Tuesday, 10 March 2015

Noda Time v1.3.1 released

It’s been a while since the last Noda Time release, and while we’re still working towards 2.0, we’ve been collecting a few bug fixes that can’t really wait. So last Friday, we released Noda Time 1.3.1.

Noda Time 1.3.1 updates the built-in version of TZDB from 2014e to 2015a, and fixes a few minor bugs, two of which were triggered by recent data changes.

Since it’s been a while since the previous release, it may be worth pointing out that new Noda Time releases are not the only way to get new time zone data: applications can choose to load an external version of the time zone database rather than use the embedded version, and so use up-to-date time zone data with any version of the Noda Time assemblies.

If you’re in a hurry, you can get Noda Time 1.3.1 from the NuGet repository (core, testing, JSON support packages), or from the links on the Noda Time home page. The rest of this post talks about the changes in 1.3.1 in a bit more detail.

End of year transitions (Bangladesh)

In the middle of 2009, Bangladesh started observing permanent daylight saving time, as an energy-saving measure. This was abandoned at the end of that year, and the country went back to permanent standard time.

Until recently, that transition back to standard time was actually recorded as happening a minute too early, at 23:59 on December 31st. TZDB 2014g fixed this by changing the transition time to “24:00” — that is, midnight at the end of the last day of the year.

Noda Time could already handle transitions at the end of the day, but would incorrectly ignore this particular transition because it occurred ‘after’ 2009. That’s now fixed, and Noda Time 1.3.1 returns the correct offset for Asia/Dhaka when using data from TZDB 2014g or later.

BCL provider: historical changes to the base offset (Russia)

In October 2014, most of Russia switched from permanent daylight saving time to permanent standard time, effectively moving local time back one hour. These changes were included in TZDB 2014f.

For people using the BCL provider instead of the TZDB provider (and using Windows), Microsoft delivered a hotfix in September 2014. However, our BCL provider depends upon the .NET framework’s TimeZoneInfo class, and the .NET framework — unlike TZDB — is unable to represent historical changes to the ‘base’ offset of a time zone (as happened here).

The result is that Noda Time (and other applications using TimeZoneInfo in .NET 4.5.3 and earlier) incorrectly compute the offset for dates before October 26th, 2014.

A future update of the .NET framework should correct this limitation, but without a corresponding change in Noda Time, the extra information wouldn’t be used; Noda Time 1.3.1 prepares for this change, and will use the correct offset for historical dates when TimeZoneInfo does.

BCL provider: time zone equality

The time zones returned by the BCL provider have long had a limitation in the way time zone equality was implemented: a BCL time zone was considered equal to itself, and unequal to a time zone returned by a different provider, but attempting to compare two different BCL time zone instances for equality always threw a NotImplementedException. This was particularly annoying for ZonedDateTime, as its equality is defined in terms of the contained DateTimeZone.

This was documented, but we always considered it a bug, as it wasn’t possible to predict whether testing for equality would throw an exception. Noda Time 1.3.1 fixes this by implementing equality in terms of the underlying TimeZoneInfo: BCL time zones are considered equal if they wrap the same underlying TimeZoneInfo instance.

Note that innate time zone equality is not really well defined in general, and is something we’re planning to reconsider for Noda Time 2.0. Rather than rely on DateTimeZone.Equals(), we’d recommend that applications that want to compare time zones for equality use ZoneEqualityComparer to specify how two time zones should be compared.

And finally…

There are a handful of other smaller fixes in 1.3.1: the NodaTime assembly correctly declares a dependency on System.Xml, so you won’t have to; the NuGet packages now work with ASP.NET’s kpm tool, and declare support for Xamarin’s Xamarin.iOS (for building iOS applications using C#) in addition to Xamarin.Android, which was already listed; and we’ve fixed a few reported documentation issues along the way.

As usual, see the User Guide and 1.3.1 release notes for more information about all of the above.

Work is still continuing on 2.0 along the lines described in our 1.3.0 release post, and we’re also planning a 1.4 release to act as a bridge between 1.x and 2.0. This will deprecate members that we plan to remove in 2.0 and introduce the replacements where feasible.

