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v2.1

Documentation:v2.1

Supported Versions:
v3.1/
v3.0/
v2.1/
v2.0/
v1.1

Other Versions:
Current(v5.0)

Memory

This section describes memory parameters.

img NOTICE: These parameters take effect only after the database restarts.

memorypool_enable

Parameter description: Specifies whether to enable a memory pool.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

  • on indicates that the memory pool is enabled.
  • off indicates that the memory pool is disabled.

Default value: off

memorypool_size

Parameter description: Specifies the memory pool size.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 128 x 1024 to INT_MAX/2. The unit is KB.

Default value: 512MB

enable_memory_limit

Parameter description: Specifies whether to enable the logical memory management module.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

  • on indicates that the logical memory management module is enabled.
  • off indicates that the logical memory management module is disabled.

Default value: on

img CAUTION:

  • If the value of max_process_memory minus shared_buffer minus cstore_buffers minus metadata size is less than 2 GB, MogDB forcibly sets enable_memory_limit to off. Metadata is the memory used in MogDB and is related to some concurrent parameters, such as max_connections, thread_pool_attr and max_prepared_transactions.

  • If this parameter is set to off, the memory used by the database is not limited. When a large number of concurrent or complex queries are performed, too much memory is used, which may cause OS OOM problems.

max_process_memory

Parameter description: Specifies the maximum physical memory of a database node.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 2 x 1024 x 1024 to INT_MAX. The unit is KB.

Default value:12GB

Setting suggestions:

The value on the database node is determined based on the physical memory of the system and the number of master database nodes deployed on a single node. The recommended calculation formula is as follows: (Physical memory - vm.min_free_kbytes) \ x 0.7 /(1 + Number of primary nodes) This parameter is used to prevent node OOM caused by memory usage increase, ensuring system reliability. vm.min_free_kbytes indicates the OS memory reserved for the kernel to receive and send data. Its value is at least 5% of the total memory. That is, max_process_memory = Physical memory x 0.665 / (1 + Number of primary nodes)

img CAUTION: If this parameter is set to a value greater than the physical memory of the server, the OS OOM problem may occur.

enable_memory_context_control

Parameter description: Specifies whether to enable the function of checking whether the amount of memory contexts exceeds the specified limit. This parameter applies only to the DEBUG version.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

  • on indicates that the function of checking the amount of memory contexts is enabled.
  • off indicates that the function of checking the amount of memory contexts is disabled.

Default value: off

uncontrolled_memory_context

Parameter description: Specifies which memory texts will not be checked when the enable_memory_context_control parameter is set to on. This parameter applies only to the DEBUG version.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

During the query, the title meaning string "MemoryContext white list:" is added to the beginning of the parameter value.

Value range: a string

Default value: empty

shared_buffers

Parameter description: Specifies the size of shared memory used by MogDB. Increasing the value of this parameter causes MogDB to request more System V shared memory than the default configuration allows.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 16 to 1073741823. The unit is 8KB.

The value of shared_buffers must be an integer multiple of BLCKSZ. Currently, BLCKSZ is set to 8 KB. That is, the value of shared_buffers must be an integer multiple of 8 KB. The minimum value changes according to BLCKSZ.

Default value: 8 MB

Setting suggestions:

Set shared_buffers to a value less than 40% of the memory. Set it to a large value for row-store tables and a small value for column-store tables. For column-store tables: shared_buffers = (Memory of a single server/Number of database nodes on the server) x 0.4 x 0.25

If shared_buffers is set to a larger value, increase the value of checkpoint_segments because a longer period of time is required to write a large amount of new or changed data.

segment_buffers

Parameter description: Specifies the memory size of the metadata page of the MogDB segment buffer.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 16 to 1073741823. The unit is 8KB.

The value of segment_buffers must be an integer multiple of BLCKSZ. Currently, BLCKSZ is set to 8 KB. That is, the value of segment_buffers must be an integer multiple of 8 KB. The minimum value changes according to BLCKSZ.

