10 ms·

Yeah, basically everything has to be that way. At the super fast speeds you start running into things like: * why won’t the devirtualizer trigger? * these ob

Yeah, basically everything has to be that way. At the super fast speeds you start running into things like: * why won’t the devirtualizer trigger? * these object headers are sure wasting a lot of cache * there’s a lot of forced pointer indirection And you just end up spending vast amounts of time and effort trying to shave off those few microseconds you’re wasting in the JVM. Once you add in all the effort trying to get around the GC, it’s bleak picture for competing with quality C++ without spending far more effort.

blackrock 6y ago
Why not just write it in C? And pre-allocate all the data structures, make them global, put them into a queue, and constantly reuse them. No more need to instantiate objects.
- chrisseaton 6y ago
  Why not just write it in assembly while you’re at it? The answer to both questions is that many people prefer writing in higher-level languages with more safety guarantees.
  - mcqueenjordan 6y ago
    This is a bit of a false equivalency -- there aren't many perf benefits in rewriting a C program in asm. The cost you pay for that rewrite is also much greater. Yes, of course $HIGH_LEVEL_LANG is preferable for many many use cases. In this context, we're discussing "high speed trading systems", for which native implementations are going to be the favorite.
    - chrisseaton 6y ago
      > for which native implementations are going to be the favorite ...but the point of the article is they aren't always the favourite.
  - opportune 6y ago
    They mention not even using most of the Java features (exceptions, built in GC) that make it safe. So I think it's a pretty fair question, since they are essentially using a very stripped down version of the language that removes most of the compelling reasons to use it, and seemingly fighting the language runtime along the way.
    - chrisseaton 6y ago
      The reason these people still use Java is that everything else can be nice high-level Java code. So they use regular Java for the build system, deployment, testing, logging, loading configuration, debugging, etc etc. There's a small core written in this strange way... but everything else is easier. And things like your profiler and debugger still work on the core as well.
- vgatherps 6y ago
  This is why C++ is used over Java for very fast stuff, or “just” fast stuff if you don’t want to bother fighting the JVM
- acqq 6y ago
  I write C++ for now almost 30 years, and always for the workloads where the cost of allocations was plainly visible, especially in the critical parts of the applications. So I have never stopped writing "non idiomatic" C++, at least from the point of view of typical language lawyers (and C++ attracted them a lot through the years). And I'm surely not the only one: there were different environments where it was recognized, through the times, first, that creating and destroying much objects was very bad, and later, with the growth of the C++ standard library, that even not everything in the C++ standard library should be treated the same, and that are better solutions than what's already available, and that the third party libraries are often bringing even more potential danger. Depending on the goal, one has to be very careful when choosing what one uses. The good side is, C++ kept its C features: if I'm deciding how I'll do something, I don't have to follow the rules of the "language lawyers." I can do my work producing what is measurably efficient. And compiler can still help me avoiding some types of errors -- others can anyway be discovered only with testing (and additional tools). At the end, knowing good what one wants is the most important aspect of the whole endeavor.
  - SomeoneFromCA 6y ago
    Yep, the same thing I do - just write in C++ as if it was good old C, with very occasional use of templates, containers and exceptions.
    - acqq 6y ago
