RFR (M) 8164921: Memory leaked when instrumentation.retransformClasses() is called repeatedly

[Coleen Phillimore]

Mikael, thanks again ...   embedded replies and 2 questions:

On 10/7/16 10:39 AM, Mikael Gerdin wrote:
> Hi Coleen,
>
> On 2016-10-06 19:54, Coleen Phillimore wrote:
>>
>> Here is an update to the Metadata leak change.  There was a bug
>> introduced when cleaning this up, which Mikael also found.
>>
>> open webrev at http://cr.openjdk.java.net/~coleenp/8164921.02/webrev
>>
>> Changes include adding logging for report_metadata_oome, which
>> necessitated removing ResourceMarks in ClassLoaderData::dump because the
>> stream passed in already required a ResourceMark, so it got a nested
>> ResourceMark message for the stream.
>>
>> I changed logging for tracing block allocations to log_trace(gc,
>> metaspace, freelist, blocks).
>>
>> In BlockFreelist::get_block and BlockFreelist::return_block() assert
>> that it's not called for a size smaller than the minimum allocation
>> (which was the bug).  Renamed get_raw_word_size() to
>> get_allocation_word_size().  This rounds up to the minimum allocation
>> size which is the same as small_block_min_size.
>>
>> Also, I added a test that takes a long time to execute to verify this,
>> and excluded it from JPRT.  I could skip adding this test if people
>> don't want it.  Also, the test verifies that continuously redefining the
>> same class gets memory for the new class that was released because the
>> block sizes are the same.   When the test exits, it gets a metaspace OOM
>> because loading new classes and allocating metadata can't use the blocks
>> returned (wrong size). There is still fragmentation in this
>> implementation, but it's better that things deallocated < 12 words are
>> actually freed.  I'll file an RFE to work on a perfect algorithm, or to
>> investigate finding a better one, although I consider this a stress test
>> that uses all of metaspace to MaxMetaspaceSize, leaving allocation only
>> to the block fragments left.  This isn't a typical use case.
>>
>> Some comments and corrections to my responses to Mikael below:
>>
>> On 10/4/16 12:15 PM, Coleen Phillimore wrote:
>>>
>>> Hi Mikael,
>>>
>>> Thanks for looking at this change.
>>>
>>> On 10/4/16 8:32 AM, Mikael Gerdin wrote:
>>>> Hi Coleen,
>>>>
>>>> On 2016-09-30 21:02, Coleen Phillimore wrote:
>>>>> Summary: Return Metablocks smaller than dictionary's dark matter.
>>>>>
>>>>> This change contributed by Jon Masamitsu and myself.  To reclaim 
>>>>> "dark
>>>>> matter" this change adds an array of small blocks by size, created
>>>>> lazily, to return Metablocks smaller than the BinaryTreeDictionary
>>>>> entry's minimum size.   This change also fixed a bug in small object
>>>>> double free and adds debugging code to check for this case.
>>>>>
>>>>> With this change, the submitted test case runs indefinitely. Also
>>>>> passed rbt tier 1-5 testing.
>>>>>
>>>>> open webrev at http://cr.openjdk.java.net/~coleenp/8164921.01/webrev
>>>>> bug link https://bugs.openjdk.java.net/browse/JDK-8164921
>>>>
>>>> I'd prefer it if SmallBlocks didn't expose its implementation by
>>>> returning its FreeLists by reference, could you change it to have
>>>> * return_chunk()
>>>> * get_chunk()
>>>> * num_chunks(word_size)
>>>>
>>>> and get rid of list_at?
>>>
>>> Okay, I refactored this so small_chunks()->get_block() and
>>> return_block() are used rather than getting list_at.  I didn't see
>>> where you got num_chunks, but list_at is hidden.
>
> Thanks, that's much better!
>
>>>>
>>>> -- 
>>>>
>>>> For how long do you plan to keep BlockFreelist::_all_blocks?
>>>> I see that it's debug only but I fear that it could case problems
>>>> with running out of native memory in our internal testing of debug
>>>> builds.
>>>
>>> I thought about taking it out, but it helped me find the double free
>>> bug.   I think if we add new code to metadata and have to call
>>> deallocate_contents on it, we risk re-introducting these double free
>>> bugs.   I could take it out.  I don't think this gets that big but I'd
>>> hate to introduce some sort of OOM bug in our testing.
>
> Perhaps a trade off could be to have a bound on the size of the array 
> and deallocate and disable it if it grows too large?