Wednesday, 16 July 2014

Micro-optimization: the surprising inefficiency of readonly fields

Introduction

Recently I've been optimizing the heck out of Noda Time. Most of the time this has been a case of the normal measurement, find bottlenecks, carefully analyse them, lather, rinse, repeat. Yesterday I had a hunch about a particular cost, and decided to experiment... leading to a surprising optimization.

Noda Time's core types are mostly value types - date/time values are naturally value types, just as DateTime and DateTimeOffset are in the BCL. Noda Time's types are a bit bigger than most value types, however - the largest being ZonedDateTime, weighing in at 40 bytes in an x64 CLR at the moment. (I can shrink it down to 32 bytes with a bit of messing around, although it's not terribly pleasant to do so.) The main reason for the bulk is that we have two reference types involved (the time zone and the calendar system), and in Noda Time 2.0 we're going to have nanosecond resolution instead of tick resolution (so we need 12 bytes just to store a point in time). While this goes against the Class Library Design Guidelines, it would be odd for the smaller types (LocalDate, LocalTime) to be value types and the larger ones to be reference types. Overall, these still feel like value types.

A lot of these value types are logically composed of each other:

  • A LocalDate is a YearMonthDay and a CalendarSystem reference
  • A LocalDateTime is a LocalDate and a LocalTime
  • An OffsetDateTime is a LocalDateTime and an Offset
  • A ZonedDateTime is an OffsetDateTime and a DateTimeZone reference

This leads to a lot of delegation, potentially - asking a ZonedDateTime for its Year could mean asking the OffsetDateTime, which would ask the LocalDateTime, which would ask the LocalDate, which would ask the YearMonthDay. Very nice from a code reuse point of view, but potentially inefficient due to copying data.

Why would there be data copying involved? Well, that's where this blog post comes in.

Behaviour of value type member invocations

When an instance member (method or property) belonging to a value type is invoked, the exact behaviour depends on the kind of expression it is called on. From the C# 5 spec, section 7.5.5 (where E is the expression the member M is invoked on, and the type declaring M is a value type):

If E is not classified as a variable, then a temporary local variable of E’s type is created and the value of E is assigned to that variable. E is then reclassified as a reference to that temporary local variable. The temporary variable is accessible as this within M, but not in any other way. Thus, only when E is a true variable is it possible for the caller to observe the changes that M makes to this.

So when is a variable not a variable? When it's readonly... from section 7.6.4 (emphasis mine) :

If T is a struct-type and I identifies an instance field of that class-type:

  • If E is a value, or if the field is readonly and the reference occurs outside an instance constructor of the struct in which the field is declared, then the result is a value, namely the value of the field I in the struct instance given by E.

(There's a very similar bullet for T being a class-type; the important part is that the field type is a value type

The upshot is that if you have a method call of:

int result = someField.Foo();

then it's effectively converted into this:

var tmp = someField;
int result = tmp.Foo();

Now if the type of the field is quite a large value type, but Foo() doesn't modify the value (which it never does within my value types), that's performing a copy completely unnecessarily.

To see this in action outside Noda Time, I've built a little sample app.

Show me the code!

Our example is a simple 256-bit type, composed of 4 Int64 values. The type itself doesn't do anything useful - it just holds the four values, and exposes them via properties. We then measure how long it takes to sum the four properties lots of times.

using System;
using System.Diagnostics;

public struct Int256
{
    private readonly long bits0;
    private readonly long bits1;
    private readonly long bits2;
    private readonly long bits3;
    
    public Int256(long bits0, long bits1, long bits2, long bits3)
    {
        this.bits0 = bits0;
        this.bits1 = bits1;
        this.bits2 = bits2;
        this.bits3 = bits3;
    }
    
    public long Bits0 { get { return bits0; } }
    public long Bits1 { get { return bits1; } }
    public long Bits2 { get { return bits2; } }
    public long Bits3 { get { return bits3; } }
}

class Test
{
    private readonly Int256 value;

    public Test()
    {
        value = new Int256(1L, 5L, 10L, 100L);
    }
    
    public long TotalValue 
    { 
        get 
        {
            return value.Bits0 + value.Bits1 + value.Bits2 + value.Bits3; 
        }
    }
    
    public void RunTest()
    {
        // Just make sure it's JITted...
        var sample = TotalValue;
        Stopwatch sw = Stopwatch.StartNew();
        long total = 0;
        for (int i = 0; i < 1000000000; i++)
        {
            total += TotalValue;
        }
        sw.Stop();
        Console.WriteLine("Total time: {0}ms", sw.ElapsedMilliseconds);
    }
    
    static void Main()
    {
        new Test().RunTest();
    }
}

Building this from the command line with /o+ /debug- and running (in a 64-bit CLR, but no RyuJIT) this takes about 20 seconds to run on my laptop. We can make it much faster with just one small change:

class Test
{
    private Int256 value;

