However, this default configuration may not be optimal when the number of streaming jobs running in the cluster exceeds 300. In such cases, resource sharing and contention among multiple streaming jobs can arise. Also, due to the high number of tasks scheduled and communicating on the compute node, the default configuration can lead to poor performance and increased risks of Out of Memory (OOM) errors. This guide describes and explains some do’s and don’ts in this case. Even with the correct cluster settings in place, RisingWave may still experience overload, leading to performance issues such as high latency and OOM errors. In such cases, we recommend users to consider increasing resources to address these issues. RisingWave offers the flexibility to scale up, down, in, or out, making it easy and quick to adjust resources as needed. We advise users to adjust resources through a trial and error approach. For clusters that are expected to create more than 300 materialized views, these configurations can help optimize the performance and resource utilization of the cluster.

​
Set the default parallelism

Add or update the parameter setting in risingwave/src/config/<your-config>.yaml. 
[meta]
disable_automatic_parallelism_control=true
default_parallelism=8
The adaptive parallelism feature in version 1.7.0 ensures that every streaming job can fully utilize all available CPUs. Therefore, we need to disable it. The default_parallelism setting determines the parallelism for newly created streaming jobs. Change the streaming_parallelism before creating streaming jobs can achieve the same effect. For guidance on how to set this value, refer to How to adjust the resources allocated to each streaming query?.

​
Limit the concurrency of creating streaming jobs

If you want to create multiple streaming jobs at once using scripts or tools such as DBT, the system parameter max_concurrent_creating_streaming_jobs is helpful. It controls the maximum number of streaming jobs created concurrently. However, please do not set it too high, as it may introduce excessive pressure on the cluster.

​
Tuning an existing cluster

You can check the total number of actors using the following SQL command. It indicates how many actors are running in your cluster. 
select count(*) from rw_actors;
If the number exceeds 50000, please pay close attention and check the following items.

​
Limit parallelism for less intensive streaming jobs

If you want to limit the parallelism for tables, indexes, materialized views, sinks, or sources, you can set the corresponding session parameters to define specific limits for each type. 
/* set the global parallelism*/
dev=> set streaming_parallelism = 1;
SET_VARIABLE
dev=> create table t_normal(v int);
CREATE_TABLE
/* specify the parallelism for the table type*/
dev=> set streaming_parallelism_for_table = 2;
SET_VARIABLE
dev=> create table t(v int);
CREATE_TABLE
/* specify the parallelism for the index type*/
dev=> set streaming_parallelism_for_index = 3;
SET_VARIABLE
dev=> create index i on t(v);
CREATE_INDEX
dev=> select * from rw_streaming_parallelism;
 id |   name   | relation_type | parallelism | max_parallelism
----+----------+---------------+-------------+-----------------
  7 | t        | table         | FIXED(2)    |             256
  6 | t_normal | table         | FIXED(1)    |             256
  8 | i        | index         | FIXED(3)    |             256
(3 rows)
By setting the parallelism strategy for a specific job type, you can control the exact parallelism used when the job is created. If you want to reset to the default strategy, simply set it to default. The parallelism will then fall back to the global streaming_parallelism setting. 
dev=> set streaming_parallelism = adaptive;
SET_VARIABLE
dev=> set streaming_parallelism_for_table = 1;
SET_VARIABLE
dev=> create table t(v int);
CREATE_TABLE
dev=> set streaming_parallelism_for_table = default;
SET_VARIABLE
dev=> create table t_default(v int);
CREATE_TABLE
dev=> select * from rw_streaming_parallelism;
 id |   name    | relation_type | parallelism | max_parallelism
----+-----------+---------------+-------------+-----------------
  9 | t_default | table         | ADAPTIVE    |             256
  8 | t         | table         | FIXED(1)    |             256
(2 rows)
You can also apply the configuration globally using the following syntax: 
ALTER SYSTEM SET streaming_parallelism_for_table TO 4;

​
Decrease the parallelism

When the total number of actors in the cluster is large, excessive parallelism can be counterproductive. After v1.7.0, you can check the parallelism number of the running streaming jobs in the system table rw_fragment_parallelism, and you can alter the streaming jobs’s parallelism with the ALTER statement. For more information, refer to Cluster scaling. Here is an example of how to adjust the parallelism. 

