FastSort

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Short Description

FastSort sorts input records using a sort key. FastSort is faster than ExtSort but requires more system resources and sorting is not stable.

Component	Same input metadata	Sorted inputs	Inputs	Outputs	Java	CTL	Auto-propagated metadata
FastSort	-		1	1-N	-	-

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Ports

Port type	Number	Description	Metadata
Input	0	for input data records	the same input and output metadata
Output	0	for sorted data records
Output	1-N	for sorted data records

Metadata

Metadata can be propagated through this component. All output metadata must be the same.

FastSort Attributes

Attribute	Req	Description	Possible values
Basic
Sort key		List of fields (separated by semicolon) the data records are to be sorted by, including the sorting order for each data field separately, see Sort Key
In memory only		If `true`, internal sorting is forced and all attributes except Sort key and Run size are ignored.	false (default) \| true
Advanced
Run size (records)		Number of records sorted at once in memory; the size of one read buffer. Largely affects speed and memory requirements, see Run Size. Multiplies with the Number of read buffers (which depends on the Number of sorting threads). Reasonable Run sizes vary from 5,000 to 200,000 based on the record size and the total number of records.	10,000 (default) \| 1000 - N
Max open files		Limits the number of temp files that can be created during the sorting. Too low number (500 or less) significantly reduces the performance, see Max Open Files. 0 denotes that the number of temp files is unlimited.	1000 (default) \| 1-N \| 0 (unlimited)
Number of sorting threads		Number of worker threads to do the job. Setting this value too high may even slow the graph run down, see Number of Sorting Threads. Also affects memory requirements.	1 (default) \| 1-N
Number of read buffers		How many chunks of data (each the size of Run size) will be held in memory at a time, see Number of Read Buffers. Defaults to Number of sorting threads + 2.	auto (default) \| 1-N
Tape buffer size (bytes)		Buffer used by a worker for filling the output. Affects the performance slightly, see Tape Buffer Size.	8192 (default) \| 1-N
Compress temporary files		If `true`, temporary files are compressed. For more information see Compress Temporary Files.	false (default) \| true
Deprecated
Estimated record count	^[1]	Estimated number of input records to be sorted.	auto (default) \| 1-N
Average record size (bytes)	^[2]	Guess on average byte size of records.	auto (default) \| 1-N
Maximum memory (MB, GB)	^[2]	Rough estimate of maximum memory that can be used.	auto (default) \| 1-N
Sorting locale		Locale used for correct sorting order	none (default) \| any locale
Case sensitive		By default (Sorting locale is `none`), upper-case characters are sorted separately and precede lower-case characters that are sorted separately too. If Sorting locale is set, upper- and lower-case characters are sorted together - if Case sensitive is `true`, a lower-case precedes corresponding upper-case while `false` preservers the order, data strings appears in the input in. Case sensitive attribute value is taken into account only if Locale is set.	false (default) \| true
^[1] Estimated record count is a helper attribute which is used for calculating (rather unnatural) Run size automatically as approximately Estimated record count to the power 0.66. If Run size set explicitly, Estimated record count is ignored. ^[2] These attributes affect automatic guess of Run size. Generally, the following formula must be true: Number of read buffers * Run size * Average record size < Maximum memory

Details

FastSort is a high performance sort component reaching the optimal efficiency when enough system resources are available. FastSort can be up to 2.5 times faster than ExtSort but consumes significantly more memory and temporary disk space.

The component takes input records and sorts them using a sorting key - a single field or a set of fields. You can specify sorting order for each field in the key separately. The sorted output is sent to all connected ports.

Pretty good results can be obtained with the default settings (just the sorting key needs to be specified). However, to achieve the best performance, a number of parameters is available for tweaking.

	Warning
	FastSort does not preserve order of records with equal key value (sorting algorithm is not stable). If stability is required, please use ExtSort instead.

Sorting Null Values

In ascending order, null values are sorted before strings, numbers, booleans, or dates. If you sort data in descending order, null values are sorted after strings, numbers, booleans, or dates.

Remember that FastSort processes the records in which the same fields of the Sort key attribute have null values as if these nulls were equal.

FastSort Tweaking

Basically, you do not need to set any of these attributes, however, sometimes you can increase performance by setting them. You may have a limited memory or you need to sort a large number of records, or these records are too big. In similar cases, you can adjust FastSort to your needs.

