Redshift workload management basics

Configuring workload management (WLM) for a Redshift cluster is one of the most impactful things you can do to improve the overall performance of your queries.

The goal is, roughly speaking, to have as less slots per queue as possible with as less — ideally none — wait time in each queue as possible. This will ensure that queries have the most amount of memory available (which helps with query execution speed as intermediate results don’t have to be written to disk) while, at the same time, they execute immediately.

There’s no golden rule on how to configure WLM queues, as it is really use-case specifics. I recommend starting very simple. By default, there’s a single queue with concurrency level of 5. This is, most probably, insufficient — queries won’t be executed immediately, but will be waiting for a slot to free up. Increase it (say, to 15) and monitor the wait time over the next few days.

You can use the v_check_wlm_query_trend_hourly admin view from the tremendously useful amazon-redshift-utils and plot it on a graph.

wlm-execution-and-wait-times-main-q

You are only interested in those with a service_class > 5 as first five are internal and you cannot change their configuration.

In the graph above you can see that there’s pretty much no wait time on the queue, which is a good thing. In such a case you can experiment with reducing the concurrency level to increase the memory-per-slot of a queue. Use this query to inspect the memory allocation and concurrency level of your queues:

SELECT service_class, query_working_mem as mem_mb_per_slot, num_query_tasks as concurrency_level
FROM stv_wlm_service_class_config
WHERE service_class > 5
ORDER BY 1;

Finally, make sure to set a query timeout (maximum time it can run) on your WLM queues. A runaway query can bring your cluster to a halt.

Figuring out the sweet spot for your WLM setup takes a while and you should revisit it regularly as your system evolves. The great thing about changing WLM config is that tweaking the properties of a queue does not require a cluster reboot so you won’t disrupt the work of your colleagues by experimenting with the setup.

There is a lot of fine-grained parameters you can adjust and tons more to learn about WLM (my favourite gem is wlm_query_slot_count). Yet already a very basic setup will help with the overall cluster performance. It is absolutely worth the effort to understand and implement WLM.

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