Posts about Internals

Stopwatch under the hood

Update: You can find an updated and significantly improved version of this post in my book "Pro .NET Benchmarking".

In the previous post, we discussed DateTime. This structure can be used in situations when you don't need a good level of precision. If you want to do high-precision time measurements, you need a better tool because DateTime has a small resolution and a big latency. Also, time is tricky, you can create wonderful bugs if you don't understand how it works (see Falsehoods programmers believe about time and More falsehoods programmers believe about time).

In this post, we will briefly talk about the Stopwatch class:

  • Which kind of hardware timers could be a base for Stopwatch
  • High precision timestamp API on Windows and Linux
  • Latency and Resolution of Stopwatch in different environments
  • Common pitfalls: which kind of problems could we get trying to measure small time intervals

If you are not a .NET developer, you can also find a lot of useful information in this post: mainly we will discuss low-level details of high-resolution timestamping (probably your favorite language also uses the same API). As usual, you can also find useful links for further reading.

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DateTime under the hood

Update: You can find an updated and significantly improved version of this post in my book "Pro .NET Benchmarking".

DateTime is a widely used .NET type. A lot of developers use it all the time, but not all of them really know how it works. In this post, I discuss DateTime.UtcNow: how it's implemented, what the latency and the resolution of DateTime on Windows and Linux, how the resolution can be changed, and how it can affect your application. This post is an overview, so you probably will not see super detailed explanations of some topics, but you will find a lot of useful links for further reading.

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Blittable types

Challenge of the day: what will the following code display?

public struct UInt128
    public ulong Value1;
    public ulong Value2;
public struct MyStruct
    public UInt128 UInt128;
    public char Char;
class Program
    public static unsafe void Main()
        var myStruct = new MyStruct();
        var baseAddress = (int)&myStruct;
        var uInt128Adress = (int)&myStruct.UInt128;
        Console.WriteLine(uInt128Adress - baseAddress);
        Console.WriteLine(Marshal.OffsetOf(typeof(MyStruct), "UInt128"));

A hint: two zeros or two another same values are wrong answers in the general case. The following table shows the console output on different runtimes:

uInt128Adress - baseAddress 480
Marshal.OffsetOf(typeof(MyStruct), "UInt128")000

If you want to know why it happens, you probably should learn some useful information about blittable types.

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