Hi Bengt,

On 19/09/2016 7:14 AM, Bengt Rutisson wrote:


Hi Serviceability,

Not sure, but I hope this is the correct list to post this on.

Sure is.

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?

Did you also notice this code:

  protected long[] getThreadAllocatedBytes(long[] ids) {
        boolean verified = verifyThreadAllocatedMemory(ids);

        long[] sizes = new long[ids.length];

so we're allocating  another array for the return value(s).

Bit difficult to use a pre-allocated array - when would you allocate it? when would you release it? Would you have one per-thread or a freelist of them? It all gets a bit too hard to manage.

A better API would allow the user to pass in the result array as well - to allow for array allocation and reuse outside of the region of code that is being measured.

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?

I'd call it a quality of implementation issue. It would be better if the allocation could be avoided, but that requires two entry points to the VM code. Certainly the query for a single thread should avoid the need for any array allocation. But I think this pattern is common throughout the management code.

You should a file a RFE for 10.

Cheers,
David


Thanks,
Bengt

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