Default value: 8MB

Setting suggestions:

segment_buffers is used to cache segment headers, which is key metadata information. To improve performance, it is recommended that the segment headers of common tables are cached to buffers and are not swapped. You are advised to set this parameter in terms of the number of tables (including index and toast tables) x number of partitions x 3 + 128. Generally, a table includes 3 metadata segments, so multiplying 3 is needed. +128 is needed because segment-based tablespace management requires a certain number of buffers.

bulk_write_ring_size

Parameter description: Specifies the size of a ring buffer used for parallel data import.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 16384 to 2147483647. The unit is KB.

Default value: 2GB

Setting suggestions: Increase the value of this parameter on database nodes if a huge amount of data will be imported.

standby_shared_buffers_fraction

Parameter description: Specifies the shared_buffers proportion used on the server where a standby instance is deployed.

This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: a double-precision floating-point number ranging from 0.1 to 1.0

Default value: 0.3

temp_buffers

Parameter description: Specifies the maximum size of local temporary buffers used by a database session.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

This parameter can be modified only before the first use of temporary tables within each session. Subsequent attempts to change the value of this parameter will not take effect on that session.

A session allocates temporary buffers based on the value of temp_buffers. If a large value is set in a session that does not require many temporary buffers, only the overhead of one buffer descriptor is added. If a buffer is used, additional 8192 bytes will be consumed for it.

Value range: an integer ranging from 100 to 1073741823. The unit is 8 KB.

Default value: 1MB

max_prepared_transactions

Parameter description: Specifies the maximum number of transactions that can stay in the prepared state simultaneously. Increasing the value of this parameter causes MogDB to request more System V shared memory than the default configuration allows.

When MogDB is deployed as an HA system, set this parameter on standby servers to a value greater than or equal to that on the primary server. Otherwise, queries will fail on the standby servers.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 0 to 262143

Default value: 10

img NOTE: Generally, explicit PREPARE operations are not required for transactions. If explicit PREPARE operations are performed for transactions, increase the value of this parameter to be greater than the number of concurrent services that require PREPARE to prevent preparation failures.

work_mem

Parameter description: Specifies the amount of memory to be used by internal sort operations and hash tables before they write data into temporary disk files. Sort operations are required for ORDER BY, DISTINCT, and merge joins. Hash tables are used in hash joins, hash-based aggregation, and hash-based processing of IN subqueries.

In a complex query, several sort or hash operations may run in parallel; each operation will be allowed to use as much memory as this parameter specifies. If the memory is insufficient, data will be written into temporary files. In addition, several running sessions could be performing such operations concurrently. Therefore, the total memory used may be many times the value of work_mem.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 64 to 2147483647. The unit is KB.

Default value: 64MB

  • Default value: 64 MB

    img NOTICE: Setting suggestions: If the physical memory specified by work_mem is insufficient, additional operator calculation data will be written into temporary tables based on query characteristics and the degree of parallelism. This reduces performance by five to ten times, and prolongs the query response time from seconds to minutes.

    • For complex serial queries, each query requires five to ten associated operations. Set work_mem using the following formula: work_mem = 50% of the memory/10.
    • For simple serial queries, each query requires two to five associated operations. Set work_mem using the following formula: work_mem = 50% of the memory/5.
    • For concurrent queries, set work_mem using the following formula: work_mem = work_mem for serial queries/Number of concurrent SQL statements.

query_mem

Parameter description: Specifies the memory used by a query. If the value of query_mem is greater than 0, the optimizer adjusts the memory cost estimate to this value when generating an execution plan.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: 0 or an integer greater than 32 MB. The default unit is KB. If the value is set to a negative value or is less than 32 MB, the default value 0 is used. In this case, the optimizer does not adjust the estimated query memory.

Default value: 0

img NOTICE:

  • If the value of query_mem is greater than 0, the optimizer adjusts the memory cost estimate to this value when generating an execution plan.
  • If the value is set to a negative value or a positive integer less than 32 MB, the default value 0 is used. In this case, the optimizer does not adjust the estimated query memory.

query_max_mem

Parameter description: Specifies the maximum memory that can be used by a query. If the value of query_max_mem is greater than 0, an error is reported when the query memory usage exceeds the value.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: 0 or an integer greater than 32 MB. The default unit is KB. If the value is set to a negative value or a positive integer less than 32 MB, the default value 0 is used. In this case, the optimizer does not limit the query memory.

