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Comment by anarcticpuffin | Hacker News Reader

This is very similar to Java's CMS collector[1]. Unfortunately, 10ms is still far too long for some applications. I have hopes for something like Azul's pauseless GC[2] eventually becoming a common GC strategy, but I'm not holding my breath. In the meantime, there's always sun.misc.Unsafe[3] in the JVM world :-(

[1] - https://docs.oracle.com/javase/8/docs/technotes/guides/vm/gc... [2] - http://www.azulsystems.com/zing/pgc [3] - http://mishadoff.com/blog/java-magic-part-4-sun-dot-misc-dot...

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Go already gives better tools than Java for managing native memory through cgo, which is a far less painful interface than JNI (believe me, I've written a lot of JNI for Hadoop). Go also has value types which is a huge win for managing memory. (And even if Java gets value types later, the whole Java standard library will still take years to change to use them, if it ever does.)

Plus, sun.misc.Unsafe is probably going away, according to Oracle. http://www.infoq.com/news/2015/07/oracle-plan-remove-unsafe

From what I've heard, Azul has a great GC, but the throughput is extremely low. It's really only a practical solution for high frequency finance and places like that where latency is everything, and throughput is nothing (can buy another 100 servers or high end hardware.) Note: I'm talking about their software product which runs on vanilla hardware, not their hardware product, which I understand is far superior.

A lot of people on HN also seem to be taking the statement that maximum GC latency will be 10ms as a statement that there will often be 10ms pauses. Hopefully, the average latency will be far less, in the 1 or 2ms, and 10ms will be something that only happens on huge heaps in certain conditions. This should be similar to what has happened on Android, where GC pauses are pretty rare and typically only 1 or 2ms.

> the whole Java standard library will still take years to change to use them, if it ever does.

All Java collections (which are what really matters) are being retrofitted alongside with the introduction of value types. As soon as value types are introduced (and the HotSpot team is working hard on that right now), all collections will be fully value-ready.

> Go already gives better tools than Java for managing native memory through cgo, which is a far less painful interface than JNI

JNI is being replaced by Project Panama: http://openjdk.java.net/projects/panama/ (you can already use a similar FFI already with JNR, which serves as a blueprint for Panama's FFI: https://github.com/jnr I've used JNR to write a FUSE filesystem in Java without a line of C, and unlike JNA, it's fast!)

Besides, HotSpot runs C now, too, and quite well:

http://www.chrisseaton.com/rubytruffle/cext/

https://dl.dropboxusercontent.com/u/292832/useR_multilang_de...

> Plus, sun.misc.Unsafe is probably going away, according to Oracle.

... only to be replaced by something much better: https://www.youtube.com/watch?v=ycKn18LtNtk (Unsafe isn't going away until replacements are available).

> From what I've heard, Azul has a great GC, but the throughput is extremely low.

Not at all. Just a little lower than with HotSpot's throughput collector, and possibly higher than with G1 (although G1 changes a lot, so that might not be true).

It feels weird to say this, but Oracle's stewardship of the jvm is making me really hopeful as an occasional ml developer.

Consider the wish list: I want a garbage collected language where, for a handful of large/important data structures, I can sidestep gc and carefully control memory layouts for cache friendliness. I'd also like direct interop with blas and my aforementioned data structures.

It looks like I may get all of this!

And yes, I've done a bunch of work with misc.unsafe but it's nowhere near as nice as it could be. What the jvm really buys you is not having to build once for each platform; I distributed code that relied on c++11 features on 3 platforms while there was mixed compiler support and it was a bloody nightmare.

> I can sidestep gc and carefully control memory layouts for cache friendliness

Memory layout and GC are two completely orthogonal issues. You will be able to control memory layout quite well with Valhalla (value types) and even on a finer-grained level with Panama if you need C interoperability. VarHandles (hopefully in Java 9) will give you safe access to off-heap memory. Currently you can do that with Unsafe, which is more work but still less than C++.

> What the jvm really buys you is not having to build once for each platform

Oh, I'd say it buys you a lot more: seamless polyglotism, exceptional performance even for dynamic stuff (dynamic languages, esp. w/ Graal, but even cool bytecode manipulation in Java or even simple code loading/swapping), and you get all that performance with unprecedented observability into the running platform.