    // Code as before
}

The same test now takes about 4 seconds - a 5-fold speed improvement, just by making a field non-readonly. If we look at the IL for the TotalValue property, the copying becomes obvious. Here it is when the field is readonly:

.method public hidebysig specialname instance int64 
        get_TotalValue() cil managed
{
  // Code size       60 (0x3c)
  .maxstack  2
  .locals init (valuetype Int256 V_0,
           valuetype Int256 V_1,
           valuetype Int256 V_2,
           valuetype Int256 V_3)
  IL_0000:  ldarg.0
  IL_0001:  ldfld      valuetype Int256 Test::'value'
  IL_0006:  stloc.0
  IL_0007:  ldloca.s   V_0
  IL_0009:  call       instance int64 Int256::get_Bits0()
  IL_000e:  ldarg.0
  IL_000f:  ldfld      valuetype Int256 Test::'value'
  IL_0014:  stloc.1
  IL_0015:  ldloca.s   V_1
  IL_0017:  call       instance int64 Int256::get_Bits1()
  IL_001c:  add
  IL_001d:  ldarg.0
  IL_001e:  ldfld      valuetype Int256 Test::'value'
  IL_0023:  stloc.2
  IL_0024:  ldloca.s   V_2
  IL_0026:  call       instance int64 Int256::get_Bits2()
  IL_002b:  add
  IL_002c:  ldarg.0
  IL_002d:  ldfld      valuetype Int256 Test::'value'
  IL_0032:  stloc.3
  IL_0033:  ldloca.s   V_3
  IL_0035:  call       instance int64 Int256::get_Bits3()
  IL_003a:  add
  IL_003b:  ret
} // end of method Test::get_TotalValue

And here it is when the field's not readonly:

.method public hidebysig specialname instance int64 
        get_TotalValue() cil managed
{
  // Code size       48 (0x30)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  ldflda     valuetype Int256 Test::'value'
  IL_0006:  call       instance int64 Int256::get_Bits0()
  IL_000b:  ldarg.0
  IL_000c:  ldflda     valuetype Int256 Test::'value'
  IL_0011:  call       instance int64 Int256::get_Bits1()
  IL_0016:  add
  IL_0017:  ldarg.0
  IL_0018:  ldflda     valuetype Int256 Test::'value'
  IL_001d:  call       instance int64 Int256::get_Bits2()
  IL_0022:  add
  IL_0023:  ldarg.0
  IL_0024:  ldflda     valuetype Int256 Test::'value'
  IL_0029:  call       instance int64 Int256::get_Bits3()
  IL_002e:  add
  IL_002f:  ret
} // end of method Test::get_TotalValue

Note that it's still loading the field address (ldflda) four times. You might expect that copying the field onto the stack once via a temporary variable would be faster, but that ends up at about 6.5 seconds on my machine.

There is an optimization which is even faster - moving the totalling property into Int256. That way (with the non-readonly field, still) the total time is less than a second - twenty times faster than the original code!

Conclusion

This isn't an optimization I'd recommend in general. Most code really doesn't need to be micro-optimized this hard, and most code doesn't deal with large value types like the ones in Noda Time. However, I regard Noda Time as a sort of "system level" library, and I don't ever want someone to decide not to use it on  performance grounds. My benchmarks show that for potentially-frequently-called operations (such as the properties on ZonedDateTime) it really does make a difference, so I'm going to go for it.

I intend to apply a custom attribute to each of these "would normally be readonly" fields to document the intended behaviour of the field - and then when Roslyn is fully released, I'll probably write a test to validate that all of these fields would still compile if the field were made readonly (e.g. that they're never assigned to outside the constructor).

