Suite Partitioning

Suites can be partitioned, that is split into multiple parts, for instance to run each partition in parallel. By running the parts in parallel the total execution time can be reduced.
FitNesse is not thread-safe so each partition's run should be executed by a separate Java process. But this can be a very effective way to speed up the time needed for testing when tests do not interfere which each other. In CI/CD servers one can spread test execution across multiple workers and get very quick tests results, even when test sizes grow.

There are multiple ways to select certain parts of a suite. Sub Wiki Suites and Tags And Filters both allow suites to be split, but these are static. And if the goal is only to improve a test suite's execution time, it could be considered improper use of these mechanisms as they are intended to define functional aspects of tests.
A purpose-build mechanism to partition a suite is also available. When running a suite one can provide a partitionCount and partitionIndex parameter (zero-based) to indicate in how many parts the suite should be split, and which part to run now.
An example usage of this mechanism is a call to http://<host>:<port>/<suite path>?suite&partitionCount=2&partitionIndex=0. This would partition the suite in two, executing the first.

So one can start multiple runs for the same suite, all using the same partition count, but a different partition index.
By starting 'partition count' runs all tests are run, without the need to manually define and maintain exactly which tests should go in which partition, even when tests are added and removed from the suite.

Partition Preview

A specific partition responder is available which will list the partitions (and test order in them) that will be created for a specific suite and partition count. It will list all test pages in HTML (or tab-separated text) format indicating in which partition the test will be placed and what its order in the partition will be.
So one could for instance do http://<host>:<port>/<suite path>?partition&partitionCount=5 to so see how that suite would be divided in 5 partitions.

Tab separated format can be requested by supplying the format parameter with value tsv, e.g. http://<host>:<port>/<suite path>?partition&format=tsv&partitionCount=5.

Partition Index File

One can also supply a file containing a partitioning in the tab-separated format created by the partition preview responder when running a suite. This file can list the desired partitioning for all known test pages. Any other pages found in the suite will be spread evenly across all partitions.
The filename for such a file must be supplied to the suite responder using the partitionIndexFile parameter. The file will be looked up inside the wiki's files section.

So for example a call to http://<host>:<port>/<suite path>?suite&partitionCount=2&partitionIndex=1&partitionIndexFile=partitions.tsv. This would partition the tests in the suite in two groups using the division described in a file called partitions.tsv in the files section, executing the second group.
The file's content could look something like this (where whitespace between columns should be tab characters):
Page	Partition	Test System	Order
FitNesse.SuiteAcceptanceTests.SuiteAuthenticationTests.AlwaysSecureOperation	0	slim	0
FitNesse.SuiteAcceptanceTests.SuiteAuthenticationTests.SecureReadOperations	1	slim	0
FitNesse.SuiteAcceptanceTests.SuiteAuthenticationTests.SecureTestOperations	1	slim	1

Partitioning Algorithm

The standard partitioning algorithm tries to make partitions with the same number of tests. It will, for instance, for a partition count of 4:

Plugins can tailor this behaviour by registering a custom 'test run factory'. For instance when historic run times of tests are known a plugin could use this information to create partitions with an equal expected run time.

Usage via FitNesseRunner

When running tests from jUnit using the FitNesseRunner one can use this feature by applying the @Partition, and possibly the @PartitionFile, annotation to the test class.