Memory

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.

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.

Value range: an integer ranging from 128 x 1024 to 1073741823. The unit is KB.

Default value: 512 MB

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.

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

• If the value of max_process_memory minus shared_buffer minus cstore_buffers minus metadata size is less than 2 GB, GaussDB forcibly sets enable_memory_limit to off. Metadata is the memory used in GaussDB 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.

Value range: an integer ranging from 2097152 to 2147483647. The unit is KB.

Default value:

Independent deployment: 360 GB (60-core CPU/480 GB memory); 192 GB (32-core CPU/256 GB memory); 96 GB (16-core CPU/128 GB memory); 40 GB (8-core CPU/64 GB memory); 20 GB (4-core CPU/32 GB memory); 10 GB (4-core CPU/16 GB memory)

Setting suggestions:

On DNs, the value of this parameter is determined based on the physical system memory and the number of primary DNs deployed on a single node. Parameter value = (Physical memory – vm.min_free_kbytes) x 0.7/(n + Number of primary DNs). 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. Therefore, the value of max_process_memory is: Physical memory x 0.665/(n + Number of primary DNs). When the number of nodes in the cluster is less than or equal to 256, n = 1. When the number of nodes in the cluster is greater than 256 and less than or equal to 512, n = 2. When the number of nodes in the cluster is greater than 512, n = 3.

You can set this parameter on CNs to the same value as that on DNs.

RAM is the maximum memory allocated to the cluster. It equals the physical memory of servers.

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

local_syscache_threshold

Parameter description: Specifies the size of system catalog cache in a session.

Value range: an integer ranging from 1 x 1024 to 512 x 1024. The unit is KB.

Default value:

• Independent deployment: 16 MB

enable_memory_context_control

Parameter description: Enables 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.

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.

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 GaussDB. Increasing the value of this parameter causes GaussDB 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.

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

The minimum value changes according to BLCKSZ.

Default value:

Independent deployment:

CN: 4 GB (60-core CPU/480 GB memory); 2 GB (32-core CPU/256 GB memory, 16-core CPU/128 GB memory); 1 GB (8-core CPU/64 GB memory); 512 MB (4-core CPU/32 GB memory) 256 MB (4-core CPU/16 GB memory)

DN: 140 GB (60-core CPU/480 GB memory); 76 GB (32-core CPU/256 GB memory); 40 GB (16-core CPU/128 GB memory); 16 GB (8-core CPU/64 GB memory); 8 GB (4-core CPU/32 GB memory); 4 GB (4-core CPU/16 GB memory)

Setting suggestions:

1. Set this parameter on DNs to a value greater than that on CNs because most queries in GaussDB are pushed down.
2. 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 DNs on the single server) x 0.4 x 0.25
3. 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.
4. If the process fails to be restarted after the value of shared_buffers is changed, perform either of the following operations based on the error information:
   1. Adjust the kernel.shmall, kernel.shmmax, and kernel.shmmin OS parameters. For details, see "Preparing for Installation > Modifying OS Configuration > Configuring Other OS Parameters" in Installation Guide.
   2. Run the free -g command to check whether the available memory and swap space of the OS are sufficient. If the memory is insufficient, manually stop other user programs that occupy much memory.
   3. Do not set shared_buffers to an excessively large or small value.

segment_buffers

Parameter description: Specifies the memory size of a GaussDB segment-page metadata page.

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

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

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: 8 MB

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.

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

Default value: 2 GB

Setting suggestions: Increase the value of this parameter on DNs 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.

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

Default value: 1

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.

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: 1 MB

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 GaussDB to request more System V shared memory than the default configuration allows.

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

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

Value range: an integer ranging from 0 to 262143

Default value:

• Independent deployment:

  1200 (60-core CPU/480 GB memory and 32-core CPU/256 GB memory); 800 (16-core CPU/128 GB memory); 400 (8-core CPU/64 GB memory); 300 (4-core CPU/32 GB memory); 200 (4-core CPU/16 GB memory)

To avoid failures in the preparation step, the value of this parameter must be greater than the number of worker threads in thread_pool_attr in thread pool mode. In non-thread pool mode, the value of this parameter must be greater than or equal to the value of max_connections.

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.

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

Default value:

• Independent deployment:

  128 MB (60-core CPU/480 GB memory, 32-core CPU/256 GB memory, and 16-core CPU/128 GB memory); 64 MB (8-core CPU/64 GB memory); 32 MB (4-core CPU/32 GB memory); 16 MB (4-core CPU/16 GB memory)

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.
• BitmapScan hash tables are also restricted by work_mem, but will not be forcibly flushed to disks. In the case of complete lossify, every 1-MB memory occupied by the hash table corresponds to a 16 GB page of BitmapHeapScan. After the upper limit of work_mem is reached, the memory increases linearly with the data access traffic based on this ratio.

query_mem

Parameter description: Specifies the memory used by a query.

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

Value range: 0 or an integer greater than 32 MB. The default unit is KB.

Default value: 0

• 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.

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

Value range: 0 or an integer greater than 32 MB. The default unit is KB.

Default value: 0

• 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.

Value range: an integer ranging from 1024 to 2147483647. The unit is KB.

Default value:

• Independent deployment:

  CN: 1 GB (60-core CPU/480 GB memory); 512 MB (32-core CPU/256 GB memory); 256 MB (16-core CPU/128 GB memory); 128 MB (8-core CPU/64 GB memory); 64 MB (4-core CPU/32 GB memory);32 MB (4-core CPU/16 GB memory)

  DN: 2 GB (60-core CPU/480 GB memory); 1 GB (32-core CPU/256 GB memory); 512 MB (16-core CPU/128 GB memory); 256 MB (8-core CPU/64 GB memory); 128 MB (4-core CPU/32 GB memory); 64 MB (4-core CPU/16 GB memory)

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 having 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.