I think I disagree about the observability -- vtune is a lot easier to use when just tuning straight C++ rather than java

Are you familiar with JMH's perfasm? http://psy-lob-saw.blogspot.com/2015/07/jmh-perfasm.html

And for profiling apps on production, I've yet to encounter a more thorough, low-overhead profiler than Java Flight Recorder.

no but i'll check it out this afternoon. Thanks!

VTune is not too dissimilar to JProfiler.

And as pron mentioned plenty of tools exist for lower level access.

Vtune gives access to PMU counters as well as attributing them to assembly. JProfiler is a purely java level profiler (it won't even tell you Hotspots in the JVM itself, nevermind assembly). They're not really comparable.

jprofiler, at least for my use cases, isn't really similar to vtune at all. I know what my hot spots are: it's the inner bits of algorithms that run a few billion to a few trillion times. What I need to do is understand, as granularly as possible, the exact instructions and how the various caches and memory are operating. Convex and tree optimizers are generally memory speed limited and my goal is to have this code run at eg 0.9+ of memory b/w speed.

Value types will provide ability to allocate storage embedded in heap object or stack, but it doesn't provide layout control (i.e. order of fields in the layout). It's a good change, but let's not exaggerate.

As the requirement was "layout control for cache friendliness" value types are all you need (or 99.99% of what you can possibly need). For interop, there's Panama. Let's not nitpick.

99.99% is perhaps your estimate, but not necessarily others. This is also not likely what people would consider "layout control" if they're coming from a language that allows field-level layout control. Being able to place frequently used together fields manually is quite useful in quite a few circumstances.

> Being able to place frequently used together fields manually is quite useful in quite a few circumstances.

I have almost never seen this make a difference outside of, say, GPU programming. The fact that Java's optimizer is much better than that of Go will make a much larger difference in execution speed.

This is mostly an issue for large objects (i.e. span multiple cache lines), but have different access patterns for various fields (i.e. clusters of fields accessed together).

The other aspect of layout control is cacheline padding, which is also not present in the JVM. There's @Contended, but it's a blunt tool and not currently a public API (it's in sun.misc).

>The fact that Java's optimizer is much better than that of Go will make a much larger difference in execution speed

Yes, but that's orthogonal.

Both grouping and padding are possible with Java's value types. The only thing you don't have full guaranteed (i.e. on multiple JVMs) control over is ordering and alignment.

Padding is possible with today's java (at least Hotspot), although it's ugly and relies on Hotspot's existing layout implementation.

Yes, you can group but as I mentioned in the other reply, you get "blob" granularity/control. This is unlikely to be an issue for a value type instance itself, but say you want to have multiple value types (each with several fields) embedded into a heap allocated object. Assume this total embedded blog spans multiple cachelines, but you have clustered access patterns. So you've grouped some data together into value types, but you have no control over how that will be laid out in the "host" object.

Again, I'm happy that Oracle will be adding value types -- they will definitely help. I contend, however, that calling it "layout control" is a slight stretch in the general sense; it's perhaps fair to call it that when comparing against today's java, but not against languages that allow explicit field-by-field layout (along with padding, alignment, etc).

> it's perhaps fair to call it that when comparing against today's java, but not against languages that allow explicit field-by-field layout (along with padding, alignment, etc).

It's fair to call it that when comparing against any language or runtime targetting general-purpose development, but not compared to languages designed for different purposes. I agree with that.

However, we're not discussing those languages here so I don't see the relevance. I'm also sure that many statements are wrong when compared to quantum computing, but I don't think there's any need to make that qualification. It's pretty clear from the context.

>It's fair to call it that when comparing against any language or runtime targetting general-purpose development

Unless you consider, e.g., C, C++, and Rust as not general-purpose. Even C# has StructLayout where you can control the struct layout. Heck, Go and C# allow you to embed a fixed size array into a struct (this facility is not in the works for Java at the moment).

> Unless you consider, e.g., C, C++, and Rust as not general-purpose.

Of course they're not! Even Stroustrup now calls C++ "a language for those who need or want to work as close to the hardware as possible". None of these languages -- unlike Java or Go -- has simplicity as a major design goal.

> Even C# has StructLayout where you can control the struct layout.

