=============
Tuning Presto
=============

The default Presto settings should work well for most workloads. The following
information may help you if your cluster is facing a specific performance problem.

When setting up a cluster for a specific workload it may be necessary to adjust the
following properties to ensure optimal performance:

  * :ref:`distributed-joins-enabled <tuning-pref-general>`
  * :ref:`query.max-memory <tuning-pref-query>`
  * :ref:`query.max-memory-per-node <tuning-pref-query>`
  * :ref:`query.initial-hash-partitions <tuning-pref-query>`
  * :ref:`task.concurrency <tuning-pref-task>`

Those and other Presto properties are described in :doc:`properties article<properties>`.

As an example, on a 11-node cluster dedicated to Presto (1 Coordinator + 10 Workers) with 8-core CPU and 128GB of RAM per node, you might want to start tuning with following values:

  * `query.max-memory = 200GB`
  * `query.max-memory-per-node = 32GB`
  * `query.initial-hash-partitions = 10`
  * `task.concurrency = 8`

If your workload consists of queries that have very quick processing time of the splits you might encounter situation when whole cluster is under utilized (very small usage of CPU, network and disks).
For example this can happen when your queries are highly selective and most splits can be filtered out just by looking into theirs headers (like bloom filters or min/max statistics in ORC files) without reading the actual data.
In such case you might want to increase values of the following properties:

  * :ref:`node-scheduler.max-pending-splits-per-node-per-stage <tuning-pref-node>`
  * :ref:`node-scheduler.max-splits-per-node <tuning-pref-node>`

If this guide does not suit your needs, You may look for more tuning options on
:doc:`/admin/properties` page.

JVM Settings
------------

The following can be helpful for diagnosing GC issues:

.. code-block:: none

    -XX:+PrintGCApplicationConcurrentTime
    -XX:+PrintGCApplicationStoppedTime
    -XX:+PrintGCCause
    -XX:+PrintGCDateStamps
    -XX:+PrintGCTimeStamps
    -XX:+PrintGCDetails
    -XX:+PrintReferenceGC
    -XX:+PrintClassHistogramAfterFullGC
    -XX:+PrintClassHistogramBeforeFullGC
    -XX:PrintFLSStatistics=2
    -XX:+PrintAdaptiveSizePolicy
    -XX:+PrintSafepointStatistics
    -XX:PrintSafepointStatisticsCount=1