Hi Bengt,
I'm not Serviceability, but you know I can't leave them micro-
optimizations alone! :-)
So, reusing cached arrays could be made to work but would require
some synchronization to keep things thread-safe and tidy[1].
This will complicate the code, especially since there's another implied
allocation in getThreadAllocatedBytes. Not to mention that caching
objects which are cheap to allocate is a bit of an performance
anti-pattern.
Adding synchronization also comes with it's own risks, especially as
we're calling into JNI and the VM code takes a somewhat shady mutex
already (Threads_lock).
Generally I don't think there's ever any behavioral guarantees about how
much - or little - a method won't allocate anything, so calling this a
bug is a bit of a stretch IMO, although it's a bit unfortunate in this
particular case.
TL;DR: I'm a bit skeptic, but if it's important to you to fix this, I
wouldn't think it's impossible.
Thanks!
/Claes
[1] Alternatively we could of course implement a JNI method taking a
long rather than a long[], which would be consistent with other methods
in ThreadImpl.java, but I think we want to avoid going that far.
On 2016-09-18 23:14, Bengt Rutisson wrote:
Hi Serviceability,
Not sure, but I hope this is the correct list to post this on.
I wanted to use the ThreadMXBean.getThreadAllocatedBytes() method to get
some information about how much memory some Java code allocated.
When I dug into the results they didn't properly add up until I realized
that the call to getThreadAllocatedBytes() actually allocates memory.
This was a surprise to me.
I'm attaching a small example to illustrate what I mean.
Running the example renders this output:
$ javac AllocMeasure.java
$ java AllocMeasure
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
Bytes allocated: 48
What I would have expected was that it would say "Bytes allocated: 0"
since I would like to add my own code between line 9 and 10 in the
example and get the value for how much memory it allocates. As it is now
I have to deduct the bytes that the getThreadAllocatedBytes() allocates
to get the correct result.
The problem is that getThreadAllocatedBytes() is implemented this way:
public long getThreadAllocatedBytes(long id) {
long[] ids = new long[1];
ids[0] = id;
final long[] sizes = getThreadAllocatedBytes(ids);
return sizes[0];
}
http://hg.openjdk.java.net/jdk9/jdk9/jdk/file/32d957185656/src/java.management/share/classes/sun/management/ThreadImpl.java#l345
I was surprised to see the "new long[1]". I realize that it is nice to
reuse getThreadAllocatedBytes(long []) method, but maybe a pre-allocated
array can be used instead of allocating a new one for each call?
I know the specification for this method is kind of fuzzy, but is this
to be considered a bug or does it work as intended?
Thanks,
Bengt