Yeah, that's precisely my point of giving you something you don't need. That C# does it doesn't mean it's actually significant for any significant number of programs.

> this facility is not in the works for Java at the moment

Paul Sandoz is working on something similar with one of the VarHandles variants (he said there's a prototype already), namely, indexed access to object fields, which would make this a language, rather than a JVM concern.

But value types do let you group fields (with sub-component values).

Also, I've noticed that whenever I say Java does X, you say, "Oh, no! It does X - ε!" Now, to me, that's nitpicking, especially considering that a perfect general-purpose language/runtime designed to be simple (for some definition of simple) should give you 90+% performance in 99% of general-purpose use cases (or 95% in 95% etc.). If it does any better then one of two possibilities is true: 1/ it's magic, or 2/ it's not a perfect simple language/runtime because it could have been made simpler (by whatever definition of simple it's chosen).

Anyone who can't settle for anything less than 100% performance or does something that's outside 95% of the use cases knows not to use such a general-purpose language/runtime, and, instead, uses a more domain-specific language/runtime or one that's not designed to be simple.

>But value types do let you group fields (with sub-component values).

They let you treat the fields of a value type as a "blob", you have no control over how they're laid out within that blob.

>Also, I've noticed that whenever I say Java does X, you say, "Oh, no! It does X - ε!" Now, to me, that's nitpicking, especially considering that a perfect general-purpose language/runtime designed to be simple (for some definition of simple) should give you 90+% performance in 99% of general-purpose use cases (or 95% in 95% etc.). If it does any better then one of two possibilities is true: 1/ it's magic, or 2/ it's not a perfect simple language/runtime because it could have been made simpler (by whatever definition of simple it's chosen).

Nothing personal, but I find your JVM related posts as borderline fanboyism (and I say this as someone that greatly respects the engineering in Hotspot, despite certain things bugging me). It's not about 100% or 95% performance; it's about not making exaggerated claims since, as you say, there's no magic.

> I find your JVM related posts as borderline fanboyism

I think they're much less fanboyish than any other language/runtime discussion on HN.

> it's about not making exaggerated claims

I am not making exaggerated claims because perfect for me (and, by definition 95% of people) is precisely how I've defined the requirements from a runtime like Java (or Go, or Python, for that matter). So, if I say "it gives you all the layout control you need", I don't mean all the layout control you need to write a DSP, or all the layout control you need to get 100% performance -- only 99% performance. I think most people make the same assumption (because they're not writing DSPs), and I find your posts nitpicky and possibly misleading for the target audience (who, by and large, don't write DSPs or medical devices or particle accelerator beam controllers -- at least certainly on threads not discussing those interesting but extreme use cases). I don't think it's reasonable to qualify every statement for some negligible (possibly nonexistent) portion of the participants, especially considering that that minority already knows the statements don't completely apply to them. Not doing so doesn't qualify as exaggeration IMO, but reasonable expressiveness, or else every discussion will be bogged down in irrelevant detail that will only distract from the main point.

So, yes, I stand behind my claim that value types give you all the control you need to get 99% performance for 99% of the use cases people would normally use Java for.

X - ε, for very large values of ε.

Which of the JNR projects is the one you used? There are three that sound like they would do the exact same thing:

https://github.com/jnr/jnr-invoke https://github.com/jnr/jnr-ffi https://github.com/jnr/jffi

jnr-ffi is the high-level API (and the one I used). jffi is a low-level ffi used by jnr-ffi. Unfortunately, jnr-ffi is still missing one piece (which can be done with the low-level FFI), namely returning creating a function pointer to a Java method dynamically and passing it to C code.

I don't know if Azul Zing throughput is "extremely low" but it does sacrifice it for latency. I had a discussion with Gil Tene on this recently where he dove into some details: https://groups.google.com/forum/m/#!topic/mechanical-sympath...

In particular, the LVB is akin to an array range check on each reference read.

> "...From what I've heard, Azul has a great GC, but the > throughput is extremely low. It's really only a practical > solution for high frequency finance and places like that > where latency is everything, and throughput is nothing..."

Well, you heard wrong.

