Regex and Speed

In the series of these posts, I take a rather regular task, and implement it multiple times in an iteratively manner, while also improving performance.

The task is the following: given a string input with placeholders in it, and a set of placeholders to actual value mappings. Replace the placeholders in the input string with the actual text values from the map and return the final string. For example, given input string Hello [%placeholder%]! and map [%placeholder%] => World should return Hello World!. The placeholder's [%placeholder%] part is referred as the key, and World part as the value in this post.

To further clarify the task, these constraints are also true:

  • placeholder keys are delimited with [% at the beginning and %] at the end

  • placeholders are non-recursive (a placeholder's value may contain another (or the same) placeholder's key, but this should not be processed recursively)

  • input text contains no placeholder that are embedded by other placeholders (such as hello [%outer[%inner%]example%])

  • the key of placeholders contain letters and underscores

  • the same fixed set of placeholders are used on multiple input strings

  • placeholders are used at most once per input text

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GC Allocate Array

In this post I will examine the GC Allocate Array methods along with other array creation alternatives.

Previously, I have looked into array allocation with when investigating SkipInitLocal. In that post it turned out the larger the array was, the larger the gain was to use stack allocation with [SkipInitLocal] attribute.

However during exercise I noticed that the current implementation of ArrayPool has been using GC.AllocateUninitializedArray<T> to create the arrays for its internal pool.

Creating Arrays

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What is New here? - aka 4 ways or creating structs

In this post I will investigate 4 different approaches for creating structs and I will measure their performance characteristics.

The release of C# 10 removed a previous restriction on struct constructors: structs are allowed have default constructors. This means the code below compiles without a build error.

public struct Person
    public int Age { get; set; } = 1;

    public string Name { get; set; } = "Foo";

In this case the default constructor will set the property values as 1 and Foo respectively. This behavior is really similar to classes. The 4 ways this post explores creating structs:

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SkipInitLocal or ArrayPool

Many data processing algorithms require a temporary data storage for certain operations. I have been recently working with one such algorithm in the sound processing domain. In my case a fixed sized temporary byte array has been required for a method execution. In a performance sensitive code, one would prefer to achieve a non-allocating solution for this problem.

In the particular case it was a tight loop for processing live sound data. Allocating a new array for every iteration of the loop is excessive. However, I could also not use a pre-allocated array for the purpose, due to the structure of the code. There are still a few options to acquire a temporary array:

ArrayPool type provides a shared pool of arrays, from which a developer might rent an array, and return it once it is no longer needed. This array exists on the heap as a regular array, and the ArrayPool makes sure to provide an array when renting which is at least the size asked for. Because the array is returned to the pool after usage, the next iteration of the algorithm may re-use the same array, avoiding any excess allocation.

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The wrong answer for a performance question

At a recent event I have challenged a design and implementation from performance point of view. My concern was using the C# decimal type over double when there was no need for it from number- precision, calculation, or representation side. I was curious if the developers have made their "due diligence" and compared the performance impact of choosing one type over another.

To summarize the answer to my questions in short, no measurements were made by the developers. However, to address my curiosity, I was told that the operation in question "only takes a few seconds" and it was also demonstrated by clicking a button that triggers the operation and a few seconds later the results were displayed. All good, business stakeholders were happy, and the event continued. It was not a forum to get into a deeper discussion about this answer. I have seen similar responses in the past, but unfortunately, I cannot think of a single aspect of the response that would be answering the question.

First, a single click and a demonstration of the operation is not a performance measurement. No numbers were involved, no resource utilization was measured, no throughput and latency has been calculated. Moreover, you cannot even reason about these numbers from a single request. Second, the business has not set any performance goals. Third, the demonstration happened on a test environment with minimal test data and a single user putting load on the system. Finally, I asked for a performance comparison between two implementations, but the answer and the demonstration did not touch this aspect.

Because I do not have the environment and the code to run and measure such operation, I will have to rely on generic reasoning and assumptions to explain why I was eager to see such performance comparison.

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