Fix and extend existing properties description
@@ -18,8 +18,8 @@ General Properties
require redistributing both tables using a hash of the join key. This can
be slower (sometimes substantially) than broadcast joins, but allows much
larger joins. Broadcast joins require that the tables on the right side of
- the join fit in memory on each machine, whereas with distributed joins the
- tables on the right side have to fit in distributed memory. This can also be
+ the join after filtering fit in memory on each node, whereas distributed joins
+ only need to fit in distributed memory across all nodes. This can also be
specified on a per-query basis using the ``distributed_join`` session property.
@@ -34,8 +34,9 @@ Tasks managment properties
Sets the number of threads used by workers to process splits. Increasing this number
can improve throughput if worker CPU utilization is low and all the threads are in use,
- but will cause increased heap space usage. The number of active threads is available via
- the ``com.facebook.presto.execution.executor.TaskExecutor.RunningSplits`` JMX stat.
+ but will cause increased heap space usage. Setting the value too high may cause a drop
+ in performance due to a context switching. The number of active threads is available
+ via the ``com.facebook.presto.execution.executor.TaskExecutor.RunningSplits`` JMX stat.
Node Scheduler Properties
@@ -49,5 +50,7 @@ Node Scheduler Properties
* **Default value:** ``legacy``
Sets the network topology to use when scheduling splits. ``legacy`` will ignore
- the topology when scheduling splits. ``flat`` will try to schedule splits on the same
- host as the data is located by reserving 50% of the work queue for local splits.
+ the topology when scheduling splits. ``flat`` will try to schedule splits on the host
+ where the data is located by reserving 50% of the work queue for local splits.
+ It is recommended to use ``flat`` for clusters where distributed storage runs on
+ the same nodes as Presto workers.
