Please implement client switch in 64-bit server JDK 14 builds
david.holmes at oracle.com
Fri Jul 19 03:28:29 UTC 2019
I'm moving this discussion to hotspot-dev as it's more appropriate.
On 19/07/2019 12:46 pm, Ty Young wrote:
> I'm requesting that the long unimplemented "client" java switch be
> implemented in Java 14.
Background: the client VM is historically only supported on 32-bit
platforms explicitly, so the memory issues you are seeing are a
combination of factors based on the ergonomic selections made by the VM
during startup. The "client VM" is predominantly a 32-bit JVM that only
supports the C1 JIT-compiler. The "server VM" in contrast supports the
C2 JIT-compiler. For a while now this distinction has blurred because
the JIT uses tiered-compilation so that it starts by acting similar to
the C1 compiler (for faster startup) and progresses into a mode that
acts like C2 (for throughput optimisation). Though there are flags you
can set to get it to act just like C1 or just like C2.
Whether a machine is considered "server class" only partially relates to
this. The startup ergonomics for a "server class" machine will configure
subsystems to use more memory than a "non-server class" machine. Again
these days (and for a while) we do not use this classification when
starting the JVM. Various ergonomic selections are made based on the
default settings for a range of components (mainly GC and JIT) together
with the characteristics of the actual runtime environment (available
memory and processors etc).
The JVM is highly tunable in this regard, but of course it needs to have
a reasonable out-of-the-box configuration - and that has evolved over
the years, but is, at least for 64-bit systems, skewed towards
server-style systems. So we cannot please everybody with the out-of-box
default configuration. It's been suggested in the past that perhaps we
should support a number of different initial configurations to make it
easy(er) to adapt to specific user requirements, but this quickly breaks
down as you can't get consensus on what those settings should be, and
anyone who really cares will do their own tuning anyway.
I can't go through your email point by point in detail sorry. Perhaps
others can focus on specific memory issues. In particular if JavaFX is a
source of problems then that will need to be discussed with the JavaFX folk.
A very strong "business case" would need to be made for the community to
look at supporting something like "-client" in the current OpenJDK.
> (Note: this entire request is based on the assumption that a JVM with
> -client is equivalent to a client JVM variant. If this is wrong, I
> apologies. There isn't much documentation to go on.)
> Since there aren't many google results or any kind of mention of this
> feature/ability even existing, i'll give an explanation to the best of
> my knowledge and personal observations:
> A "client" JVM variant is geared towards graphical end-user
> applications. According to a URL link found in the man entry for java
> this supposedly results in faster startups. While this *may* be true, a
> much larger and more important benefit is a massive committed memory
> reduction in the range of about 25% to 50% when running a JavaFX
> application. At minimum with similar heap sizes, that is a 75 MB memory
> savings at 300MB (a somewhat typical peak usage with JavaFX
> applications) with a typical server JVM. That's huge.
> The downside to this however is that at most, the maximum amount of
> (committed?) memory that a client JVM variant can use is somewhere
> around 300MB by default. For the intended purpose of the client JVM
> switch/variant this is *probably* fine. Server JVM variants only seems
> to allocate more memory to boost performance, which really isn’t that
> much of a difference with the intended use case of the client JVM
> switch/variant… especially considering the more appealing memory savings.
> So why should this be implemented?
> The answer is simple: using more memory then is necessary is bad, angers
> users, and frustrates developers who want to be responsible by not
> wanting to eat up their users's memory when it isn't needed.
> Even if you've have never heard anyone complain about Java's memory
> usage, you've most likely heard someone complain about a similar
> cross-platform software: Electron. People hate Electron applications for
> their absurd memory usage and will actively avoid them by using
> alternatives if possible.
> For reference, Etcher, an Electron application that allows users to
> easily create bootable USB drives on Windows, Linux, and probably Mac OS
> uses around 298 MB just at launch on Linux. Electron is both comparable
> in both goals(cross-platform solutions, JavaFX vs. Electron) and in
> memory usage.
> Java may not be a native language and there may be *some* unavoidable
> penalty for that but being wasteful and consuming resources where not
> necessary is, well, unnecessary. This can help reduce the amount of
> memory a java application uses significantly when used.
> With that all said, since JEPs include risks/impact/problems, it's best
> to mention some that come to mind:
> Because of the default lower memory limit, applications which go beyond
> this will fail. The easiest and best workaround would be to simply make
> the client JVM switch/variant opt-in. This would allow all existing Java
> applications to continue to work as expected.
> The only other issue that I can think of is people launching
> applications with -client without knowing the limitations of it and
> filing bogus bug reports to app developers. This can be mitigated with
> better documentation and awareness in places like the man page for Java.
> Since no one seems to really have used or knew about it before it's more
> likely end developers that will be passing the switch to their
> applications via scripts then end users will be.
> All in all, this is pretty safe as long as server JVM switch/variant
> remains the default. Maybe others can think of other
> And finally addressing the two questions/comments I imagine someone at
> some point are going to ask/say:
> Why not just compile a client JVM variant from source and use jLink?
> If heap and garbage collection is healthy, who cares?
> For the first one, yes, this is a route that could be taken. It has some
> problems however, namely:
> - You have to be the developer or have source code access to use jLink.
> - jLink -from my understanding- requires a **fully** modular Java
> application. Some used libraries may not be modular yet.
> - A full JDK source code compile is required - something that is really
> easy to do under Linux but might not be under Windows and takes
> considerable CPU power to do. No one that I’m aware of (on Linux anyway)
> provides client JVM variant builds. Presumably This is because the
> server JVM variant is the most versatile.
> and as for the second: just because there is say, 5.8GB out of 8GB
> available doesn't mean you should or have the right to use it as you see
> fit. People do more than use Java applications. If you are running a web
> browser with lots of tabs open, a Java application could realistically
> cause major system stuttering as memory is moved to swap/pagefile. While
> I used 300MB above as an easy realistic example, i've seen JavaFX
> applications consume as much as 700MB and even 1GB committed memory.
> Just opening Scene Builder and playing around with the GUI consumes
> 400MB easily on a server JVM variant(Oracle JDK/JRE 10 to be exact).
> While memory usage may never be as good as native, the current amount of
> memory being consumed is insane and any normal user with standard amount
> of memory(6-8GB) *will* feel this. Adding this switch could potentially
> help a lot here and give Java a slight edge over similar software
> Can this feature please be implemented? Likewise, could the
> documentation on what a "client" JVM and other JVM variants be updated
> and improved?
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