Parallel LSD Radix Sort

I’ve taken several attempts at parallelizing the LSD Radix Sort algorithm. This blog provides the key concepts and details about the latest attempt, which has succeeded and is beginning to pay off dividends of higher performance.

Performance Summary

The following table shows performance of three variations of the LSD Radix Sort on two different multi-core machines, in Millions of UInt32’s per second, implemented in C#

Algorithm Random Presorted Constant Computer
LSD Radix Sort 104 45 90 1-core i7-9750H
LSD Radix Sort (Partially Parallel) 118 48 112 6-core i7-9750H
LSD Radix Sort (Parallel) 195 203 225 6-core i7-9750H
LSD Radix Sort (Parallel) 316 360 370 48-core AWS

C++ implementation of the serial and fully parallel algorithms perform about 40% faster, on 6-core and 48-core processors. Currently, Parallel Merge Sort reaches higher performance on the 48-core processor (see, for now.

Partially Parallel Algorithm

My HPCsharp open source C# Nuget package, available on, contains a partially and fully parallel implementations of the LSD Radix Sort algorithm. The partially parallel version splits the algorithm into two phases:

  • counting phase
  • permutation phase

where only the counting phase uses multiple cores to gain 25% higher performance. This idea is described in my blog The permutation phase runs on a single core, and is the reason the overall algorithm is only partially parallel.

The partially parallel algorithm performs well, running at 120 Million Int32/sec on a 6-core laptop. An equivalent C++ implementation, available in a git repo runs at nearly 150 Million Int32/sec.

Fully Parallel Algorithm Requires Parallel Permutation

To parallelize more of the LSD Radix Sort algorithm, the permutation phase needs to be parallelized. To accomplish this, we split the input array into several sub-arrays. Each worker (e.g. CPU core) permutes elements of a sub-array it is given. All available workers do this in parallel.

To be able to permute each sub-array correctly, additional information needs to be provided along with the sub-array input of array elements. For each of the 256 bins, we need to know where to place elements of each sub-array as we permute them. In other words, the start index within each bin is needed, for each sub-array. This is simple to determine, during the counting phase of the LSD Radix Sort algorithm. As we count elements of the input array, instead of counting the entire input array, we count elements of each sub-array.

From the counts of each sub-array, we can create the additional information that we need for each sub-array – where to place the elements of that sub-array within each bin. In other words, instead of the start index for each bin, we determine the start index for each sub-array within each bin. This requires a 2-D array, where the upper dimension specifies the sub-array index, and the lower dimension holds the starting indexes for each of the 256 bins. This way each sub-array knows where its elements need to go within each bin, when it permutes them.

De-randomization of Writes

The permutation phase of the algorithm is the most time consuming portion of the LSD Radix Sort. During this phase, the input array is read sequentially, which is efficient. However, the writes are to one of the 256 bins, potentially in random order. This order is dependent on the input data itself. These possibly random writes, are more than 10X slower than sequential writes. This performance drop was shown in my blog on memory accesses Truly random writes were shown to be 350X slower than sequential writes.

Random write accesses put significant stress on modern system memory sub-system, reducing the overall available bandwidth, reducing the ability for many cores to write in parallel. This reduction in bandwidth, limits scaling of the algorithm on multi-core processors. For example, 27 GBytes/sec of write memory bandwidth is reduced to 2.7 GBytes/sec.

To improve write performance we can de-randomize writes, by adding buffering for each of the 256 bins. Each bin would have a buffer of some number of elements (e.g. 64). We would write to each of these buffers instead of writing directly to system memory. Once a particular buffer has filled up, it is written sequentially to system memory. This type of a buffering scheme helps parallel performance scale significantly.

One nice benefit of using de-randomization thru buffering, is that performance of the LSD Radix Sort algorithm gets more consistent for different input distributions. For example, arrays that are filled with random data perform very similarly to arrays filled with already presorted data, or constant value data. This performance improvement holds for parallel and serial algorithms. It drops performance for serial algorithm slightly for random input data, but brings up performance for presorted, or incrementing input data distribution.