Zing is used in plenty of throughput-intesive and throughout-centric applications, and sustainable throughput on Zing tends to be higher (not lower) than with other JVMs. E.g. Cassandra clusters tend to carry higher production loads per machine when powered by Zing (compared to OpenJDK or HotSpot on the same hardware). All while dramatically improving their latency behavior and consistency.

Specifically, on similarly sized heaps and workloads, the C4 collector's throughout is better than CMS's and close to ParallelGC's. And since it's throughput scales linearly with the amount of empty heap configured and since (unlike OpenJDK/Hotspot) Zing places no practical pause-related caps on how much memory can be applied, it tends to beat both on efficiency in actual configurations.

The notion that good latency behavior has to come at the expense of throughput is just a silly myth. There are plenty of examples that disprove it. Zing/C4 is just one of many.

I thought Zing was basically HotSpot licensed and modified to use C4 (plus a few other minor things). How comes it's faster than HotSpot overall? Are there a lot of Azul-specific compiler optimisations there too?

- Zing is based on HotSpot, and it's biggest visible change is C4, but it changes a lot more than just the collector. E.g. it addresses pretty much all the causes of JVM glitches. (You can see a discussion of the many other reasons JVMs pause here: https://www.youtube.com/watch?v=Y39kllzX1P8).

- The reason Zing tends to to carry higher throughput in production is that in most Java-based systems, production throughput levels are limited not by system capacity, but by how far you can drive the JVMs before the glitches start being unbearable, and what looks like occasional small hiccups at lower throughputs starts looking more like epileptic seizures at load. Capacity planning and sizing usually aim to keep peak production loads below the levels that lead to these "I don't want to go there" behaviors. By taking out the various glitching/pausing/stalling behaviors typically associated with JVMs under load, Zing extends the smooth operating range such that it comes much closer to the traditional "how much can this hardware handle?" capacity and sizing behavior people are used to in non-Java and non-GC'ed environments.

- When you compare raw throughput or speed (with no SLAs, e.g. "how long does this multi-hour batch job take to complete?"), with similar configurations Zing is usually comparable to OpenJDK/HotSpot [Where comparable typically means within +/-15-20% range. Sometimes faster, sometimes slower.] But once people apply simple knob twists in the applications (like turning up heap sizes, caches, and other using-memory-no-longer-hurts related settings) they often get more raw throughout per instance or machine through simple efficiency benefits (like the elimination of raw work that comes from higher in-process, on-heap cache hit rates).

Thanks, fascinating answer.

In this old thread [1] one of the Azul developers outlines a potential path to having a C4 type GC for Go. Interestingly he mentions moving to a generational collector first, before moving to concurrent. It seems the Go developers have decided to go the other way, i.e. make the collector concurrent, but not generational. One of the reasons given for going generational first is implementing a write-barrier, but from the blog post it sounds like that is implemented for this concurrent collector anyway, so maybe it doesn't really matter.

[1] - https://groups.google.com/d/msg/golang-dev/GvA0DaCI2BU/SmEel...

+1 on Azul. They've pretty much solved it by improving on past methods combining hardware and software. Go could do the same thing. I keep wondering about putting a dedicated FPGA on the memory bus that does nothing but concurrent GC. Have a mechanism to keep the processor (s) and it from stepping on each others' toes. Might work wonders.

Why would an FPGA be a better solution than software running on another core?

I'm guessing since it's sitting on the memory bus it could intercept pointer modifications and synchronously update it's graph?

Memory bus is just for speed. You don't want it doing stuff like that lol. See these two LISP machines for where my inspiration of putting it on memory bus came from:

http://diyhpl.us/~bryan/papers2/paperbot/Design%20of%20a%20L...

(See section 7 for a radical... err realy old... way to do concurrent GC. Full paper available at ACM/IEEE or if you Google LISP processors Guy Steele enough.)

Scheme machine by Burger 1995

http://citeseerx.ist.psu.edu/viewdoc/download;jsessionid=569...

(See the main graphic and specifics in storage section later. Once again, GC-like stuff is handled in memory management part of the processor. This processor knows that, though, to assist GC a bit. Also different in that it was specified and then synthesized to heterogenous hardware with DDD "correct-by-construction" toolkit.)

So, have fun with those. Plus, Google hardware-assisted or hardware garbage collection to get lots of interesting results already done.

