Tuning Realtime Performance

See the section on Ingesting Realtime Data before reading this section.

Pinot servers ingest rows into a consuming segment that resides in volatile memory. Therefore, pinot servers hosting consuming segments tend to be memory bound. They may also have long garbage collection cycles when the segment is completed and memory is released.

Controlling memory allocation

You can configure pinot servers to use off-heap memory for dictionary and forward indices of consuming segments by setting the value of pinot.server.instance.realtime.alloc.offheap to true. With this configuration in place, the server allocates off-heap memory by memory-mapping files. These files are never flushed to stable storage by Pinot (the Operating System may do so depending on demand for memory on the host). The files are discarded when the consuming segment is turned into a completed segment.

By default the files are created under the directory where the table’s segments are stored in local disk attached to the consuming server. You can set a specific directory for consuming segments with the configuration pinot.server.consumerDir. Given that there is no control over flushing of pages from the memory mapped for consuming segments, you may want to set the directory to point to a memory-based file system, eliminating wasteful disk I/O.

If memory-mapping is not desirable, you can set pinot.server.instance.realtime.alloc.offheap.direct to true. In this case, pinot allocates direct ByteBuffer objects for consuming segments. Using direct allocation can potentially result in address space fragmentation.

Note

We still use heap memory to store inverted indices for consuming segments.

Controlling number of rows in consuming segment

The number of rows in a consuming segment needs to be balanced. Having too many rows can result in memory pressure. On the other hand, having too few rows results in having too many small segments. Having too many segments can be detrimental to query performance, and also increase pressure on the Helix.

The recommended way to do this is to use the realtime.segment.flush.desired.size setting as described in StreamConfigs Section. You can run the administrative tool pinot-admin.sh RealtimeProvisioningHelper that will help you to come up with an optimal setting for the segment size.

Moving completed segments to different hosts

This feature is avaialble only if the consumption type is LowLevel.

The structure of the consuming segments and the completed segments are very different. The memory, CPU, I/O and GC characteristics could be very different while processing queries on these segments. Therefore it may be useful to move the completed segments onto differnt set of hosts in some use cases.

You can host completed segments on a different set of hosts using the tagOverrideConfig as described in Table Config. Pinot will automatically move them once the consuming segments are completed.

Controlling segment build vs segment download on Realtime servers

This feature is available only if the consumption type is LowLevel.

When a realtime segment completes, a winner server is chosen as a committer amongst all replicas by the controller. That committer builds the segment and uploads to the controller. The non-committer servers are asked to catchup to the winning offset. If the non-committer servers are able to catch up, they are asked to build the segment and replace the in-memory segment. If they are unable to catchup, they are asked to download the segment from the controller.

Building a segment can cause excessive garbage and may result in GC pauses on the server. Long GC pauses can affect query processing. In order to avoid this, we have a configuration that allows you to control whether

It might become desirable to force the non-committer servers to download the segment from the controller, instead of building it again. The completionConfig as described in Table Config can be used to configure this.

Fine tuning the segment commit protocol

This feature is available only if the consumption type is LowLevel.

Once a committer is asked to commit the segment, it builds a segment, and issues an HTTP POST to the controller, with the segment. The controller than commits the segment in Zookeeper and starts the next consuming segment.

It is possible to conifigure the servers to do a split commit, in which the committer performs the following steps:

  • Build the segment
  • Start a transaction with the lead controller to commit the segment (CommitStart phase)
  • Post the completed segment to any of the controllers (and the controller posts it to segment store)
  • End the transaction with the lead controller (CommentEnd phase). Optionally, this step can be done with the segment metadata.

This method of committing can be useful if the network bandwidth on the lead controller is limiting segment uploads.

In order to accomplish this, you will need to set the following configurations:
  • On the controller, set pinot.controller.enable.split.commit to true (default is false).
  • On the server, set pinot.server.enable.split.commit to true (default is false).
  • On the server, set pinot.server.enable.commitend.metadata to true (default is false).