5 ms·

First, a GC allows you to abstract over memory allocations (i.e. hide them). This makes maintaining code over time significantly easier, as allocation becomes a

by pron 6y ago

First, a GC allows you to abstract over memory allocations (i.e. hide them). This makes maintaining code over time significantly easier, as allocation becomes a hidden implementation detail. Second, while there have been similar claims made in the past, they always apply for a certain target throughput and latency. GCs are constantly making great strides in that regard. Next year, ZGC will have a <1ms worst-case latency for pretty serious allocation rates, and with an acceptable hit to throughput. As you can see in this post and previous ones, G1 offers great throughput with acceptable latencies.

jakewins 6y ago
This GC you speak of that hides allocations as an implementation detail sounds excellent! Every GC I've worked with - and my day job is orchstrating several thousand JVMs - simply moves the time when you need to think of allocations somewhere else. Either it moves it into PagerDuty, like G1, or it moves it into your GCP bill like Shenandoa and ZGC. The trade-off has always been latency-throughput-footprint, nothing I've seen yet has changed that. The innovation in Rust is realizing you can do all the tracing work at compile time.
- pron 6y ago
  An implementation detail doesn't mean that you don't think about it. We think about implementations all the times. It means that it's a detail, and changing a subroutine's allocation pattern does not create a ripple-effect throughout the code in its transitive consumers. > The innovation in Rust is realizing you can do all the tracing work at compile time. This is spectacularly false. For all non-trivial allocation/deallocation patterns, Rust also uses a runtime reference-counting GC, which is significantly slower than the tracing GCs you find in OpenJDK. The benefit comes from not relying on it too much, but this comes at a considerable cost of lost abstraction, which means more costly maintenance over the years. This is the same for all low-level languages; the difference Rust brings is that it (conservatively!) checks for memory access errors. Another difference is that most people who talk about Rust haven't actually written a significant application in it and had to maintain it for years. I'm not saying it's impossible -- people do this for C and C++, which make a similar tradeoff in this regard -- but it does come at a substantial cost.
  - jakewins 6y ago
    At this point I think we are arguing somewhat subjective things. I used to be a hard-core zealot for the JVM as the performance platform of the future - and did talks arguing just as you are here why HotSpot outperforms $LANGUAGE in real applications. I feel like I'm hearing myself in your argument.. I wrote a signficant portion of the Neo4j storage engine, which is in Java. Now I'm writing another database engine in Rust (sidenote: not for replacing the Neo4j engine, just because it's interesting). Arguably database engines qualify as "significant applications". I find - subjectively: - Maintaining performant code in Rust is easier. I do the same patterns as I did in Java, except it doesn't rely on easy-to-break assumptions of how HotSpot happens to work (ex: stack allocations) - Like you said elsewhere, the issue is often about fragmentation, ultimately stemming from object churn. I find that Rust makes it, culturally perhaps, easier to maintain low allocation code than Java. (sidenote: I think these two points is also why Go code generally has "better GC behavior"; Go doesn't have a better GC, it has a language that encourages less heap allocation) - The engine I'm writing in Rust is faster than anything I've written in Java and - critically - runs for days and months without notable stalls. Come to the dark side!
    - cogman10 6y ago
