Memory

enable_memory_limit

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

Type: POSTMASTER

Value range: Boolean

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

Default value: on

• If the value of max_process_memory-max_shared_memory-cstore buffers is less than 2 GB, forcibly set enable_memory_limit to off.
• The max_shared_memory parameter is closely related to the shared_buffer, max_connections, and max_prepared_transactions parameters. If the value of max_shared_memory is too large, you can decrease the values of the three parameters.
• The dynamic load management function depends on the memory management function. After the enable_memory_limit parameter is disabled, the dynamic load management and TopSQL functions become invalid.

max_process_memory

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

Type: POSTMASTER

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

Default value: The value is automatically adapted on non-secondary DNs. The formula is (Physical memory size) x 0.8/ (1 + Number of primary DNs). If the result is less than 2 GB, 2 GB is used by default. The default size of the secondary DN is 12 GB.

Setting suggestions:

On DNs, the value of this parameter is determined based on the physical system memory and the number of DNs deployed on a single node. Parameter value = (Physical memory – vm.min_free_kbytes) x 0.8/(n + Number of primary DNs). This parameter aims to ensure system reliability, preventing node OOM caused by increasing memory usage. vm.min_free_kbytes indicates OS memory reserved for kernels to receive and send data. Its value is at least 5% of the total memory. That is, max_process_memory = Physical memory x 0.8/ (n + Number of primary DNs). If the cluster scale (number of nodes in the cluster) is smaller than 256, n=1; if the cluster scale is larger than 256 and smaller than 512, n=2; if the cluster scale is larger than 512, n=3.

Set this parameter on CNs to the same value as that on DNs.

RAM is the maximum memory allocated to the cluster.

shared_buffers

Parameter description: Specifies the size of shared memory used by GaussDB(DWS). If this parameter is set to a large value, GaussDB(DWS) may require more System V shared memory than the default setting.

Type: POSTMASTER

Value range: an integer ranging from 128 to INT_MAX. The unit is 8 KB.

Changing the value of BLCKSZ will result in a change in the minimum value of the shared_buffers.

Default value: The value of CN is half of the value of DN. The value of DN is calculated using the following formula: POWER(2,ROUND(LOG(2, max_process_memory/18),0)). If the maximum value allowed by the OS is smaller than 32 MB, this parameter will be automatically changed to the maximum value allowed by the OS during database initialization.

Setting suggestions:

You are advised to set this parameter for DNs to a value greater than that for CNs, because GaussDB(DWS) pushes its most queries down to DNs.

It is recommended that shared_buffers be set 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

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

bulk_write_ring_size

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

Type: USERSET

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

Default value: 2 GB

Setting suggestions: Increase the value of this parameter on DNs if a huge amount of data is to be imported.

buffer_ring_ratio

Parameter description: ring buffer threshold for parallel data export

Type: USERSET

Value range: integer in the range 1–1000

Default value: 250

• The default value indicates that the threshold is 250/1000 (a quarter) of shared_buffers.
• The minimum value is 1/1000 of the value of shared_buffers.
• The maximum value is the value of shared_buffers.

Setting suggestions: If the cache hit ratio is not as expected during export, you are advised to configure this parameter on DNs.

temp_buffers

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

Type: USERSET

Value range: an integer ranging from 800 to INT_MAX/2. The unit is 8 KB.

Default value: 8 MB

• 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.
• Based on the value of temp_buffers, a session allocates temporary buffers as required. The cost of setting a large value in sessions that do not require many temporary buffers is only a buffer descriptor. If a buffer is used, 8192 bytes will be consumed for it.

max_prepared_transactions

Parameter description: Specifies the maximum number of transactions that can stay in the prepared state simultaneously. If this parameter is set to a large value, GaussDB(DWS) may require more System V shared memory than the default setting.

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

Type: POSTMASTER

Value range: an integer ranging from 0 to 536870911. The value of CN set to 0 indicates that the prepared transaction feature is disabled.

Default value: 800 for both CNs and DNs

Set this parameter to a value greater than or equal to that of max_connections to avoid failures in preparation.

work_mem

Parameter description: Specifies the memory capacity to be used by internal sort operations and Hash tables before writing to temporary disk files. Sort operations are used for ORDER BY, DISTINCT, and merge joins. Hash tables are required for Hash joins as well as Hash-based aggregations and IN subqueries.

For a complex query, several sort or Hash operations may be running in parallel; each operation will be allowed to use as much memory as this value specifies. If the memory is insufficient, data is 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.

Type: USERSET

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

Default value: 512 MB for small-scale memory and 2 GB for large-scale memory (If max_process_memory is greater than or equal to 30 GB, it is large-scale memory. Otherwise, it is small-scale 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.

