Hi Claes and David,
Thanks for the quick responses!
Yes, a pre-allocated array is probably not the way to go. Letting the user
pass a result array is much better or just specialising the "long
getThreadAllocatedBytes(long id)" to avoid allocations instead of re-using
the "long[] getThreadAllocatedBytes(long[] ids)" version.
As a user of the API I am not surprised that the "long[]
getThreadAllocatedBytes(long[] ids)" version needs to allocate the result
array. But since I know that "long getThreadAllocatedBytes(long id)" pretty
much just needs to read a field in the thread it surprises me that it needs
to allocate.
I'll file an RFE for 10.
Thanks again for the help with this!
Cheers,
Bengt
2016-09-19 1:45 GMT+02:00 David Holmes <david.hol...@oracle.com>:
> 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
>>
>