      I personally like both languages and think they both have a place. My 2 cents. The JVM works exceptionally well when you are dealing with long lived apps which deal with a lot of allocations and the machine running it has a substantial amount of memory. Things like webservers, for example, are near perfect fits for the JVM. Rust does really well when you need high performance, low memory, and ultra fast startup times. It won't necessarily outperform the JVM when you talk about doing a lot of heap allocations (due to heap fragmentation) and it unfortunately suffers from the same heap fragmentation if you are dealing with a long lived server that does a lot of allocations. But then, maybe that performance loss is acceptable for the ability to very quickly scale up and down servers. Now, the JVM is making great strides towards getting faster startup times and even fast performance at startup (AppCDS). However, those strides often involve trade offs with either build complexity or performance losses (such as Graal's AOT). The benefit for rust is that it is as fast as it ever will be without any special build steps or tweaks. Oh, and let's not forget diagnostics. Flight recorder for the JVM is simply AMAZING. The ability to hook up to a poorly behaving production server, start flight recording, and getting detailed information about things like "where are allocations happening" or "what are the hot methods" is simply amazing. No other platform that I know of has the level of detail you can get right out of the box with flight recorder. Certainly not without restarting the application with additional configuration. For example, you'd need a special build of rust with profiling turned on to even start to get the same level of info, doing such also significantly negatively impacts performance.
haxen 6y ago
> ZGC will have a <1ms worst-case latency In our benchmarks we never saw a GC pause of more than 2 ms on either ZGC or Shenandoah, but the end-to-end latency, the one the user cares about, is impacted by much more than a single GC pause. Sometimes there would be several pauses in a rapid sequence, or just the background GC thread would do too much work at once. Even after dedicating a core or two to the GC, you still face the issues of cache pollution and RAM throughput stealing that heap walking incurs.
- pron 6y ago
  Yep, that's because concurrent stack processing (https://openjdk.java.net/jeps/376 https://openjdk.java.net/jeps/376) hasn't been merged yet, nor has the changes to the VM's internal references. Both of those are coming within a year. Until then, there's still significant work done by the GC at safepoints.
  - haxen 6y ago
    A month ago we benchmarked against the master of the ZGC repo. There wasn't a big difference.
    - pron 6y ago
      Sorry, I misread your previous comment. The planned improvements that would make the biggest difference are making ZGC generational, and improving scalar-replacement (or doing some sort of stack allocation).
CyberDildonics 6y ago
> This makes maintaining code over time significantly easier, as allocation becomes a hidden implementation detail. You say that, but there seem to be no end to the stories of people spending enormous amount of time fighting the gc. It is not difficult to avoid heap allocations in other languages as well as freeing them deterministically.
- pron 6y ago
  First, you say it as if there aren't event bigger struggles without a GC. So the comparison is not "I'm fighting with the GC vs. I'm not" but "I'm fighting with the GC vs fighting other things, which is better?" So you hear those stories because those are the stories people who use a GC can tell. Second, as this blog post series shows, the "fight" is not what it used to be. You don't really need to control allocation any more until your rates are really high. Java's GCs have just gotten so much better in JDK 14 and beyond.
  - CyberDildonics 6y ago
    > First, you say it as if there aren't event bigger struggles without a GC. I'm extremely skeptical about that. My experience is that with modern C++ you lose very little elegance and gain a huge amount of control by giving up a garbage collector. Memory management becomes a very minor problem. The vast majority of memory allocations are avoided and those that need to be there can be done ahead of time. I have never heard anyone writing a latency sensitive program in C++ (games, trading etc.) say that their life would be easier if they were using a gc or that they wished they could do it in java. I have however seen decades of people talking about all the extreme lengths and rabbit holes they go down to deal with the java gc. From a broader perspective, pretty much any language with a gc ends up having a perpetual conversation around how the next gc will solve the problems with the current gc. You can see it in java, go, julia, and D. The only one I never hear about is LuaJIT, but maybe I just haven't seen it or maybe the expectations are lower.
    - imtringued 6y ago
      Lua without a JIT is just a glue language. The expectations are low and LuaJIT is easily smashing them because a lot of Lua code is/was written for the original interpreter.

