From this article http://blogs.msdn.com/b/ericlippert/archive/2010/04/08/precision-and-accuracy-of-datetime.aspx:
Now, the question “how much time has elapsed from start to finish?” is a completely different question than “what time is it right now?” If the question you want to ask is about how long some operation took, and you want a high-precision, high-accuracy answer, then use the StopWatch class. It really does have nanosecond precision and accuracy that is close to its precision.
The question is - what should I use if I need elapsed time from start to finish and I need 20 ms precision?
StopWatch has much better precision so I'm afraid that I will pay processor time for "extra-precision" which I don't need.
DateTime.Now has exactly precision I need but it also has a lot of extra-stuff like Month, Year etc. and I'm again afraid that this makes it much slower.
This article should help you:
http://www.dijksterhuis.org/timing-function-performance-stopwatch-class/
In general it says that the Stopwatch is the better choice, which is even my personal opinion.
The only (considerable?) overhead of the StopWatch is the incorporation of QueryPerformanceFrequency determining if the StopWatch will go on with high resolution frequency or not. Actually it is an overhead only if it will go without high resolution frequency. Otherwise is a faster option as it gets the timestamp with a WIN32 QueryPerformanceCounter call instead of DateTime.UtcNow.Ticks.
From what I read on the internet untill now StopWatch class from System.Diagnostics namespace is the best class used for timing.
If you want precision under 1 ms remember to use the Elapsed.TotalMilliseconds property to retrieve elapsed time, sw.ElapsedMilliseconds jumps in increments of 1 ms.
using System;
using System.Diagnostics;
namespace MySolution
{
class Program
{
static void Main(string[] args)
{
var sw = Stopwatch.StartNew();
// code to be timed here
sw.Stop();
Console.WriteLine("Run time in ms = {0}", sw.Elapsed.TotalMilliseconds);
}
}
}
Related
I just ran into some unexpected behavior with DateTime.UtcNow while doing some unit tests. It appears that when you call DateTime.Now/UtcNow in rapid succession, it seems to give you back the same value for a longer-than-expected interval of time, rather than capturing more precise millisecond increments.
I know there is a Stopwatch class that would be better suited for doing precise time measurements, but I was curious if someone could explain this behavior in DateTime? Is there an official precision documented for DateTime.Now (for example, precise to within 50 ms?)? Why would DateTime.Now be made less precise than what most CPU clocks could handle? Maybe it's just designed for the lowest common denominator CPU?
public static void Main(string[] args)
{
var stopwatch = new Stopwatch();
stopwatch.Start();
for (int i=0; i<1000; i++)
{
var now = DateTime.Now;
Console.WriteLine(string.Format(
"Ticks: {0}\tMilliseconds: {1}", now.Ticks, now.Millisecond));
}
stopwatch.Stop();
Console.WriteLine("Stopwatch.ElapsedMilliseconds: {0}",
stopwatch.ElapsedMilliseconds);
Console.ReadLine();
}
Why would DateTime.Now be made less precise than what most CPU clocks could handle?
A good clock should be both precise and accurate; those are different. As the old joke goes, a stopped clock is exactly accurate twice a day, a clock a minute slow is never accurate at any time. But the clock a minute slow is always precise to the nearest minute, whereas a stopped clock has no useful precision at all.
Why should the DateTime be precise to, say a microsecond when it cannot possibly be accurate to the microsecond? Most people do not have any source for official time signals that are accurate to the microsecond. Therefore giving six digits after the decimal place of precision, the last five of which are garbage would be lying.
Remember, the purpose of DateTime is to represent a date and time. High-precision timings is not at all the purpose of DateTime; as you note, that's the purpose of StopWatch. The purpose of DateTime is to represent a date and time for purposes like displaying the current time to the user, computing the number of days until next Tuesday, and so on.
In short, "what time is it?" and "how long did that take?" are completely different questions; don't use a tool designed to answer one question to answer the other.
Thanks for the question; this will make a good blog article! :-)
DateTime's precision is somewhat specific to the system it's being run on. The precision is related to the speed of a context switch, which tends to be around 15 or 16 ms. (On my system, it is actually about 14 ms from my testing, but I've seen some laptops where it's closer to 35-40 ms accuracy.)
Peter Bromberg wrote an article on high precision code timing in C#, which discusses this.