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: 512 MB

max_loaded_cudesc

Parameter description: Specifies the number of cudesc cached in each 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.

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 GaussDB 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.

Value range: an integer ranging from 100 to 2147483647. The unit is KB.

Default value:

• If the value of ulimit -s minus 640 KB is greater than or equal to 2 MB, the default value of this parameter is 2 MB.
• If the value of ulimit -s minus 640 KB is less than 2 MB, the default value of this parameter is the value of ulimit -s minus 640 KB.

When setting this parameter, comply with the following principles:

• The database needs to reserve 640 KB stack depth. Therefore, the maximum value of this parameter is the actual stack size limit enforced by the OS kernel (as set by ulimit -s) minus 640 KB.
• If the value of this parameter is greater than the value of ulimit -s minus 640 KB before the database is started, the database fails to be started. During database running, if the value of this parameter is greater than the value of ulimit -s minus 640 KB, this parameter does not take effect.
• If the value of ulimit -s minus 640 KB is less than the minimum value of this parameter, the database fails to be started.
• Setting this parameter to a value greater than the actual kernel limit means that a running recursive function may crash an individual backend process.
• Since not all OSs provide this function, you are advised to set a specific value for this parameter.
• The default value is 2 MB, which is relatively small and does not easily 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.

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

Default value: 32 MB

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.

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

Default value: 16 MB

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.

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

memory_trace_level

Parameter description: Specifies the control level for recording memory allocation information after the dynamic memory usage exceeds 90% of the maximum dynamic memory. This parameter takes effect only when the GUC parameters use_workload_manager and enable_memory_limit are enabled. This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: enumerated values

• none: indicates that memory application information is not recorded.
• level1: After the dynamic memory usage exceeds 90% of the maximum dynamic memory, the following information is recorded and the recorded memory information is saved in the $GAUSSLOG/mem_log directory.
  • Global memory overview.
  • Memory usage of the top 20 memory contexts of the instance, session, and thread types.
  • The totalsize and freesize columns for each memory context.
• level2: After the dynamic memory usage exceeds 90% of the maximum dynamic memory, the following information is recorded and the recorded memory information is saved in the $GAUSSLOG/mem_log directory.
  • Global memory overview.
  • Memory usage of the top 20 memory contexts of the instance, session, and thread types.
  • The totalsize and freesize columns for each memory context.
  • Detailed information about all memory applications in each memory context, including the file where the allocated memory is located, line number, and size.

Default value: level1

• If this parameter is set to level2, the memory allocation details (file, line, and size) of each memory context are recorded, which greatly affects the performance. Therefore, exercise caution when setting this parameter.
• The recorded memory snapshot information can be queried by using the system function gs_get_history_memory_de....
• If the use_workload_manager parameter is disabled and the bypass_workload_manager parameter is enabled, this parameter also takes effect. The bypass_workload_manager parameter is of the SIGHUP type; therefore, after the reload mode is set, you need to restart the database for the setting to take effect.
• The recorded memory context is obtained after all memory contexts of the same type with the same name are summarized.

resilience_memory_reject_percent

Parameter description: Specifies the dynamic memory usage percentage for escape from memory overload. This parameter takes effect only when the GUC parameters use_workload_manager and enable_memory_limit are enabled. This parameter is a SIGHUP parameter. Set it based on instructions provided in Table 1.

Value range: a string, consisting of one or more characters

This parameter consists of recover_memory_percent and overload_memory_percent. The meanings of the two parts are as follows:

• recover_memory_percent: Percentage of the dynamic memory usage when the memory recovers from overload to the maximum dynamic memory. When the dynamic memory usage is less than the maximum dynamic memory multiplied by the value of this parameter, the overload escape function is disabled and new connections are allowed. The value ranges from 0 to 100. The value indicates a percentage.
• overload_memory_percent: Percentage of the dynamic memory usage to the maximum dynamic memory when the memory is overloaded. When the dynamic memory usage is greater than the maximum dynamic memory multiplied by the value of this parameter, the current memory is overloaded. In this case, the overload escape function is triggered to kill sessions and new connections are prohibited. The value ranges from 0 to 100. The value indicates a percentage.

Default value: '0,0', indicating that the escape from memory overload function is disabled.

Example:

resilience_memory_reject_percent = '70,90'

When the memory usage exceeds 90% of the upper limit, new connections are forbidden and stacked sessions are killed. When the memory usage is less than 70% of the upper limit, session killing is stopped and new connections are allowed.

• You can query the maximum dynamic memory and used dynamic memory in the pv_total_memory_detail view. max_dynamic_memory indicates the maximum dynamic memory, and dynamic_used_memory indicates the used dynamic memory.
• If this parameter is set to a small value, the escape from memory overload process is frequently triggered. As a result, ongoing sessions are forcibly logged out, and new connections fail to be connected for a short period of time. Therefore, exercise caution when setting this parameter based on the actual memory usage.
• If the use_workload_manager parameter is disabled and the bypass_workload_manager parameter is enabled, this parameter also takes effect. The bypass_workload_manager parameter is of the SIGHUP type; therefore, after the reload mode is set, you need to restart the database for the setting to take effect.
• The values of recover_memory_percent and overload_memory_percent can be 0 at the same time. In addition, the value of recover_memory_percent must be smaller than that of overload_memory_percent. Otherwise, the setting does not take effect.