	Tip
	The memory requirements of the component can be estimated as follows: heap memory = Number of read buffers × Run size × estimated record size direct memory = Max open files × Tape buffer size

Run Size
The core attribute for FastSort; determines how many records form a "run" (i.e., a bunch of sorted records in temp files). The lower Run size, the more temp files get created, less memory is used and greater speed is achieved. On the other hand, higher values might cause memory issues. There is no rule of thumb as to whether Run size should be high or low to get the best performance. Generally, the more records you are about to sort the bigger Run size you might want. The rough formula for Run size is Estimated record count^0.66. Note that memory consumption multiplies with the Number of read buffers, which in turn grows with Number of sorting threads. So, higher Run sizes result in much higher memory footprints.
Max Open Files
FastSort uses relatively large numbers of temporary files during its operation. By default, the number of temporary files is limited to 1,000. For production systems, it is recommended to set the limit as high as possible because there is no speed sacrifice, see Performance Bottlenecks. On the other hand, you can lower the limit even further to prevent hitting the user quota or other OS-specific limits and runtime limitations. The following table should give you a better idea:
Dataset size Number of temp. files Default Run size Note
1,000,000 ~100 ~10,000
10,000,000 ~250 ~45,000
1,000,000,000 20,000 to 2,000 50,000 to 500,000 Depends on available memory
Note that numbers in the table above are not exact and might be different on your system.
Number of Sorting Threads
Tells FastSort how many runs (chunks) should be sorted at a time in parallel. By default, it is automatically set to 1 or 2 based on the number of CPU cores in your system. Overriding this value makes sense if your system has lots of CPU cores and you think your disk performance can handle working with so many parallel data streams.
Number of Read Buffers
This setting corresponds tightly to the Number of sorting threads - must be equal to or greater than the Number of sorting threads. The more read buffers the lower chance the workers will block each other. Defaults to Number of sorting threads + 2
Compress Temporary Files
Along with Temporary files charset this option lets you reduce the space required for temporary files. Compression can save a lot of space but decreases the performance by up to 30% so be careful with this setting.
Tape Buffer Size
Size (in bytes) of a file output buffer. The default value is 8kB. Decreasing this value might avoid memory exhaustion for large numbers of runs (e.g. when Run size is very small compared to the total number of records). However, the impact of this setting is quite small.

Dataset size	Number of temp. files	Default Run size	Note
1,000,000	~100	~10,000
10,000,000	~250	~45,000
1,000,000,000	20,000 to 2,000	50,000 to 500,000	Depends on available memory

Best Practices

Be sure you have dedicated enough memory to your Java Virtual Machine (JVM). Having plenty of memory available, FastSort is capable of doing astonishing job. Remember that the default JVM heap space 64MB can cause FastSort to crash. Don't hesitate to increase the memory value up to 2 GB (but still leaving some memory for the operating system). It is well worth it. How to set the JVM is described in Chapter 14, Runtime Configuration.

Performance Bottlenecks

Sorting big records (long string fields, tens or hundreds of fields, etc.): FastSort is greedy for both memory and CPU cores. If the system does not have enough of either, FastSort can easily crash with out-of-memory. In this case, use the ExtSort component instead.
Utilizing more than 2 CPU cores: Unless you you are able to use really fast disk drives, overriding the default value of Number of sorting threads to more than 2 threads does not necessarily help. It can even slow the process back down a bit as extra memory is loaded for each additional thread.
Coping with quotas and other runtime limitations: In complex graphs with several parallel sorts, even with other graph components also having huge number of open files, Too many open files error and graph execution failure may occur. There are two possible solutions to this issue:
1. increase the limit (quota)
  This option is recommended for production systems since there is no speed sacrifice. Typically, setting limit to higher number on Unix systems.
2. force FastSort to keep the number of temporary files below some limit
  For regular users on large servers increasing the quota is not an option. Thus, Max open files must be set to a reasonable value. FastSort then performs intermediate merges of temporary files to keep their number below the limit. However, setting Max open files to values, for which such merges are inevitable, often produces significant performance drop. So keep it at the highest possible value. If you are forced to limit FastSort to less than a hundred temporary files, even for large datasets, consider using ExtSort instead which is designed for performance with limited number of tapes.

Compatibility

FastSort is available since CloverETL Designer version 2.2.0.