Default value: 0

img NOTICE:

  • If the value of query_max_mem is greater than 0, an error is reported when the query memory usage exceeds the value.
  • If the value is set to a negative value or a positive integer less than 32 MB, the default value 0 is used. In this case, the optimizer does not limit the query memory.

maintenance_work_mem

Parameter description: Specifies the maximum amount of memory to be used by maintenance operations, such as VACUUM, CREATE INDEX, and ALTER TABLE ADD FOREIGN KEY. This parameter may affect the execution efficiency of VACUUM, VACUUM FULL, CLUSTER, and CREATE INDEX.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 1024 to INT_MAX. The unit is KB.

  • Default value: 16MB

    img NOTICE: Setting suggestions:

    • The value of this parameter must be greater than that of work_mem so that database dumps can be more quickly cleared or restored. In a database session, only one maintenance operation can be performed at a time. Maintenance is usually performed when there are not many running sessions.
    • When the Automatic Vacuuming process is running, up to autovacuum_max_workers times this memory may be allocated. In this case, set maintenance_work_mem to a value greater than or equal to that of work_mem.
    • If a large amount of data is to be clustered, increase the value of this parameter in the session.

psort_work_mem

Parameter description: Specifies the memory capacity to be used for partial sorting in a column-store table before writing to temporary disk files. This parameter can be used for inserting tables with a partial cluster key or index, creating a table index, and deleting or updating a table.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

img NOTICE: Several running sessions could be performing such operations concurrently. Therefore, the total memory used may be many times the value of psort_work_mem.

Value range: an integer ranging from 64 to 2147483647. The unit is KB.

Default value: 512MB

max_loaded_cudesc

Parameter description: Specifies the number of loaded CuDescs per column when a column-store table is scanned. Increasing the value will improve query performance and increase memory usage, particularly when there are many columns in the column-store table.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

img NOTICE: If max_loaded_cudesc is set to a large value, memory may be insufficient.

Value range: 100 to 1073741823

Default value: 1024

max_stack_depth

Parameter description: Specifies the maximum safe depth of the MogDB execution stack. The safety margin is required because the stack depth is not checked in every routine in the server, but only in key potentially-recursive routines, such as expression evaluation.

This parameter is a SUSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 100 to INT_MAX. The unit is KB.

Default value: 2 MB

img NOTICE: When setting this parameter, comply with the following principles:

  • The database needs to reserve 640 KB stack depth. Therefore, the ideal setting for this parameter is the actual stack size limit enforced by the OS kernel (as set by ulimit -s) minus 640 KB.
  • Setting this parameter to a value larger than the actual kernel limit means that a running recursive function may crash an individual backend process. In an OS where MogDB can determine the kernel limit, this parameter cannot be set to an unsafe value.
  • Since not all OSs provide this function, you are advised to set a specific value for this parameter.
  • 2 MB is a small value and will hardly cause system breakdown,

cstore_buffers

Parameter description: Specifies the shared buffer size used in column-store tables.

This parameter is a POSTMASTER parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 16384 to 1073741823. The unit is KB.

Default value: 1 GB

Setting suggestions:

Column-store tables use the shared buffer specified by cstore_buffers instead of that specified by shared_buffers. When column-store tables are mainly used, reduce the value of shared_buffers and increase that of cstore_buffers.

bulk_read_ring_size

Parameter description: Specifies the ring buffer size used for parallel data export.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: an integer ranging from 256 to 2147483647. The unit is KB.

Default value: 16MB

enable_early_free

Parameter description: Specifies whether the operator memory can be released in advance.

This parameter is a USERSET parameter. Set it based on instructions provided in Table 1 GUC parameters.

Value range: Boolean

  • on indicates that the operator memory can be released in advance.
  • off indicates that the operator memory cannot be released in advance.

Default value: on

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