Performance Tips¶

Use a single persistent session¶

Sessions are expensive objects to create in both time and resources because they maintain a pool of connections to your ScyllaDB/Cassandra cluster. An application should create a minimal number of sessions and maintain them for the lifetime of an application.

Use token-aware and latency-aware policies¶

The token-aware load balancing can reduce the latency of requests by avoiding an extra network hop through a coordinator node. When using the token-aware policy requests are sent to one of the nodes which will retrieved or stored instead of routing the request through a proxy node (coordinator node).

The latency-aware load balancing policy can also reduce the latency of requests by routing requests to nodes that historical performing with the lowest latency. This can prevent requests from being sent to nodes that are underperforming.

Both latency-aware and token-aware can be use together to obtain the benefits of both.

Use paging when retrieving large result sets¶

Using a large page size or a very high LIMIT clause can cause your application to delay for each individual request. The driver’s paging mechanism can be used to decrease the latency of individual requests.

Choose a lower consistency level¶

Ultimately, choosing a consistency level is a trade-off between consistency and availability. Performance should not be a large deciding factor when choosing a consistency level. However, it can affect high-percentile latency numbers because requests with consistency levels greater than ONE can cause requests to wait for one or more nodes to respond back to the coordinator node before a request can complete. In multi-datacenter configurations, consistency levels such as EACH_QUORUM can cause a request to wait for replication across a slower cross datacenter network link. More information about setting the consistency level can be found here.

Increasing core connections¶

In some workloads, throughput can be increased by increasing the number of core connections. By default, the driver uses a single core connection per host. It’s recommended that you try increasing the core connections to two and slowly increase this number while doing performance testing. Two core connections is often a good setting and increasing the core connections too high will decrease performance because having multiple connections to a single host inhibits the driver’s ability to coalesce multiple requests into a fewer number of system calls.

Coalesce delay¶

The coalesce delay is an optimization to reduce the number of system calls required to process requests. This setting controls how long the driver’s I/O threads wait for requests to accumulate before flushing them on to the wire. Larger values for coalesce delay are preferred for throughput-based workloads as it can significantly reduce the number of system calls required to process requests.

In general, the coalesce delay should be increased for throughput-based workloads and can be decreased for latency-based workloads. Most importantly, the delay should consider the responsiveness guarantees of your application.

Note: Single, sporadic requests are not generally affected by this delay and are processed immediately.

New request ratio¶

The new request ratio controls how much time an I/O thread spends processing new requests versus handling outstanding requests. This value is a percentage (with a value from 1 to 100), where larger values will dedicate more time to processing new requests and less time on outstanding requests. The goal of this setting is to balance the time spent processing new/outstanding requests and prevent either from fully monopolizing the I/O thread’s processing time. It’s recommended that your application decrease this value if computationally expensive or long-running future callbacks are used (via cass_future_set_callback()), otherwise this can be left unchanged.