I would like a precise Datetime.Now :), so I cooked this up:
public class PreciseDatetime
{
// using DateTime.Now resulted in many many log events with the same timestamp.
// use static variables in case there are many instances of this class in use in the same program
// (that way they will all be in sync)
private static readonly Stopwatch myStopwatch = new Stopwatch();
private static System.DateTime myStopwatchStartTime;
static PreciseDatetime()
{
Reset();
try
{
// In case the system clock gets updated
SystemEvents.TimeChanged += SystemEvents_TimeChanged;
}
catch (Exception)
{
}
}
static void SystemEvents_TimeChanged(object sender, EventArgs e)
{
Reset();
}
// SystemEvents.TimeChanged can be slow to fire (3 secs), so allow forcing of reset
static public void Reset()
{
myStopwatchStartTime = System.DateTime.Now;
myStopwatch.Restart();
}
public System.DateTime Now { get { return myStopwatchStartTime.Add(myStopwatch.Elapsed); } }
}
From MSDN you'll find that DateTime.Now has an approximate resolution of 10 milliseconds on all NT operating systems.
The actual precision is hardware dependent. Better precision can be obtained using QueryPerformanceCounter.
For what it's worth, short of actually checking the .NET source, Eric Lippert provided a comment on this SO question saying that DateTime is only accurate to approx 30 ms. The reasoning for not being nanosecond accurate, in his words, is that it "doesn't need to be."
From MSDN documentation:
The resolution of this property
depends on the system timer.
They also claim that the approximate resolution on Windows NT 3.5 and later is 10 ms :)
The resolution of this property depends on the system timer, which
depends on the underlying operating system. It tends to be between 0.5
and 15 milliseconds.
As a result, repeated calls to the Now property in a short time interval, such as in a loop, may return the same value.
MSDN Link
I have an application which monitors a particular event and then starts to calculate things once it happens. Events are irregular and can come in any pattern from bunches in a sec to none for long time..
I want to measure %% of time the application is busy (similar to CPU % Usage)
I want to use Timer100Ns counter
Two questions:
Do I increment it by hardware ticks or by DateTime ticks (e.g. if I use Stopwatch - do I use sw.ElapsedTicks or sw.Elapsed.Ticks) ?
Do I need a base counter for it?
so I am about to write something like this:
Stopwatch sw = new Stopwatch();
sw.Start();
// Do some operation which is irregular by nature
sw.Stop();
// Measure utilization of the application
myCounterOfTypeTimer100Ns.IncrementBy(sw.Elapsed.Ticks);
Will it do ?
EDIT : I experimented with it a bit and now its even more confusing.. It actually shows the values I increment it by. Not %%.
The mystery unravelled. It currently appears that I don't use it in the way it was supposed to be used (or rather I didn't read TFM properly). If the sampling interval is 1s (as in perf mon live window) and you intervals are more than 1s then it shows you a nonsense number... To achieve smoothness, the activity you are trying to measure must be really fractions of 1s.. Otherwise this counter is not a good idea..
The answer for this kind of problem (although its not obvious, but still disturbing that nobody suggested it in a week) is actually SampleCounter.
im actually doing a tool, where every ms counts, and i want to debug my program to see where is it slow and where its fast enough, cause somewhere it goes slowly, the program is like 600lines, is there any way to see execution time of EVERY function in my program? i could only find this method
Stopwatch stopWatch = Stopwatch.StartNew();
Thread.Sleep(10000);
stopWatch.Stop();
// Get the elapsed time as a TimeSpan value.
TimeSpan ts = stopWatch.Elapsed;
but i saw its not actually so accurate and i need it to be as accurate as possible. thanks!
I ran the following:
var stopWatch = Stopwatch.StartNew();
Thread.Sleep(1000);
stopWatch.Stop();
Console.WriteLine(stopWatch.Elapsed);
The output was 00:00:01.0002463, please note that in this case, you might expect a second (reasonable expectation), but, it's slightly over a second. Why? Thread.Sleep() isn't particularly accurate! Stopwatch is accurate enough to demonstrate that, and is in fact, generally accurate to nanosecond resolution.
That said, you're going about this the wrong way if you're trying to use stopwatches, you should instead profile your application with something like dotTrace and see what the hot spots are.
A just need a stable count of the current program's progression in milliseconds in C#. I don't care about what timestamp it goes off of, whether it's when the program starts, midnight, or the epoch, I just need a single function that returns a stable millisecond value that does not change in an abnormal manner besides increasing by 1 each millisecond. You'd be surprised how few comprehensive and simple answers I could find by searching.