by pron 6y ago

jakewins 6y ago

This GC you speak of that hides allocations as an implementation detail sounds excellent! Every GC I've worked with - and my day job is orchstrating several thousand JVMs - simply moves the time when you need to think of allocations somewhere else. Either it moves it into PagerDuty, like G1, or it moves it into your GCP bill like Shenandoa and ZGC. The trade-off has always been latency-throughput-footprint, nothing I've seen yet has changed that. The innovation in Rust is realizing you can do all the tracing work at compile time.

pron 6y ago
An implementation detail doesn't mean that you don't think about it. We think about implementations all the times. It means that it's a detail, and changing a subroutine's allocation pattern does not create a ripple-effect throughout the code in its transitive consumers. > The innovation in Rust is realizing you can do all the tracing work at compile time. This is spectacularly false. For all non-trivial allocation/deallocation patterns, Rust also uses a runtime reference-counting GC, which is significantly slower than the tracing GCs you find in OpenJDK. The benefit comes from not relying on it too much, but this comes at a considerable cost of lost abstraction, which means more costly maintenance over the years. This is the same for all low-level languages; the difference Rust brings is that it (conservatively!) checks for memory access errors. Another difference is that most people who talk about Rust haven't actually written a significant application in it and had to maintain it for years. I'm not saying it's impossible -- people do this for C and C++, which make a similar tradeoff in this regard -- but it does come at a substantial cost.
- jakewins 6y ago
  At this point I think we are arguing somewhat subjective things. I used to be a hard-core zealot for the JVM as the performance platform of the future - and did talks arguing just as you are here why HotSpot outperforms $LANGUAGE in real applications. I feel like I'm hearing myself in your argument.. I wrote a signficant portion of the Neo4j storage engine, which is in Java. Now I'm writing another database engine in Rust (sidenote: not for replacing the Neo4j engine, just because it's interesting). Arguably database engines qualify as "significant applications". I find - subjectively: - Maintaining performant code in Rust is easier. I do the same patterns as I did in Java, except it doesn't rely on easy-to-break assumptions of how HotSpot happens to work (ex: stack allocations) - Like you said elsewhere, the issue is often about fragmentation, ultimately stemming from object churn. I find that Rust makes it, culturally perhaps, easier to maintain low allocation code than Java. (sidenote: I think these two points is also why Go code generally has "better GC behavior"; Go doesn't have a better GC, it has a language that encourages less heap allocation) - The engine I'm writing in Rust is faster than anything I've written in Java and - critically - runs for days and months without notable stalls. Come to the dark side!
  - cogman10 6y ago
    I personally like both languages and think they both have a place. My 2 cents. The JVM works exceptionally well when you are dealing with long lived apps which deal with a lot of allocations and the machine running it has a substantial amount of memory. Things like webservers, for example, are near perfect fits for the JVM. Rust does really well when you need high performance, low memory, and ultra fast startup times. It won't necessarily outperform the JVM when you talk about doing a lot of heap allocations (due to heap fragmentation) and it unfortunately suffers from the same heap fragmentation if you are dealing with a long lived server that does a lot of allocations. But then, maybe that performance loss is acceptable for the ability to very quickly scale up and down servers. Now, the JVM is making great strides towards getting faster startup times and even fast performance at startup (AppCDS). However, those strides often involve trade offs with either build complexity or performance losses (such as Graal's AOT). The benefit for rust is that it is as fast as it ever will be without any special build steps or tweaks. Oh, and let's not forget diagnostics. Flight recorder for the JVM is simply AMAZING. The ability to hook up to a poorly behaving production server, start flight recording, and getting detailed information about things like "where are allocations happening" or "what are the hot methods" is simply amazing. No other platform that I know of has the level of detail you can get right out of the box with flight recorder. Certainly not without restarting the application with additional configuration. For example, you'd need a special build of rust with profiling turned on to even start to get the same level of info, doing such also significantly negatively impacts performance.

haxen 6y ago

> ZGC will have a <1ms worst-case latency In our benchmarks we never saw a GC pause of more than 2 ms on either ZGC or Shenandoah, but the end-to-end latency, the one the user cares about, is impacted by much more than a single GC pause. Sometimes there would be several pauses in a rapid sequence, or just the background GC thread would do too much work at once. Even after dedicating a core or two to the GC, you still face the issues of cache pollution and RAM throughput stealing that heap walking incurs.

pron 6y ago
Yep, that's because concurrent stack processing (https://openjdk.java.net/jeps/376 https://openjdk.java.net/jeps/376) hasn't been merged yet, nor has the changes to the VM's internal references. Both of those are coming within a year. Until then, there's still significant work done by the GC at safepoints.
- haxen 6y ago
  A month ago we benchmarked against the master of the ZGC repo. There wasn't a big difference.
  - pron 6y ago
    Sorry, I misread your previous comment. The planned improvements that would make the biggest difference are making ZGC generational, and improving scalar-replacement (or doing some sort of stack allocation).

CyberDildonics 6y ago

> This makes maintaining code over time significantly easier, as allocation becomes a hidden implementation detail. You say that, but there seem to be no end to the stories of people spending enormous amount of time fighting the gc. It is not difficult to avoid heap allocations in other languages as well as freeing them deterministically.

pron 6y ago
First, you say it as if there aren't event bigger struggles without a GC. So the comparison is not "I'm fighting with the GC vs. I'm not" but "I'm fighting with the GC vs fighting other things, which is better?" So you hear those stories because those are the stories people who use a GC can tell. Second, as this blog post series shows, the "fight" is not what it used to be. You don't really need to control allocation any more until your rates are really high. Java's GCs have just gotten so much better in JDK 14 and beyond.
- CyberDildonics 6y ago
  > First, you say it as if there aren't event bigger struggles without a GC. I'm extremely skeptical about that. My experience is that with modern C++ you lose very little elegance and gain a huge amount of control by giving up a garbage collector. Memory management becomes a very minor problem. The vast majority of memory allocations are avoided and those that need to be there can be done ahead of time. I have never heard anyone writing a latency sensitive program in C++ (games, trading etc.) say that their life would be easier if they were using a gc or that they wished they could do it in java. I have however seen decades of people talking about all the extreme lengths and rabbit holes they go down to deal with the java gc. From a broader perspective, pretty much any language with a gc ends up having a perpetual conversation around how the next gc will solve the problems with the current gc. You can see it in java, go, julia, and D. The only one I never hear about is LuaJIT, but maybe I just haven't seen it or maybe the expectations are lower.
  - imtringued 6y ago
    Lua without a JIT is just a glue language. The expectations are low and LuaJIT is easily smashing them because a lot of Lua code is/was written for the original interpreter.