See my reply to anarcticpuffin.

.. while keeping in mind that for some workloads GC is a large chunk of the app's work. For example, 10 Xeon cores worth of GC throughput (out of say 24) would be a pretty tall order for a FPGA, and as a fixed resource it easily becomes an Amdahl's law bottleneck.

It would be a cool thing to try still, and maybe doable with COTS hw: https://www-ssl.intel.com/content/www/us/en/embedded/technol... + http://www.hotchips.org/wp-content/uploads/hc_archives/hc21/...

It's a tall order because you set it up to be. Real system design would call for a balancing act, as usual. Remember that you can put a bunch of GC's on one FPGA that all run concurrently with access to shitloads of I/O and/or fast memory bus. Amdahl's law shouldn't kick in any more than with concurrent GC's in general. The parallelism, simplicity, and tech like in your link should make it faster than an on-board collector. The concept isn't speculation as it's already been done in two different ways:

Fine-Grained Parallel Compacting Garbage Collection through Hardware-Supported Synchronization (2010) http://www.ikr.uni-stuttgart.de/Content/Publications/Archive...

Stall-free, real-time collector for FPGA's (2012) http://researcher.watson.ibm.com/researcher/files/us-bacon/B...

The question is, "Can modern CPU's and off-chip FPGA's keep in sync without performance getting dragged down?" The FPGA's have gotten faster. The CPU's I/O have gotten faster. So, I'm sure it can be done but it might be difficult enough to be someone's Master thesis. ;)

Besides, I call for replacing current chips with open ones easy to modify for acceleration and security. Gaisler LEON4 SPARC, Rocket RISC-V, Cambrige's BERI/CHERI MIPS64... these all come to mind. Plan was to put them onto a high-end FPGA w/ concurrent GC's to test the scheme. Once it worked, ASIC conversion time baby. S-ASIC's are $200-500k on average with resulting production & packaging being way cheaper after that. Just hoping there's a few companies that would split the cost to eliminate most memory and control flow issues forever. ;)

Azul have a similar concept with their Java co-processors. I think they originally went the co-processor route because up until recently (i.e. last few years) the CPU instructions they take advantage of didn't exist on commodity chips.

http://www.azulsystems.com/products/vega/overview

I'm aware of it. My main recommendation, partly to eliminate x86 risks, was to switch to better, multi-core RISC processors while adding onboard garbage collection to them. One of best was a Scheme machine that had it in the hardware memory subsystem transparent to the CPU or application. I wanted to see about building something like that with Java compatibility and accidentally found the awesome Vega systems in process.

The advantage of implementing it in raw hardware are many: easy to build into a state machines with high clock-rate (even HLS should work); won't waste CPU time or cache; can be setup to analyse segments of memory in parallel because it's hardware; latency with right algorithm can be lower. That combined with multicore RISC and some concurrency extensions I know of would give plenty performance while knocking out tons of errors. Can't exactly do that with Intel/AMD chips unless we buy their semi-custom designs for untold millions, now can we?

So, next idea (for Intel/AMD) customers is to put it on the memory bus with some synchronization mechanism between CPU task and FPGA. This will use far less CPU time than a CPU that's doing GC stuff non-stop. That's on top of above benefits. Plus, if a workload doesn't need low-latency GC, it has a whole FPGA to use for acceleration. :)

I enjoy comments such as this because I enjoy putting a thought, an idea in my head and rolling it around, seeing where it goes and what problems it has. This reply may be a bit rambling as a result.

If the FPGA on the bus is doing concurrent GC, we'd need a way to mark pointers and integers reliably. It means breaking all C code that does pointer manipulation, and breaking custom tagged pointer implementations. Niche languages and applications wouldn't have the buy in, and justifiably so, to set global policy on memory. On a developer machine you could experiment, but on a consumer device you would need to use whatever the OS and the bulk of applications have decided on.

Maybe that's not a bad thing though? The OS would need to provide implementations of malloc and free, and other primitives, that are tied to the hardware. But I suppose that's not unreasonable.

Except when it comes to virtualization. The FPGA isn't fixed function, but on the timescales that modern VMMs provide slices of CPU time to virtual machines, reprogramming an FPGA is an eternity. So we would gain in potential performance but lose in flexibility, substantially.