I just removed it.  I had run all the tests with the debugging code and 
they passed.  If we have to, we can add it back again pretty easily.
>
>>>>
>>>> BlockFreelist::min_size() is a bit of a misnomer since it returns the
>>>> minimum size of blocks to be put on the BlockTreeDictionary, not the
>>>> minimum size of blocks which are reusable.
>>>
>>> How about min_dictionary_size() ?
>
> That's fine.
>
>>>
>>>>
>>>> Is there any particular reason behind _small_blocks being lazy
>>>> allocated and _dictionary not?
>>>
>>> We lazily create the BlockFreelists with this change.
>>>
>>>   // Lazily create a block_freelist
>>>   if (block_freelists() == NULL) {
>>>     _block_freelists = new BlockFreelist();
>>>   }
>>>
>>> So the small_blocks are lazily created in the BlockFreelists but the
>>> dictionary is not.  I guess if we're going to create the
>>> BlockFreelists here, we'll most likely need both and maybe
>>> small_blocks need not be lazily created.  Was that your suggestion?
>>>
>>> My concern with this change was all the space used by the
>>> small_blocks() but if we're doing any deallocation within the
>>> metaspace, at least one of the things will be <12 words.
>>>
>>> I'll make small_blocks() not be lazily allocated since BlockFreelist
>>> are.  These are pretty expensive, but should be limited to a few
>>> metaspaces.
>>
>> Class SpaceManager doesn't need small_blocks() so I left small_blocks as
>> lazily allocated.
>
> Ok, I agree that the size of small_blocks() motivates its lazy 
> allocation scheme.
>
>>>>
>>>> I would prefer if BlockFreelist::return_block would perform the
>>>> checks in reverse order instead of having a return inside the first
>>>> if block, something like
>>>>
>>>> if (word_size > small_block_max_size) {
>>>> dict()->return_chunk(c)
>>>> } else if (word_size > small_block_min_size) {
>>>> small_blocks()->return_chunk(c)
>>>> } else {
>>>>   // dark matter
>>>> }
>>>
>>> Why?  We don't want to cast Metablock into dark matter so check if
>>> word_size < small_block_min_size first.
>>>
>>>   Metablock* free_chunk = ::new (p) Metablock(word_size);
>>>   if (word_size < SmallBlocks::small_block_max_size()) {
>>>     small_blocks()->return_chunk(word_size);
>>>   } else {
>>>     dictionary()->return_chunk(free_chunk);
>>>   }
>>>
>>
>> There is no dark matter in these functions anymore.
>
> Oh! Right.
>
>
>>>>
>>>>
>>>> For BlockFreelist::get_block I realize that the code is a bit more
>>>> complex but this also raises a few questions.
>>>> * When we allocate from the dictionary we search for a block which is
>>>> at least as large as we ask for and split it, returning whatever was
>>>> left back to the free list. We don't search for "large enough" blocks
>>>> from the small blocks manager, is that intentional or just to keep
>>>> the code simple (I like simple)?
>>>
>>> I'm glad you asked about this so I could give background.   It turns
>>> out that we deallocate metaspace items better this way.  I had a
>>> version that did exactly what you said.  It was a simple sorted linked
>>> list of returned blocks < min_dictionary_size (12) where get_block
>>> returned the first block where the item would fit.  It had some best
>>> fit algorithm so if the block returned was a lot bigger, it wouldn't
>>> pick it.
>>>
>>> My implementation could get through 69300 retransformations before the
>>> list didn't work anymore (too many small block fragments of the wrong
>>> size) and metaspace was exhausted (metaspace was limited to 12M in
>>> this test).  Jon's implementation ran this test indefinitely. So it's
>>> somewhat simple but it worked really well.
>>>
>>>>
>>>> In the last part of get_block where we return the unused part of a
>>>> block retrieved from the dictionary uses compares with
>>>> BlockFreelist::min_size() which, as I mentioned above, is the min
>>>> size for blocks in the dictionary, not the min size for blocks to be
>>>> reusable.
>>>> I think this code can just call return_block unconditionally for any
>>>> nonzero value of "unused" and let return_block deal with dark matter.
>>>>
>>> yes, I'll make that change.
>>
>> This change has to be conditional because I assert that
>> BlockFreelist::return_block() is never called for < 
>> small_block_min_size.
>
> Ok.
>
>
> There's one more thing, though.
> In SpaceManager::retire_current_chunk the last piece of the current 
> allocation chunk is made reusable through a call to 
> SpaceManager::deallocate. There are two problems with this:
>
> * retire_current_chunk only calls deallocate for wasted blocks larger 
> than the min dictionary size, this means that wasted blocks which 
> would fit in SmallBlocks will be wasted instead of reusable.

Yeah.   I didn't know if this was worth doing for 12 words or less. Do you?

> * the remaining_words parameter which retire_current_chunk passes to 
> deallocate is _exact_ and must not be aligned up through 
> get_allocation_word_size.
>

Since it's already greater than the allocation_word_size, this isn't a 
problem.

>
> Also,
> are you sure about logging the output of ClassLoaderData::dump on the 
> info level? Perhaps logging the cld dump on the debug level is a good 
> trade-off? It looks like the trace level causes the VSM to print the 
> entire free lists to the log so that's probably a bit much.

It is a lot of stuff I didn't want to see.  I could change it back to 
trace though because the thing I really wanted to see in 
MetaspaceAux::dump() is guarded by trace

   if (log_is_enabled(Trace, gc, metaspace, freelist)) {
     if (block_freelists() != NULL) block_freelists()->print_on(out);
   }

It seems like debug would be a better level though for both.
>
>
>>
>> Thanks,
>> Coleen
>>
>>>>
>>>>
>>>> Are the changes to Method and ConstMethod the "bug fix to small
>>>> object double free"?
>>>
>>> Yes.
>>>> Is the problem that when a method was freed its annotations were
>>>> deallocated as well? Could the annotations fields in the "old"
>>>> ConstMethod be cleared instead so that the old annotation arrays
>>>> could be kept or is that just needless complexity?
>>>
>>> I'd rather copy the annotations, because I don't know how the old
>>> method, which could still be running, might use the annotations. I
>>> don't want to mess with that but I see your point.
>
> That's fine I think, I wouldn't want to mess around with it either. I 
> just wanted to understand the reasoning behind the change, thanks for 
> confirming my suspicion.
>

thanks,
Coleen

> /Mikael
>
>>>
>>> Coleen
>>>
>>>>
>>>> Thanks
>>>> /Mikael
>>>>
>>>>
>>>>
>>>>>
>>>>> Thanks,
>>>>> Coleen and Jon
>>>
>>