information. 
     In a distributed environment where the script is running on a number of systems, the execution of the script on the systems may be further improvised by using the profiling information of the script executing on each of the systems in the distributed environment. The profiling information generated by each of the systems is aggregated and an executable control region is generated for the portion of the script corresponding to the aggregated profiling information.  FIGS. 11-14  describe various systems and techniques for optimizing the execution of a script in a distributed environment. 
       FIG. 11  is a distributed environment in which an embodiment of the invention may operate. The distributed environment  1100  includes a number of online systems  100  executing a script code such as script  500  of  FIG. 5 . The execution engine  240  in each of the online systems  100  generates profiling information for the script based on the execution of the script in the respective online systems. A master online system  1105  in the distributed environment  1100  obtains the profiling information from each of the online systems  100  executing the script, and aggregates the profiling information to create an aggregated profile. The master online system  1105  identifies the executable blocks of the script corresponding to the aggregated profile, and generates a master executable control region for the portion of the script corresponding to the identified executable blocks. Further, the generated master executable control region is transmitted to each of the online systems  100 , where it is stored, and used for executing in response to subsequent requests for executing the script. 
     Additionally or alternatively, the master executable control region can be generated by each of the online systems  100  instead of the master online system  1105 . In such an embodiment, the master online system  1105  transmits the aggregated profile to each of the online systems  100  which further generates the master executable control region based on the aggregated profile. 
     In at least some embodiments, the master executable control region is more efficient compared to the executable control region generated for each of the online systems  100  separately because the master executable control region is generated considering the execution pattern of the script in the entire distributed environment  1100  rather than an individual online system  100 . 
     The profiling information of the script can be aggregated based on various aggregation criteria. The aggregation criteria includes aggregating profiling information (a) from online systems  100  having a particular type of processor, (b) from online systems  100  executing a particular operating system, (c) from online systems  100  in a particular geography, (d) from each of the online systems  100  based on a frequency of execution of executable blocks, (e) from each of the online systems  100  based on a frequency of transitions between executable blocks, (f) from each of the online systems  100  based on a frequency of execution of a path, or (g) a combination of any of the above criteria. 
     For example, the master online system  1105  may obtain profiling information such as a list of executable blocks that have executed more than a predefined threshold number of times from each of the online systems  100 , and aggregate them to obtain an overall list of executable blocks that have executed most number of times in the distributed environment  1100 . In another example, the master online system  1105  may obtain a path (list of executable blocks along the path) that has executed more than a predefined threshold number of times from each of the online systems  100 , and aggregate them to obtain an overall list of paths that have executed most number of times in the distributed environment  1100 . In yet another example, the master online system  1105  may obtain both a path and a list of executable blocks that have executed more than a predefined threshold number of times from each of the online systems  100 , and aggregate them to obtain an overall list of paths and executable blocks that have executed most number of times in the distributed environment  1100 . 
       FIG. 12  illustrates examples of master executable control regions generated based on various aggregation criteria, according to an embodiment of the disclosed technique.  FIG. 12  shows executable blocks C1-C6 and the transitions e1-e6 between the executable blocks generated for a portion of a script. The executable blocks C1-C6 may be generated using the techniques described in  FIGS. 4 and 5 . Consider that the script is executing on three online systems such as online systems  100  of  FIG. 11 . Each of the three online systems  100  generates profiling information for the script based on the execution of the script in the respective online systems. 
     In example  1205 , the profiling information of three online systems  100  indicate that transitions e2, e4 and e4, respectively, executed more than a predefined number of times. The profiling information from the three online systems  100  is aggregated based on transitions, and a master executable control region  1210  is generated. 
     In example  1215 , the profiling information of three online systems  100  indicate that paths C1-C2-C3, C1-C5 and C1-C4, respectively, executed more than a predefined number of times. The profiling information from the three online systems  100  is aggregated based on paths, and a master executable control region  1220  is generated. 
     In example  1225 , the profiling information of three online systems  100  indicate that path C1-C2-C3, transitions e1 and e5, respectively, executed more than a predefined number of times. The profiling information from the three online systems  100  is aggregated based on a combination of path and transitions, and a master executable control region  1230  is generated. 