Edit: Why did you remove the C# from my title? I'd figure that's a pretty important piece of information.
When your program starts create a StopWatch and Start() it.
private StopWatch sw = new StopWatch();
public void StartMethod()
{
sw.Start();
}
At any point you can query the Stopwatch:
public void SomeMethod()
{
var a = sw.ElapsedMilliseconds;
}
If you want something accurate/precise then you need to use a StopWatch, and please read Eric Lippert's Blog (formerly the Principal Developer of the C# compiler Team) Precision and accuracy of DateTime.
Excerpt:
Now, the question “how much time has elapsed from start to finish?” is a completely different question than “what time is it right now?” If the question you want to ask is about how long some operation took, and you want a high-precision, high-accuracy answer, then use the StopWatch class. It really does have nanosecond precision and accuracy that is close to its precision.
If you don't need an accurate time, and you don't care about precision and the possibility of edge-cases that cause your milliseconds to actually be negative then use DateTime.
Do you mean DateTime.Now? It holds absolute time, and subtracting two DateTime instances gives you a TimeSpan object which has a TotalMilliseconds property.
You could store the current time in milliseconds when the program starts, then in your function get the current time again and subtract
edit:
if what your going for is a stable count of process cycles, I would use processor clocks instead of time.
as per your comment you can use DateTime.Ticks, which is 1/10,000 of a millisecond per tick
Also, if you wanted to do the time thing you can use DateTime.Now as your variable you store when you start your program, and do another DateTime.Now whenever you want the time. It has a millisecond property.
Either way DateTime is what your looking for
It sounds like you are just trying to get the current date and time, in milliseconds. If you are just trying to get the current time, in milliseconds, try this:
long milliseconds = DateTime.Now.Ticks / TimeSpan.TicksPerMillisecond;
I made a small test application in C# that sets DateTime.Now and starts a StopWatch. Every ten seconds I print _stopwatch.Elapsed.TotalMilliseconds and (DateTime.Now - _startTime).TotalMilliseconds.
While I don't expect the two to be identical, I was surprised to see them diverge linearly by about one millisecond per 20 seconds. I assume DateTime.Now calls the system clock, while the StopWatch does some kind of accumulation?
Sample output:
StopWatch : 0,2 DateTime : 1,0 Diff : 0,81
StopWatch : 10000,5 DateTime : 10002,6 Diff : 2,04
(...)
StopWatch : 2231807,5 DateTime : 2231947,7 Diff : 140,13
StopWatch : 2241809,5 DateTime : 2241950,2 Diff : 140,70
Full source: https://gist.github.com/knatten/86529563122a342de6bb
Output: https://gist.github.com/knatten/84f9be9019ee63119ee2
The answer is relatively straight forward.
Stopwatch counts processor ticks using a performance counter, a mechanism that varies between processors.
DateTime queries the system clock - the system clock is updated periodically by windows using output from the (probably quartz) crystal clock on your motherboard.
All clocks drift and these two different timing mechanisms will drift at different rates.
Under the hood, Stopwatch uses this API
The Stopwatch class assists the manipulation of timing-related
performance counters within managed code. Specifically, the Frequency
field and GetTimestamp method can be used in place of the unmanaged
Win32 APIs QueryPerformanceFrequency and QueryPerformanceCounter.
DateTime uses this API
DateTime.Now ticks every few milliseconds. Also, the rate at which it ticks is not fixed from machine to machine. On modern windows systems machines you can expect that the tick resolution will be around 100 ticks per second.
On the other hand, StopWatch queries the CPU hardware to get high precision. Actually you can get the resolution of StopWatch using Stopwatch.Frequency.
I didn't know both of the above, until I read an interesting post of Eric Lippert, about common performance benchmarks mistakes, please look here here. It's really a great post.
Stopwatch is much more precise than DateTime which would account for the discrepancy
From MSDN:
The Stopwatch measures elapsed time by counting timer ticks in the underlying timer mechanism. If the installed hardware and operating system support a high-resolution performance counter, then the Stopwatch class uses that counter to measure elapsed time. Otherwise, the Stopwatch class uses the system timer to measure elapsed time. Use the Frequency and IsHighResolution fields to determine the precision and resolution of the Stopwatch timing implementation.