Aside from anything else, I find the subtle difference in behaviour between a readonly field and a read/write field fascinating... it's something I'd been vaguely aware of in the past, but this is the first time that it's had a practical impact on me. Maybe it'll never make any difference to your code... but it's probably worth being aware of anyway.

Friday, 27 June 2014

Noda Time v1.3.0 released

Noda Time 1.3.0 came out today, bringing a healthy mix of new features and bug fixes for all your date and time handling needs. Unlike with previous releases, the improvements in Noda Time 1.3 don’t really have a single theme: they add a handful of features and tidy up some loose ends on the road to 2.0 (on which more below).

So in no particular order…
  • Noda Time 1.3 adds support for the Persian (Solar Hijri) calendar, and experimental support for the Hebrew calender. Support for the latter is “experimental” because we are not entirely convinced that calculations around leap years work as people would expect, and because there is currently no support for parsing and formatting month names. See the calendars page in the user guide for more details.
  • Speaking of parsing and formatting, both should be significantly faster in 1.3.0. Parse failures should also be much easier to diagnose, as errors now indicate which part of the input failed to match the relevant part of the pattern.
  • The desktop build of Noda Time should now be usable from partially-trusted contexts (such as ASP.NET shared hosting), as it is now marked with the AllowPartiallyTrustedCallers attribute.
  • Finally, we also fixed a small number of minor bugs, added annotations for ReSharper users, and added a few more convenience methods — ZonedDateTime.IsDaylightSavingTime() and OffsetDateTime.WithOffset(), for example — in response to user requests. There’s also a new option to make the JSON serializer use a string representation for Interval.
Again, see the User Guide and 1.3.0 release notes for more information about all of the above.

You can get Noda Time 1.3.0 from the NuGet repository as usual (core, testing, JSON support packages), or from the links on the Noda Time home page.

Onward to 2.0

Meanwhile, development has started on Noda Time 2.0. Noda Time 2.0 will not be binary-compatible with Noda Time 1.x, but it will be mostly source-compatible: we don’t plan to make completely gratuitous changes.

Among other things, Noda Time 2.0 is likely to contain:
  • Significant changes to internal representations, with consequences for overall performance (some good, some — hopefully for less-important cases — less good). To take one example: we expect to change the granularity of Instant and Duration from ticks to nanoseconds.
  • A better definition of the range of values that are supported for various types and calendars, and a defined behaviour for when those ranges are exceeded. In a similar vein, we plan to revisit how ordering and equality are implemented (mostly for edge cases).
  • A unified API for changing dates and times similar to the Java 8 “adjuster” concept. (This may replace some methods that are currently on concrete types.)
  • Removal of everything marked as obsolete in 1.x.
We don’t expect to have a release of Noda Time 2.0 until next year, so we may well make some additional releases in the 1.3.x series between now and then, but in general we’ll be focussing on 2.0. If you’re interested in helping out, come and talk to us on the mailing list.

Tuesday, 3 June 2014

Hebrew calendar cheat sheet

This post is largely for my own benefit, but I figured it might be interesting to others too, in terms of what you need to think about when coding against the Hebrew calendar. Currently I'm trying to work out what it means to add a year to a date in the Hebrew calendar, at which point it's useful to have some reference tables.

Month names

There are two month numbering systems, which Noda Time calls Civil and Scriptural. In leap years, the number/name mapping in the civil numbering system is offset for the second half of the year, due to Adar being split into Adar I and Adar II.

Number Scriptural Civil (non-leap) Civil (leap)
1 Nisan Tishri Tishri
2 Iyar Heshvan Heshvan
3 Sivan Kislev Kislev
4 Tamuz Tevet Tevet
5 Av Shevat Shevat
6 Elul Adar Adar I
7 Tishri Nisan Adar II
8 Heshvan Iyar Nisan
9 Kislev Sivan Iyar
10 Tevet Tamuz Sivan
11 Shevat Av Tamuz
12 Adar / Adar I Elul Av
13 Adar II   Elul

(Heshvan is sometimes lengthened to Marcheshvan; it's also called Cheshvan. Heshvan is the version in CLDR, which is why I've used it here. Other month names have similar variations, e.g. Tishri vs Tishrei. These are only English versions of Hebrew names, of course.)

Sample years

In unit tests it's useful to have some sample data for specific situations. Here's the data for a complete leap cycle of 19 years. (This period maps to years 1639 to 1659 in the Gregorian calendar.)