     In example  1235 , the profiling information of three online systems  100  indicate that path C1-C5-C6-C2, executable block C3 and transition e1, respectively, executed more than a predefined number of times. The profiling information from the three online systems  100  is aggregated based on a combination of path, number of times of execution of an executable block and transitions. A master executable control region  1240  is generated for the aggregated profile. 
     Accordingly, profiling information of online systems  100  may be aggregated in various other combinations and using various other criteria for which a master executable block may be created. In at least some embodiments, more than one master executable control region may be created for an aggregated profile. 
       FIG. 13  illustrates a block diagram of a master online system  1105 , according to an embodiment of the disclosed technique. The master online system  1105  can be similar to one of the online systems  100 . Additionally, the master online system  100  may include a profile receiving unit  1305 , a profile aggregation unit  1310 , an aggregated profile transmission unit  1315 , an executable control region generation unit  1320 , and an executable control region transmission unit  1325 . 
     The profile receiving unit  1305  receives profiling information from various online systems  100  in the distributed environment  1100  executing the script. The profile aggregation unit  1310  aggregates the profiling information based on a predefined aggregation criterion to create an aggregated profile. In at least some embodiments, a user may configure the profile aggregation unit  1310  to consider a particular aggregation criterion. After the aggregated profile is created, the executable control region generation unit  1320  generates the executable control region for the aggregated profile, and the executable control region transmission unit  1325  transmits the executable control region to the online systems  100  in the distributed environment. 
     In at least some embodiments, aggregation of the profiling information may be performed in response to a trigger such as a predefined time interval, a predefined time of the day, etc. The profile aggregation unit  1310  ensures that the profile receiving unit  1305  has received the most up to date profiling information from the online systems  100  before generating an aggregated profile. 
     In embodiments where the executable control region is created by the online systems  100 , the aggregated profile is transmitted to each of the online systems  100  in the distributed environment  1100  by the aggregated profile transmission unit  1315 . Thereafter, each of the online systems  100  generates the executable control region based on the received aggregated profile. 
       FIG. 14  is a flow diagram illustrating a process of creating an executable control region based on an aggregated profile in a distributed environment, according to an embodiment of the disclosed technique. The process  1400  may be executed in a distributed environment such as distributed environment  1100 . At step  1405 , the master online system  1105  receives profiling information from each of the online systems  100  of the distributed environment  1100  that is executing the script. At step  1410 , the profile aggregation unit  1310  aggregates the profiling information received from the online systems  100  based on a predefined profile aggregation criterion. 
     At step  1415 , the executable control region generation unit  1320  identifies the executable blocks of the script corresponding to the aggregated profile. At step  1420 , the executable control region generation unit  1320  combines the basic blocks of the script corresponding to the identified executable blocks to form a region of the script. At step  1425 , the executable control region generation unit  1320  generates an executable control region for the region of the script. At step  1430 , the executable control region transmission unit  1325  transmits the executable control region to each of the online systems  100  in the distributed environment  1100  that is executing the script. 
     In embodiments where the executable control regions for the aggregated profiles are created by each of the online systems  100 , the steps  1415 - 1425  may be executed by each of the online systems  100 . 
       FIG. 15  is a block diagram of an apparatus that may perform various operations, and store various information generated and/or used by such operations, according to an embodiment of the disclosed technique. The apparatus can represent any computer or processing system described herein. The processing system  1500  is a hardware device on which any of the entities, components or services depicted in the examples of  FIGS. 1-14  (and any other components described in this specification), such as client device  160 , online system  100 , master online system  1105 , etc. can be implemented. The processing system  1500  includes one or more processors  1505  and memory  1510  coupled to an interconnect  1515 . The interconnect  1515  is shown in  FIG. 15  as an abstraction that represents any one or more separate physical buses, point to point connections, or both connected by appropriate bridges, adapters, or controllers. The interconnect  1515 , therefore, may include, for example, a system bus, a Peripheral Component Interconnect (PCI) bus or PCI-Express bus, a HyperTransport or industry standard architecture (ISA) bus, a small computer system interface (SCSI) bus, a universal serial bus (USB), IIC (I2C) bus, or an Institute of Electrical and Electronics Engineers (IEEE) standard 1394 bus, also called “Firewire”. 