/* get the current parallelism of the running streaming jobs*/
dev=> select * from rw_fragment_parallelism;
  id  |    name     |   relation_type   | fragment_id | distribution_type | state_table_ids | upstream_fragment_ids |        flags        | parallelism
------+-------------+-------------------+-------------+-------------------+-----------------+-----------------------+---------------------+-------------
 1003 | table_xxx_1 | table             |           6 | HASH              | {}              | {}                    | {SOURCE,DML}        |           4
 1003 | table_xxx_1 | table             |           5 | HASH              | {1003}          | {6}                   | {MVIEW}             |           4
 1006 | table_xxx_4 | table             |          12 | HASH              | {}              | {}                    | {SOURCE,DML}        |           16
 1006 | table_xxx_4 | table             |          11 | HASH              | {1006}          | {12}                  | {MVIEW}             |           16
 1005 | table_xxx_3 | table             |          10 | HASH              | {}              | {}                    | {SOURCE,DML}        |           32
 1005 | table_xxx_3 | table             |           9 | HASH              | {1005}          | {10}                  | {MVIEW}             |           32
 1004 | table_xxx_2 | table             |           8 | HASH              | {}              | {}                    | {SOURCE,DML}        |           32
 1004 | table_xxx_2 | table             |           7 | HASH              | {1004}          | {8}                   | {MVIEW}             |           32
 1013 | other_mv    | materialized view |          16 | HASH              | {1014,1013}     | {5}                   | {MVIEW,STREAM_SCAN} |           32
 1009 | mv_xxx_2    | materialized view |          14 | HASH              | {1010,1009}     | {7}                   | {MVIEW,STREAM_SCAN} |           32
 1011 | mv_xxx_3    | materialized view |          15 | HASH              | {1012,1011}     | {9}                   | {MVIEW,STREAM_SCAN} |           32
 1007 | mv_xxx_1    | materialized view |          13 | HASH              | {1008,1007}     | {5}                   | {MVIEW,STREAM_SCAN} |           4
(12 rows)

/* generate the alter statement sql to decrease parallelism for some jobs with some condition*/
dev=> select distinct 'alter ' || fp.relation_type || ' ' || fp.name || ' set parallelism = 4;' as sql
from rw_fragment_parallelism fp
where fp.parallelism > 4 and fp.name like '%_xxx_%';
                       sql
-------------------------------------------------------
 alter table table_xxx_4 set parallelism = 4;
 alter materialized view mv_xxx_2 set parallelism = 4;
 alter materialized view mv_xxx_3 set parallelism = 4;
 alter table table_xxx_2 set parallelism = 4;
 alter table table_xxx_3 set parallelism = 4;
(5 rows)

/* alter them one by one*/
dev=>  alter table table_xxx_4 set parallelism = 4;
ALTER_TABLE
dev=>  alter materialized view mv_xxx_2 set parallelism = 4;
ALTER_MATERIALIZED_VIEW
dev=>  alter materialized view mv_xxx_3 set parallelism = 4;
ALTER_MATERIALIZED_VIEW
dev=>  alter table table_xxx_2 set parallelism = 4;
ALTER_TABLE
dev=>  alter table table_xxx_3 set parallelism = 4;
ALTER_TABLE

​
Adjust actors distribution

After adding a new compute node to a cluster with many existing streaming jobs, the distribution of actors among the compute nodes might become unbalanced, resulting in low workload on some nodes. This SQL query can display the distribution of actors on each compute node. 
select worker_id, count(*) from rw_actors a, rw_parallel_units p where a.parallel_unit_id = p.id group by p.worker_id;
Here is an example of an unbalanced result. The number of actors on worker node 12001 is much lower than the other two worker nodes. 
worker_id|count|
---------+-----+
    12001|  720|
    12002|23455|
    12003|23300|
To rebalance the actor, you can use the alter parallelism statement mentioned above after v1.7.0, and the actors will be distributed to different compute nodes automatically.
In some references, /risingwave/bin/risingwave ctl scale horizon --include-workers all is used to scale out all streaming jobs to avoid the skewed actor distribution. However, this approach may not be sufficient when dealing with a large number of streaming jobs, as it does not consider the default_parallelism parameter.

​
Other precautions for too many actors

  • Resources of meta store (etcd) and meta node: During recovering or scaling processes,there might be spikes in resource usage on the nodes. If you encounter OOM errors or observe some error logs in the meta such as {"level":"ERROR","fields":{"message":"lease keeper failed","error":"grpc request error: status: Unavailable, message: \"etcdserver: request timed out, waiting for the applied index took too long\",}, please try to scale up the nodes.
  • Resources of the prometheus or other monitoring systems: The number of metrics’ time series grows linearly with the number of actors, so please pay attention to the resource requirements of your monitoring system and ensure that it can handle the growing number of metrics efficiently.