On 2018-04-12 23:30, David Holmes wrote:
On 12/04/2018 11:33 PM, Magnus Ihse Bursie wrote:
On 2018-04-12 14:15, David Holmes wrote:
Hi Magnus,
On 12/04/2018 9:39 PM, Magnus Ihse Bursie wrote:
It is currently easy to add new JVM features to the JVM build, but
it is not possible to remove features.
With this change, features can be both added or removed from the
default set. They are added using --with-jvm-features=f1,f2 and
removed using --with-jvm-features=-f1,-f2. The syntax can be
combined, so --with-jvm-features=dtrace,-nmt will enable dtrace but
disable native memory tracking.
I need to point out that we have never tested disabling individual
VM features likes this. They are either all on, or all off for the
minimal VM! There may be implicit dependencies between features.
Well, I have. :-) However, I don't do that regularly, and changes
might very well have crept in. As always, if you build something
non-standard that is not regularly tested, you're on your own.
Feels to me like you've taken away the safety-fence and are
encouraging people to attempt these unsupported configurations.
Whether that was your intent or not.
It is always possible to configure something that does not work. :-) We
can make it more easy to do the right thing, but if we were to make
impossible everything that has not been tested, then we would also make
things impossible that are needed for some use cases.
Wrt JVM features, this has *always* been possible. If you use
"--with-jvm-variants=custom --with-jvm-features=jvmti,nmt" then you
*are* going to build an almost (or fully?) useless JVM. In fact, this
method made it *much* harder to try to get a functioning JVM without a
specific feature.
I think I should update the build documentation regarding JVM features
though, and I can definitely add some wisely worded warnings about
unsupported combinations of features, especially if removing them.
In any case, the purpose of this is not so much to disable existing
JVM features (after all, no one has really been missing this
functionality), as to pave the way for the upcoming patch for
including/excluding individual GCs.
Surely a GC selection flag would have sufficed.
It was the common agreement of both the build team and the GC folks
responsive for the upcoming selectively GC inclusion patch, that this
was better handled as JVM features than a separate GC flag.
/Magnus
David
/Magnus
David
I also included some additional code cleanup and fixes, such as
printing the JVM feature set at the summary.
Bug: https://bugs.openjdk.java.net/browse/JDK-8201483
WebRev:
http://cr.openjdk.java.net/~ihse/JDK-8201483-disable-JVM-features/webrev.01