      To those who miss the point, nobody is here denying that C++ has something to bring, it's just that what it brings isn't what those who promote some fashion would claim that is to be universally used, and, honestly, there's no actual reason to believe such claims, for they being not more true now than at the times where "making complex OOP hierarchies" was the most popular advice -- I remember these times too. Or the times when managers wanted to believe that everybody will just use Rational Rose to draw nice diagrams and the actual programming won't be needed, at all. Every time has its hypes: http://www.jot.fm/issues/issue_2003_01/column1/ http://www.jot.fm/issues/issue_2003_01/column1/ https://wiki.c2.com/?UmlCaseVultures https://wiki.c2.com/?UmlCaseVultures One size doesn't fit all. Some solutions to some problems could be and are provably better than those typically promoted or "generally known" at some point of time. If that all still doesn't mean anything to you, please read carefully and very, very slowly "The Summer of 1960", seen on HN some 9 years ago: https://news.ycombinator.com/item?id=2856567 https://news.ycombinator.com/item?id=2856567 Edit: Answering the parallel post writing "When you proclaim to ignore language lawyers, it sounds like you are knowingly breaking the rules of the C++ standard." No. The language lawyers, in my perception, religiously follow everything that enters the standard and proclaim that all that has to be used, because it's standardized. Including the standard libraries and some specific stuff there that isn't optimal for the problem I'm trying to solve. And especially that whatever is newer and more recently entered the standard is automatically better. It's understandable that they support their own existence by doing all that -- it's about becoming "more important" just by following/promoting some book or some obligatory rituals (it's an easy and time proven strategy through the centuries, and that's why I call it "religiously", of course -- and I am also not surprised that somebody who identifies themselves with being one of the "lawyers" wouldn't like this perspective -- you are free to suggest a better name). But it should also be also very obvious that it's not what's necessarily optimal for me to follow, as soon as I can decide what I'm doing. And yes, it's different in the environment where the "company policy" is sacred. There one has the company "policy lawyers", and typically every attempt of change can die if one isn't one of them.
chrisseaton 6y ago
What do you mean by ‘devirtualizer’ and why would you want that triggered? Sounds like something you wouldn’t want to trigger?
- deleted 6y ago
  [deleted]
- vgatherps 6y ago
  The devirtualizer (maybe this is the wrong JVM terminology, it’s basically what clang/GC call it) is part of the optimizer which sees that you have a virtual function call (like most in Java) where there is a unique caller, so you can replace the virtual call with a direct one and possibly inline it. In the JVM I think this can only be done speculatively (you have to double check the type), but it still matters.
  - chrisseaton 6y ago
    Ah right yes conflict of terminology. In the JVM devirtualising means making a virtual object a full object again, so the opposite of what you want to be happening. I don't think the JVM really names the optimisation you're talking about, but it does do it, through either a global assumption based on the class hierarchy, or an inline cache with a local guard.
    - gameswithgo 6y ago
      the jvm names the optimization exactly like he said
      - thu2111 6y ago
        IIRC it calls it monomorphisation.
    - deleted 6y ago
      [deleted]
    - masklinn 6y ago
arthurcolle 6y ago
Thanks for the insights - I must admit I only briefly ever even used Java so I am not an expert. Do you have any recommendations for documents or a PDF that covers the JVM memory model and scheduler in any depth that makes it appropriate, possibly with applications to hard-realtime systems? Again, thanks for the response, this was insightful and I'd love to learn more. Also, an aside - do you have any thoughts on the use of Rust in these same systems? It's a little bit more bleeding edge, but I'm curious to hear an expert's thoughts!