The biggest problem would be legacy C code. Code that treats "void*" and "long" as interchangeable values. And that includes a lot of hand-written JITs, where coercing values between pointers and word-sized integers is prevalent.

Rambling aside on potential upsides and downsides: The world would probably be a better place if pointers and integers were separated and strongly typed down to the hardware level. It would be a pain, but if the null pointer is a billion dollar mistake, placing all bets on the Princeton architecture may yet be a trillion dollar mistake this century.

This isn't working with just any C or OS code. I'd assume that. It would take a modified or clean-slate runtime whose language can work with it. It might not tolerate virtualization, either. As I was heading for bed, I decided to at least Google to see if anyone did anything with FPGA's and GC's to not leave your head with nothing to think on tonight. ;) Here's what I found:

FPGA-aware garbage collection in Java (2005) https://buytaert.net/files/fpl05-paper.pdf

(Modified Jikes VM to use FPGA coprocessor for collection. Result was good performance with around 2.32% overhead on memory-intensive benchmarks.)

Fine-Grained Parallel Compacting Garbage Collection through Hardware-Supported Synchronization (2010) http://www.ikr.uni-stuttgart.de/Content/Publications/Archive...

Stall-free, real-time collector for FPGA's (2012) http://researcher.watson.ibm.com/researcher/files/us-bacon/B...

(This is on one chip with tiny resources for GC around 1% and 4-17% overhead.)

Maybe I can get something better for you after some rest. Note that my comment to the other person has more details of where I was going with this.

https://news.ycombinator.com/item?id=10150480

Two factors require any GC support to be behind the CPU's MMU:

1) GC itself operates on virtual addresses.

2) If you want concurrent collection you're probably going to need a read barrier, and that will require some GC / MMU interaction.

The Azul Vega had a lot of interesting features to support GC (and other Java constructs), but the most important by far is the HW read barrier.

The virtual addressing part might be fine now that the GPUs are doing it, and hypervisors support programmable passthrough using the x86 IOMMU (VT-d etc) features.

Though I'm convinced custom hardware is doomed here for the usual custom hardware reasons. Maybe GPUs have gotten good enough at pointer chasing to be usable here?

> Maybe GPUs have gotten good enough at pointer chasing to be usable here?

They haven't.

Custom hardware is actually flourishing in datacenters. My preferred architecture is Cavium Octeon III style of many-core RISC, accelerators for everything, plenty I/O, and hypervisor support. Selling like hotcakes. Adapteva's stuff outperforms CPU's & GPU's at performance-per-watt-per-area with sales to HPC people. There's similarly at least a few custom hardware companies in each segment doing something that's hard or not cost-effective with existing hardware or software.

I agree that the risk is high, though, to the point that one shouldn't depend on it. So, I'd advice selling system w/ services that's profitable which just happens to use such custom hardware. A high-performance, easy-to-manage, easy-to-integrate... already worth buying... platform that also has hardware-supported GC and/or memory safety. The sales of the system & licensing of the software subsidize hardware costs, which are structured to be cheap anyway. Start with FPGA's, then S-ASIC's, then advanced S-ASIC's or finally ASIC's. The NRE stays as low as volume can support.

Relevant example of this model (and evidence for my GC idea) is Azul Systems Vega machines. Those are custom hardware for Java supporting native bytecodes, a bunch of RAM, a pauseless GC, and easy enterprise integration. So, while we're all speculating, they're selling custom hardware w/ pauseless GC's. I'm just trying to work out a different, cheaper design hopefully integrating with Intel/AMD.

http://www.azulsystems.com/products/vega/overview

Note that they support a whole range of hardware, software, and services to diversify income. Any one thing shouldn't sink them, esp unfavorable hardware. That's the model to copy.

1. I expect, if we use standard OS, that a modification might be needed where the FPGA will know the real address. The FPGA might be given enough info to figure it out with DMA or it might be something like a custom, syscall that gives FPGA details.

2. Barriers are one method. They're among the most expensive. An IBM prototype used careful lock management (supported by lock registers) and scheduling to avoid barriers. There's actually quite a few ways in the literature to avoid barriers in concurrency. I figure a team would have to leverage them plus asynchronous I/O from CPU to FPGA for optimal performance. I see it as a series of hand-offs to FPGA which, once it has necessary information, acts on those hand-offs with CPU assuming it completed after certain time, seeing a part of memory saying so, or receiving an interrupt.

