A Look at the Internals of 'Tiered JIT Compilation' in .NET Core

http://www.mattwarren.org/2017/12/15/How-does-.NET-JIT-a-method-and-Tiered-Compilation/

by matthewwarren • 9 years ago

84 52 9 years ago

polskibus2• 9 years ago

I heard so many times that JVM is much better than CLR, especially in the JIT area (Hotspot?).

If it's true, do the new .NET Core internals, allow .NET Core to beat JVM? If not, are there any additional steps planned to beat JVM at last?

.NET ecosystem is much smaller than Java's (for instance all the Apache projects) and Linux + .NET Core happened too late to make up for it in my opinion. Hopefully, at least performance can be the point that can turn other people attention back towards C#/.NET.

matthewwarren5• 9 years ago

If it's true, do the new .NET Core internals, allow .NET Core to beat JVM?

I'm hoping that 'Tiered Compilation' will help, but it's early days. We will have to wait until it is 'officially' in .NET Core before it can be properly tested

chrisseaton6• 9 years ago

I heard so many times that JVM is much better than CLR, especially in the JIT area (Hotspot?).

I wouldn't disagree with this (I work at Oracle on VM research), but I've been told by other people that Microsoft or at least the .NET just generally prefer a simpler approach, without profiling, dynamic compilation, deoptimisation, and other complexity the approach used by the JVM brings. It's much more what-you-see-is-what-you-get from their JIT, rather than the mind-binding things the JVM manages to do with some code, and maybe that makes life easier for their use cases.

I'm not sure they want to add the same kind of complexity that the JVM has, even if it did bring them some better performance. But as I say that's second hand conversations.

_ivvf• 9 years ago

I definitely agree with this stance, and would say that languages like C# and golang have a weaker runtime but try to make up for it in other ways, often surpassing Java in certain areas. For instance, both languages have stack-allocated types to help stress the GC less. C# also has a much better native interop story than Java, as well as the unsafe keyword.

At the end of the day, if you want amazing performance in Java, you have to write C-like code in Java, and that often involves things like managing your own offheap memory, sticking with arrays of numbers, etc. Java's biggest areas for improvement in the performance arena are going to come allowing for stack-allocated types, fully-continugous arrays, less overhead when calling native code, and providing official APIs for the other things you can currently do with sun.misc.Unsafe.

matthewwarren5• 9 years ago

but I've been told by other people that Microsoft or at least the .NET just generally prefer a simpler approach, without profiling, dynamic compilation, deoptimisation, and other complexity the approach used by the JVM brings. It's much more what-you-see-is-what-you-get from their JIT, rather than the mind-binding things the JVM manages to do with some code, and maybe that makes life easier for their use cases.

Interesting, it's clear after a bit of research that this is the case (i.e. .NET JIT simpler that Hotspot), but it had never occurred to me that it was a conscious decision

I'd always thought that the .NET was just playing catch-up and/or didn't need to do all the things that Hotspot does. (for instance .NET has value types or 'stucts' so some optimisations are not needed because the programmer can just use them instead of classes, if they need the perf boost)

mehrdadn4• 9 years ago

> I heard so many times that JVM is much better than CLR, especially in the JIT area (Hotspot?).
I wouldn't disagree with this

I would wholeheartedly disagree with this though. There's not even any way to write, say, a generic class (even one as simple as ArrayList<T>) and then let the user instantiate it with a primitive type like <int> in Java. To use generics in Java is to pay the speed penalty of an indirection for every single primitive type on top of the extra per-object bytes of space overhead. There's just no way Java is going to fix these problems without massive breaking changes in the type system or an extremely in-depth & expensive form of static analysis unlike anything I've seen in any real-world compiler.

kittiepryde• 9 years ago

Are there standard benchmarks used to measure and compare the two approaches (jvm vs .net)?

benaadams3• 9 years ago

The JVM jit _has_ to do a lot more for performance.

Lack of value types means greater allocation by default - so it needs escape analysis to avoid it.

Type erasure for generic types means speculative optimisation and deoptimisation steps for types.

etc

Doesn't mean the .NET Jit can't improve in these areas (e.g. now its doing devirtualization for .NET Core 2.1, and escape analysis would be great) - but it does start in a better place.

bunderbunder2• 9 years ago

I think that the "lack of value types" is probably an overrated difference these days.

Empirically, I've found that value types generally only offer a tangible perf benefit when they're the size of a pointer or smaller. There aren't a whole lot of spots where that comes into play. Primitives, sure, but Java also has those. Beyond that, maybe you'd see some difference in code that does a lot of date calculations, maybe. I've done some slightly obscene and shameful optimizations around packing data into 64-bit structures in the past, but those are pretty special cases.

forgotpassagan6• 9 years ago

The JVM has simply has a lot more resources poured into it. C# and the CLR is a more modern clone of Java and it shines in the great usability of C# and to some extent the more sane op-codes of the CLR. Transforming between the two is actually easy enough that several projects offer 100% interop between C# and pure Java libraries.

I expect performance of the JVM to beat C# for a long time, but since the designs are so similar the difference isn't important in most cases.

Having spent time in both ecosystems, I'll admit that C# is easily a superior language to Java, honestly the best designed language I've ever used. Not surprising since it has the same designer as Delphi and Typescript. It's all syntactic sugar, but so much so that programming C# is significantly faster and easier.

What really hurts the .NET ecosystem right now is an insanely large gap in open source support. Something MS is fostering now but spent more than a decade trying to kill. This will not be easy to fix, as it's more a people problem than technical. MS actively tried to hurt the open source community for over a decade and to get support they will need to change the mind of countless people. It remains to be seen if this is viable.

If I was Microsoft I would remedy this by adding first-class support for Java libraries in C# by throwing their support behind existing projects to do so. This might close a gap that could otherwise take decades to fill

pjmlp6• 9 years ago

If I was Microsoft I would remedy this by adding first-class support for Java libraries in C# by throwing their support behind existing projects to do so. This might close a gap that could otherwise take decades to fill

Apparently you never heard of J#.

https://en.wikipedia.org/wiki/J_Sharp

yazr2• 9 years ago

that several projects offer 100% interop between C# and pure Java libraries.

What are the leading/mature libs for this ?

tatersolid2• 9 years ago

My experience having managed a medium scale SaaS app’s slow transition from Java to C#: 1) the CLR has very few “nerd knobs” compared with the JVM. It mostly auto-tunes itself and just works 2) despite this “lack of features” we’ve never experienced long pauses, GC storms, or our other JVM pains despite the same high user load and cloud instances 3) performance as measured by median and 99.9% response times is better in the C# bits on the same hardware, but there are also fewer DB round trips due to eliminating Hibernate in favor of Dapper and EF 4) tuning the JVM for high load is still poorly documented whack-a-mole of switches and profiling. G1 didn’t help the “auto-tuning” aspects of the JVM much in our experience, we still had STW pauses of 10+ seconds under load occasionally 5) this comparison is likely totally unfair as the C# code is clearly “better-written” having been actually designed and drags in far fewer bloated dependencies than the Java code did

polskibus2• 9 years ago

Can you say whether you moved to .net Core or .net framework?

A Look at the Internals of 'Tiered JIT Compilation' in .NET Core

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