Year Leap? Days in Cheshvan Days in Kislev
5400 No 29 30
5401 No 29 29
5402 Yes 30 30
5403 No 29 30
5404 Yes 29 29
5405 No 30 30
5406 No 29 30
5407 Yes 30 30
5408 No 29 29
5409 No 30 30
5410 Yes 29 30
5411 No 30 30
5412 No 29 29
5413 Yes 29 30
5414 No 30 30
5415 Yes 30 30
5416 No 29 29
5417 No 29 30
5418 Yes 30 30
5419 No 30 30

Tuesday, 26 November 2013

Noda Time v1.2.0 released

Somewhat tardily, I'm happy to announce the release of Noda Time 1.2.0, which we released last Monday.

While the changes in Noda Time 1.1 were around making a Portable Class Library version and filling in the gaps from the first release, Noda Time 1.2 is all about serialization and text formatting.

On the serialization side, Noda Time now supports XML and binary serialization natively, and comes with an optional assembly to handle JSON serialization (using Json.NET). On the text formatting side, Noda Time 1.2 now properly supports formatting and parsing of the Duration, OffsetDateTime, and ZonedDateTime types.

We also fixed a few bugs, and added a some more convenience methods — Interval.Contains() and ZonedDateTime.Calendar, among others — in response to requests we received from people using the library.

Finally, it apparently wouldn’t be a proper Noda Time major release without fixing another spelling mistake in our API: we replaced Period.Millseconds in 1.1, but managed not to spot that we’d also misspelled Era.AnnoMartyrm, the era used in the Coptic calendar. That’s fixed in 1.2, and I think (hope) that we’re done now.

There’s more information about all of the above in the comprehensive serialization section of the user guide, the pattern documentation for the Duration, OffsetDateTime, and ZonedDateTime types, and the 1.2.0 release notes.

You can pick up Noda Time 1.2.0 from the NuGet repository as usual (core, testing, JSON support packages), or from the links on the Noda Time home page, which also hosts the User Guide and API reference.

Saturday, 6 April 2013

Noda Time v1.1.0 released

I'm pleased to announce the release of version 1.1.0 of Noda Time. The primary new feature is a Portable Class Library version (in the same package) which allows you to use Noda Time when writing applications for Windows Store, Windows Phone 7 and Windows Phone 8. There are additional features around the time zone data available from TZDB, including location information and fuller Windows time zone ID mappings... and a few other bits and bobs, as you might expect.

Enjoy!

Wednesday, 7 November 2012

Noda Time v1.0 released

Go get Noda Time 1.0!

Today is the end of the longest release cycle I've been personally involved in. On November 5th 2009, I announced my intention to write a port of Joda Time for .NET. The next day, Noda Time was born - with a lofty (foolhardy) set of targets.

Near the end of a talk *about* Noda Time this evening, I released Noda Time 1.0.0.

It's taken three years, but I'm immensely proud of what we've managed to achieve. We're far from "done" but I believe we're already significantly ahead of most other date/time APIs I've seen in terms of providing a clean API which reduces *incidental* complexity while highlighting the *inherent* complexity of the domain. (This is a theme I'm becoming dogmatic about on various fronts.)

There's more to do - I can't see myself considering Noda Time to be "done" any time soon - but hopefully now we've got a stable release, we can start to build user momentum.

One point I raised at the DotNetDevNet presentation tonight was that there's a definite benefit (in my very biased view) in just *looking into* Noda Time:

  • If you can't use it in your production code, use it when prototyping
  • If you can't use it in your prototype code, play with it in personal projects
  • If you can't use it in personal projects, read the user guide to understand the concepts

I hope that simply looking at the various types that Noda Time providers will give you more insight into how you should be thinking about date and time handling in your code. While the BCL API has a lot of flaws, you can work around most of them if you make it crystal clear what your data means at every step. The type system will leave that largely ambiguous, but there's nothing to stop you from naming your variables descriptively, and adding appropriate comments.

Of course, I would far prefer it if you'd start using Noda Time and raising issues on how to make it better. Spread the word.

Oh, and if anyone from the BCL team is reading this and would like to include something like Noda Time into .NET 5 as a "next generation" date/time, I'd be *really* interested in talking to you :)