That's a rough sketch.

There are plans for adding one to hotspot[1]. As I understand it it can't use the same tricks as C4 because that would require kernel support, but unlike CMS it will be compacting, thus avoiding the terrible failure modes of CMS.

There also is some work being done to make the CMS failures a little less terrible by parallelizing them.[2]

[1] http://openjdk.java.net/jeps/189 [2] https://bugs.openjdk.java.net/browse/JDK-8086706

As HotSpot's GC team is mostly working on G1 and grooming it as a CMS replacement (it has replaced the parallel GC as the default GC in current JDK9 builds), I don't think there's much further work on CMS.

The CMS improvements I've mentioned to are being contributed by a 3rd party. Based on the mailing list posts[1] I think it was google.

[1] http://mail.openjdk.java.net/pipermail/hotspot-gc-dev/2015-J...

That's really cool! I hope it gets merged...

I'm really surprised this is so new. I'm only beginning to enter the field and falling asleep the other night one of my thoughts was 'why doesn't the GC happen piecemeal?' AKA 'who said we have to collect everything, just unload 2 bins worth'. Mainly WRT use in Android, where it causes perceivable hitching

My experience is a lot of people/groups get obsessed with one metric in this case average speed. A pauseless GC is always going to be a bit slower than one that pauses once in a blue moon (meaning all the time from the users standpoint). Requires a really strong will to go against that sort of grain.

Just remembered though: Friend who is a Azul developer said that 0x86 processors did not have the correct instructions to support pausesless GC up until very recently.

So probably a chicken and the egg problem.

can you open that up more? I'm skeptical. Which instructions?

Over my pay grade, which is why I'm slopping away at ARM Cortex firmware instead of at Azul. I'd ask my friend but he's on vacation. I do know Azul originally ran their JVM on custom processors. (Target market high speed trading systems, etc) I can't see why they would have done that unless older 0x86 etc processors just couldn't do what they needed.

A little internet searching, leads to this and some other stuff. https://www.artima.com/lejava/articles/azul_pauseless_gc.htm...

I think the deal is, if the GC moves something, it can leave a bunch of dangling pointers. Those need to be fixed up to point to the new place before they get dereferenced. Except Azul uses a read barrier to trap when a dangling pointer is dereferenced, so the fix up can be lazy.

The Android Framework team has been working on improving the runtime and its garbage collector for quite some time. There have been many improvements to the garbage collection strategy. With ART on Lollipop, according to its creator, there should never be a visible GC pause when the app is in the foreground. More information here : http://www.anandtech.com/show/8231/a-closer-look-at-android-... and in various I/O talks. Extremely bad code will still cause problems (but that's true for whatever memory management strategy you rely on) but in my experience, on a nexus 5, GC pauses are no longer a real issue on Android.

Still, Samsung is here to samsung things up. Comparisons of scrolling performances between a flagship samsung device and a nexus are just as expected : https://www.reddit.com/r/GalaxyS6/comments/3ck9no/scrolling_...

That's another topic entirely, but I think that another key factor is to move almost as much as possible of the UI work to a separate thread, not just to make heavy work in the background. It is starting to happen with ripple animations on a RenderThread, but I think that this strategy could be generalized.

On my tablet (not nexus) with Android 5.0 I can surely see when everything comes to a stall.

Tablets are indeed complex.

At best they have the same SOC as a good phone but they typically have way more pixels to carry around.

NoName Chinese tablets are often a disaster and even high end tablets are hard to get right.

Even putting graphics aside, things like a slow internal memory can also wreak performances.

might be 5.0 mem leak issues and general bugs

> Still, Samsung is here to samsung things up

3rd time I've laughed with more than 60% intensity at something I read on the internet in the last 2 years. thank you!!! hahahaaha

for reference, my last 100% laugh that returned me to the days of cartoons and childhood was this comment about how to install and activate your non-rental-Cable-modem: https://www.reddit.com/r/technology/comments/27szuw/comcast_...

Interesting side note - Azul's GC isn't "pauseless" it's pause less.

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