     The processor(s)  1505  is/are the central processing unit (CPU) of the processing system  1500  and, thus, control the overall operation of the processing system  1500 . In certain embodiments, the processor(s)  1505  accomplish this by executing software or firmware stored in memory  1510 . The processor(s)  1505  may be, or may include, one or more programmable general-purpose or special-purpose microprocessors, digital signal processors (DSPs), programmable controllers, application specific integrated circuits (ASICs), programmable logic devices (PLDs), trusted platform modules (TPMs), or the like, or a combination of such devices. 
     The memory  1510  is or includes the main memory of the processing system  1500 . The memory  1510  represents any form of random access memory (RAM), read-only memory (ROM), flash memory, or the like, or a combination of such devices. In use, the memory  1510  may contain a code. In one embodiment, the code includes a general programming module configured to recognize the general-purpose program received via the computer bus interface, and prepare the general-purpose program for execution at the processor. In another embodiment, the general programming module may be implemented using hardware circuitry such as ASICs, PLDs, or field-programmable gate arrays (FPGAs). 
     Also connected to the processor(s)  1505  through the interconnect  1515  are a network adapter  1530 , a storage device(s)  1520  and I/O device(s)  1525 . The network adapter  1530  provides the processing system  1500  with the ability to communicate with remote devices, over a network and may be, for example, an Ethernet adapter or Fibre Channel adapter. The network adapter  1530  may also provide the processing system  1500  with the ability to communicate with other computers within the cluster. In some embodiments, the processing system  1500  may use more than one network adapter to deal with the communications within and outside of the cluster separately. 
     The I/O device(s)  1525  can include, for example, a keyboard, a mouse or other pointing device, disk drives, printers, a scanner, and other input and/or output devices, including a display device. The display device can include, for example, a cathode ray tube (CRT), liquid crystal display (LCD), or some other applicable known or convenient display device. 
     The code stored in memory  1510  can be implemented as software and/or firmware to program the processor(s)  1505  to carry out actions described above. In certain embodiments, such software or firmware may be initially provided to the processing system  1500  by downloading it from a remote system through the processing system  1500  (e.g., via network adapter  1530 ). 
     The techniques introduced herein can be implemented by, for example, programmable circuitry (e.g., one or more microprocessors) programmed with software and/or firmware, or entirely in special-purpose hardwired (non-programmable) circuitry, or in a combination of such forms. Special-purpose hardwired circuitry may be in the form of, for example, one or more ASICs, PLDs, FPGAs, etc. 
     Software or firmware for use in implementing the techniques introduced here may be stored on a machine-readable storage medium and may be executed by one or more general-purpose or special-purpose programmable microprocessors. A “machine-readable storage medium”, as the term is used herein, includes any mechanism that can store information in a form accessible by a machine. 
     A machine can also be a server computer, a client computer, a personal computer (PC), a tablet PC, a laptop computer, a set-top box (STB), a personal digital assistant (PDA), a cellular telephone, an iPhone, a Blackberry, a processor, a telephone, a web appliance, a network router, switch or bridge, or any machine capable of executing a set of instructions (sequential or otherwise) that specify actions to be taken by that machine. 
     A machine-accessible storage medium or a storage device(s)  1520  includes, for example, recordable/non-recordable media (e.g., ROM; RAM; magnetic disk storage media; optical storage media; flash memory devices; etc.), etc., or any combination thereof. The storage medium typically may be non-transitory or include a non-transitory device. In this context, a non-transitory storage medium may include a device that is tangible, meaning that the device has a concrete physical form, although the device may change its physical state. Thus, for example, non-transitory refers to a device remaining tangible despite this change in state. 
     The term “logic”, as used herein, can include, for example, programmable circuitry programmed with specific software and/or firmware, special-purpose hardwired circuitry, or a combination thereof.