Chapter 17 Process memory

Introduce

Processes use many different resources to achieve their goals . This includes some or all CPU cycle 、 Memory 、 External storage 、 Network bandwidth, etc . This article is about memory usage . say concretely , It handles the memory allocated for data storage , for example ：

• Public and private variables

When assigning values to them for the first time , They are allocated memory space . In the case of local arrays , The combination of local variable name plus all subscript values refers to a single variable value .

Except for variables that contain extremely long strings , Variables will take up with $STORAGE Related space . Variables containing extremely long strings are stored in different ways , And it doesn't take up $STORAGE In the space .

• Object instances

Whenever you instantiate an object , Will allocate space to save the current content of the object and the object it refers to . This space is returned when the last object reference is deleted .

• Local I/O buffer

Will be associated with the device that the process is using I/O Buffers are stored in the process space .

Manage process space

The process starts with the initial memory pool for the above entities . When applications create them , They consume memory in the pool ; When the application deletes them , Their memory will be returned to the pool . for example , When a routine starts executing , Always create local variables that consume some memory ; When the routine returns and these variables are out of range , The memory used by these variables will be returned and reused .

When an application needs memory , And the process is not large enough in its memory pool （ continuity ） Available memory area to meet the demand , The process will request additional memory blocks from the underlying operating system to be added to its pool . later , If the memory block is completely unused , It will return to the operating system . Because the order of allocating memory for entities and the order of deleting these entities from memory are not necessarily mirror images of each other , So as the execution goes on , Memory will become fragmented to some extent . This will affect the memory allocation and release of the above operating system .

$ZSTORAGE

Processes can use up to 2TB Memory . To help manage memory usage , It provides a way for administrators or applications to set small memory consumption limits . This value is stored in the system variable of each process $ZSTORAGE in , therefore $ZSTORAGE Always include the maximum allowable size of process memory （ With KB In units of ）.

$ZSTORAGE The value of 1KB Specify... For units . The minimum allowable value is 256, namely 256KB. The process can be $ZSTORAGE The maximum value set is 2TB (2^31 * 1KB) Memory . Trying to set a value less than the minimum value or greater than the maximum value will default to the minimum value or the maximum value, respectively .

Maximum memory per process

This value is set through the management portal . To access relevant pages , Please select system management > To configure > The system configuration > Memory and startup . On the page that appears , Set up Maximum Per-Process Memory (KB) The value in .

In profile (iris.cpf) in , This parameter is called bbsiz. This value is when the process starts $ZSTORAGE The initial value of the .

Be careful ： You can also go through ObjectScript JOB Set the memory limit of the process when the command starts the process .

$STORAGE

System variables $STORAGE Indicates the amount of storage available for processes that are still running . It is in bytes . When the memory request of the process is greater than $STORAGE When the value in or the request to allocate memory from the operating system fails , It will generate <STORE> error .

error

When the memory request of the process is satisfied, it will lead to $STORAGE The value of becomes negative , Or when the request of the operating system to allocate memory fails , It will generate <STORE> error . About handling $STORAGE Become negative <STORE> error , A process can be considered to be in one of two modes ： Normal mode and low memory mode .

• Normal mode

When the process is in normal mode and requests memory , Otherwise, it will lead to $STORAGE When it becomes negative , The process throws <STORE> Error and enter low memory mode .

• Low memory mode

In low memory mode , The allowed operation will $STORAGE Push to negative , In order to provide some extra memory for the application to handle errors and clean up . When a process in low memory mode releases memory ,$STORAGE The value of rises to at least 256KB（ or $ZSTORAGE Of 25%, If it is lower ）, The process will return to normal mode . In low memory mode ,$STORAGE The lower limit of is about -1MB. Otherwise, it will lead to $STORAGE Any operation below this limit will result in <STORE> error . The value of the lower limit is determined by $STORAGE Value definition , subtract 1MB.

Be careful ： The process can put $ZSTORAGE Set to any value within its allowable range . If $ZSTORAGE Set to less than the currently used value , be $STORAGE Will have a negative value . If this happens when the process is in normal mode , The next operation to allocate memory will result in the process getting <STORE> Error and enter low memory mode , The lower limit is equal to this value minus 1MB. If this happens when the process is already in low memory mode , Then the lower limit remains unchanged .

For reasons beyond -1MB Low memory mode limit or failure to allocate memory from the operating system <STORE> error , Because there is too little memory available , The behavior of the process is unpredictable . Into the <STORE> error , Or the error handler may not be called and the process may stop .

Error handlers can be resolved in one or more of the following ways <STORE> error ：

• Abort the calculation that caused the memory request , It may be possible to release calculations when they occur <STORE> Any storage space obtained before the error .
• Try to generate more available memory by deleting unwanted data .
• Carry out any necessary cleaning work （ For example, close the open file ）, And then terminate the program .
• take $ZSTORAGE Set the value of to a larger value , To allow the process to continue and request more memory in the future .

Special notes

platform

Most instances have less allocable space per process than 2TB Running on the system . On such systems , When The process ran out of available system memory （ Actual physical memory plus available swap space ） when , The underlying system may handle this situation in many ways . Some examples are ：

• On some platforms , The system will send a signal , guide Process termination .
• On some platforms （ for example Linux and AIX） On , The system uses heuristic algorithms to kill the process it considers the most aggressive . This may be a process , But it may also be another alternative process .
• Some systems generate kernels that crash the underlying operating system “ panic panic” To handle memory exhaustion .
• Some systems can handle memory exhaustion , But recovery may lead to access conflicts in the process .

Good programming practice shows that the process should not rely on the error recovery algorithm used by the underlying platform . contrary , Such processes should provide adequate design 、 Testing and logging , To allow the process to properly estimate and manage its own resource needs .