• In complex serial query scenarios, each query requires five to ten associated operations. Set work_mem using the following formula: work_mem = 50% of the memory/10.
• In simple serial query scenarios, 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, use the formula: work_mem = work_mem in serialized scenario/Number of concurrent SQL statements.

query_mem

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

Type: USERSET

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 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

query_max_mem

Parameter description: Specifies the maximum memory that can be used by query. If the value of query_max_mem is greater than 0, when generating an execution plan, the optimizer uses this value to set the available memory for operators. If job memory usage exceeds the value of this parameter, an error is reported and the job exits.

Type: USERSET

Value range: 0 or an integer greater than 32 MB. The default unit is KB. If the value is less than 32 MB, the system automatically sets this parameter to the default value 0. In this case, the optimizer does not limit the memory usage of jobs.

Default value: 0

agg_max_mem

Parameter description: Specifies the maximum memory that can be used by the Agg operator when the number of aggregation columns exceeds 5. This parameter takes effect only when the value of query_max_mem is greater than 0. (This parameter is supported only in 8.1.3.200 and later cluster versions.)

Type: USERSET

Value range: 0 or an integer greater than 32 MB. The default unit is KB. If the value is less than 32 MB, the system automatically sets this parameter to the default value 0. In this case, the memory usage of the Agg operator is not limited based on the value.

Default value:

• If the current cluster is upgraded from an earlier version to 8.1.3, the value in the earlier version is inherited. The default value is INT_MAX.
• If the current cluster version is 8.1.3, the default value is 2GB.

maintenance_work_mem

Parameter description: Specifies the maximum size of memory to be used for 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.

Type: USERSET

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

Default value: 512 MB for small-scale memory and 2 GB for large-scale memory (If max_process_memory is greater than or equal to 30 GB, it is large-scale memory. Otherwise, it is small-scale memory.)

Setting suggestions:

• You are advised to set this parameter to the same value of work_mem so that database dump can be cleared or restored more quickly. In a database session, only one maintenance operation can be performed at a time. Maintenance is usually performed when there are not much sessions.
• When the Automatic Cleanup process is running, up to autovacuum_max_workers times of this memory may be allocated. Set maintenance_work_mem to a value equal to or larger than the value of work_mem.
• If a large amount of data needs to be processed in the cluster, increase the value of this parameter in sessions.

psort_work_mem

Parameter description: Specifies the memory used for internal sort operations on column-store tables before they are written into 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.

Type: USERSET

Multiple running sessions may perform partial sorting on a table at the same time. Therefore, the total memory usage may be several times of the psort_work_mem value.

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

Default value: 512 MB

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 the query performance and increase the memory usage, particularly when there are many columns in the column tables.

Type: USERSET

Value range: an integer ranging from 100 to INT_MAX/2

Default value: 1024

When the value of max_loaded_cudesc is set to a large value, the memory may be insufficient.

max_stack_depth

Parameter description: Specifies the maximum safe depth of GaussDB(DWS) 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.

Type: SUSET

Take the following into consideration when setting this parameter:

• The ideal value of this parameter is the maximum stack size enforced by the kernel (value of ulimit -s).
• 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 GaussDB(DWS) can check the kernel limit, such as the SLES, GaussDB(DWS) will prevent this parameter from being set to a value greater than the kernel limit.
• Since not all the OSs provide this function, you are advised to set a specific value for this parameter.

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

Default value: 2 MB

2 MB is a small value and will not incur system breakdown in general, but may lead to execution failures of complex functions.

cstore_buffers

Parameter description: Specifies the size of the shared buffer used by ORC, Parquet, or CarbonData data of column-store tables and OBS or HDFS column-store foreign tables.

Type: POSTMASTER

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

Default value: The CN size is 32 MB, and the DN size is calculated as follows: POWER(2,ROUND(LOG(2, max_process_memory/18),0)).

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.

Use cstore_buffers to specify the cache of ORC, Parquet, or CarbonData metadata and data for OBS or HDFS foreign tables. The metadata cache size should be 1/4 of cstore_buffers and not exceed 2 GB. The remaining cache is shared by column-store data and foreign table column-store data.

enable_orc_cache

Parameter description: Specifies whether to reserve 1/4 of cstore_buffers for storing ORC metadata when the cstore buffer is initialized.

Type: POSTMASTER

Value range: Boolean

Default value:

• on indicates that the orc metadata cache is enabled, which improves the query performance of the HDFS table but occupies the column-store buffer resources. The column-store performance deteriorates.
• off indicates the orc metadata cache is disabled.

schedule_splits_threshold

Parameter description: Specifies the maximum number of files that can be stored in memory when you schedule an HDFS foreign table. If the number is exceeded, all files in the list will be spilled to disk for scheduling.

Type: USERSET

Value range: an integer ranging from 1 to INT_MAX

Default value: 60000

bulk_read_ring_size

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

Type: USERSET

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

Default value: 16 MB

check_cu_size_threshold

Parameter description: If the amount of data inserted to a CU is greater than the value of this parameter when data is inserted to a column-store table, the system starts row-level size verification to prevent the generation of a CU whose size is greater than 1 GB (non-compressed size).

Type: USERSET

Value range: an integer ranging from 0 to 1048576. The unit is KB.

Default value: 1 GB