The following table is for C++ implementations, not C#. C# implementations perform similarly, just a bit slower:

LSD Radix Sort Mega unsigned / sec Distribution Computer
Two-Phase Parallel 153 Random 6-core laptop
Two-Phase Parallel 140 Pre-Sorted 6-core laptop
Two-Phase Parallel 121 Constant 6-core laptop
Two-Phase Parallel De-rand 129 Random 6-core laptop
Two-Phase Parallel De-rand 105 Pre-Sorted 6-core laptop
Two-Phase Parallel De-rand 118 Constant 6-core laptop

Two Phase Strategy Is Not Always Possible

Separating the LSD Radix Sort algorithm into the counting phase and the permutation phase, is a really good optimization to reduce the number of passes over the input array. It is described in my blog ( This idea works great when the entire input array will be processed. However, this idea does not work when processing sub-arrays.

When permuting the entire array at a time, while processing each digit, from the least significant digit to the most, the counts are not affected by permuting array elements. However, they do change, when permuting sub-arrays, because the input array elements will not stay within the bounds of a sub-array being permuted. As we permute a sub-array, its elements can move anywhere within the full array – i.e. they don’t stay within the bounds of that particular sub-array. Thus, we will need to perform counting after processing the input array for each digit. In other words, for fully parallel LSD Radix Sort, two phase strategy is not correct, and we need to use the traditional strategy of counting followed by permutation, for each digit.

Our overall processing becomes:

  • outer loop is over the number of digits, starting with the least significant digit and moving toward the most significant digit
  • within the loop, perform counting for that digit of each sub-array, across the entire input array
  • compute starting index for each sub-array, for each bin
  • process all sub-arrays in parallel

Parallelism Summary

The resulting Parallel LSD Radix Sort has been parallelized more extensively. The two biggest computing parts of the algorithm have been parallelized. The counting phase for each digit has been parallelized. The permutation phase has also been parallelized during processing of each digit. Digits are still processed serially in a loop, from the least significant digit to the most significant digit.

Consistency of Performance

Performance of many sorting algorithms varies substantially across input data distributions. For example, quicksort and Introspective Sort perform about 6X slower for an input array filled with random data, versus a nearly pre-sorted array. They also perform about 3X slower for an array filled with constant data.

Typical LSD Radix Sort algorithm performs better for random data, slowing down by over 2X for pre-sorted data, and slowing down by 20% for constant data. The above implementations of this algorithm were shown to perform much more consistently across several input data distributions.

The parallel LSD Radix Sort performs more consistently across several input data distributions, with slightly faster performance for pre-sorted and constant input data distributions. De-randomization buffering of the write data is the reason for the nearly uniform performance across input distributions.

Where to Find

The fully parallel LSD Radix Sort algorithm has been implemented in C# inside HPCsharp nuget package on, starting with version 3.15.0. The source code is open and is available in the git repo inside RadixSortLsdParallel.cs. C++ version will also available.

Further Details:

Transform Histogram into Starting Indexes

Once we know the counts for each bin, for each sub-array, we can determine starting indexes for each bin, for each sub-array. For bin 0, the starting index of sub-array 0 is zero. The starting index for sub-array 1, would be the starting index of sub-array 0 plus the count for bin 0 for sub-array 0. This pattern continues for starting index of sub-array 2 and so on, where its starting index is the starting index of the previous sub-array starting index plus the count for the same bin of previous sub-array.

At this point all within each bin, all of the sub-array’s starting indexes have been adjusted to account for the counts within each bin, but not to account for the bins before them – the starting index of sub-array 0 is zero for each bin – which is wrong. We need to compute the size of the overall bin 0 and adjust bin 1 starting indexes by adding that amount. To compute the size of bin 0, we can take the starting index of the last sub-array of bin 0 and add the count for bin 0 for the last sub-array. Then all of bin 1 starting indexes need to be adjusted by adding this value to each of them.

The above descriptions could use some pictures and diagrams to make these explanations clearer 😉

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