blackrock 6y ago

Why not just write it in C? And pre-allocate all the data structures, make them global, put them into a queue, and constantly reuse them. No more need to instantiate objects.

chrisseaton 6y ago
Why not just write it in assembly while you’re at it? The answer to both questions is that many people prefer writing in higher-level languages with more safety guarantees.
- mcqueenjordan 6y ago
  This is a bit of a false equivalency -- there aren't many perf benefits in rewriting a C program in asm. The cost you pay for that rewrite is also much greater. Yes, of course $HIGH_LEVEL_LANG is preferable for many many use cases. In this context, we're discussing "high speed trading systems", for which native implementations are going to be the favorite.
  - chrisseaton 6y ago
    > for which native implementations are going to be the favorite ...but the point of the article is they aren't always the favourite.
- opportune 6y ago
  They mention not even using most of the Java features (exceptions, built in GC) that make it safe. So I think it's a pretty fair question, since they are essentially using a very stripped down version of the language that removes most of the compelling reasons to use it, and seemingly fighting the language runtime along the way.
  - chrisseaton 6y ago
    The reason these people still use Java is that everything else can be nice high-level Java code. So they use regular Java for the build system, deployment, testing, logging, loading configuration, debugging, etc etc. There's a small core written in this strange way... but everything else is easier. And things like your profiler and debugger still work on the core as well.
vgatherps 6y ago
This is why C++ is used over Java for very fast stuff, or “just” fast stuff if you don’t want to bother fighting the JVM
acqq 6y ago
I write C++ for now almost 30 years, and always for the workloads where the cost of allocations was plainly visible, especially in the critical parts of the applications. So I have never stopped writing "non idiomatic" C++, at least from the point of view of typical language lawyers (and C++ attracted them a lot through the years). And I'm surely not the only one: there were different environments where it was recognized, through the times, first, that creating and destroying much objects was very bad, and later, with the growth of the C++ standard library, that even not everything in the C++ standard library should be treated the same, and that are better solutions than what's already available, and that the third party libraries are often bringing even more potential danger. Depending on the goal, one has to be very careful when choosing what one uses. The good side is, C++ kept its C features: if I'm deciding how I'll do something, I don't have to follow the rules of the "language lawyers." I can do my work producing what is measurably efficient. And compiler can still help me avoiding some types of errors -- others can anyway be discovered only with testing (and additional tools). At the end, knowing good what one wants is the most important aspect of the whole endeavor.
- SomeoneFromCA 6y ago
  Yep, the same thing I do - just write in C++ as if it was good old C, with very occasional use of templates, containers and exceptions.
  - acqq 6y ago
    To those who miss the point, nobody is here denying that C++ has something to bring, it's just that what it brings isn't what those who promote some fashion would claim that is to be universally used, and, honestly, there's no actual reason to believe such claims, for they being not more true now than at the times where "making complex OOP hierarchies" was the most popular advice -- I remember these times too. Or the times when managers wanted to believe that everybody will just use Rational Rose to draw nice diagrams and the actual programming won't be needed, at all. Every time has its hypes: http://www.jot.fm/issues/issue_2003_01/column1/ http://www.jot.fm/issues/issue_2003_01/column1/ https://wiki.c2.com/?UmlCaseVultures https://wiki.c2.com/?UmlCaseVultures One size doesn't fit all. Some solutions to some problems could be and are provably better than those typically promoted or "generally known" at some point of time. If that all still doesn't mean anything to you, please read carefully and very, very slowly "The Summer of 1960", seen on HN some 9 years ago: https://news.ycombinator.com/item?id=2856567 https://news.ycombinator.com/item?id=2856567 Edit: Answering the parallel post writing "When you proclaim to ignore language lawyers, it sounds like you are knowingly breaking the rules of the C++ standard." No. The language lawyers, in my perception, religiously follow everything that enters the standard and proclaim that all that has to be used, because it's standardized. Including the standard libraries and some specific stuff there that isn't optimal for the problem I'm trying to solve. And especially that whatever is newer and more recently entered the standard is automatically better. It's understandable that they support their own existence by doing all that -- it's about becoming "more important" just by following/promoting some book or some obligatory rituals (it's an easy and time proven strategy through the centuries, and that's why I call it "religiously", of course -- and I am also not surprised that somebody who identifies themselves with being one of the "lawyers" wouldn't like this perspective -- you are free to suggest a better name). But it should also be also very obvious that it's not what's necessarily optimal for me to follow, as soon as I can decide what I'm doing. And yes, it's different in the environment where the "company policy" is sacred. There one has the company "policy lawyers", and typically every attempt of change can die if one isn't one of them.

chrisseaton 6y ago

What do you mean by ‘devirtualizer’ and why would you want that triggered? Sounds like something you wouldn’t want to trigger?

deleted 6y ago
[deleted]
vgatherps 6y ago
The devirtualizer (maybe this is the wrong JVM terminology, it’s basically what clang/GC call it) is part of the optimizer which sees that you have a virtual function call (like most in Java) where there is a unique caller, so you can replace the virtual call with a direct one and possibly inline it. In the JVM I think this can only be done speculatively (you have to double check the type), but it still matters.
- chrisseaton 6y ago
  Ah right yes conflict of terminology. In the JVM devirtualising means making a virtual object a full object again, so the opposite of what you want to be happening. I don't think the JVM really names the optimisation you're talking about, but it does do it, through either a global assumption based on the class hierarchy, or an inline cache with a local guard.
  - gameswithgo 6y ago
    the jvm names the optimization exactly like he said
    - thu2111 6y ago
      IIRC it calls it monomorphisation.
  - deleted 6y ago
    [deleted]
  - masklinn 6y ago

arthurcolle 6y ago

Thanks for the insights - I must admit I only briefly ever even used Java so I am not an expert. Do you have any recommendations for documents or a PDF that covers the JVM memory model and scheduler in any depth that makes it appropriate, possibly with applications to hard-realtime systems? Again, thanks for the response, this was insightful and I'd love to learn more. Also, an aside - do you have any thoughts on the use of Rust in these same systems? It's a little bit more bleeding edge, but I'm curious to hear an expert's thoughts!