[9] RFR(L): 8046809: vm/mlvm/meth/stress/compiler/deoptimize CodeCache is full.

Albert Noll albert.noll at oracle.com
Tue Oct 14 08:03:45 UTC 2014

Hi Vladimir,

I filed a CCC request and added the 3 removed flags to the 
obsolete_jvm_flags table.

Here is the updated webrev:


On 10/14/2014 04:00 AM, Vladimir Kozlov wrote:
> Looks good but you need to wait CCC approval for product flags removal.
> And you need to add them to obsolete_jvm_flags table in arguments.cpp.
> Thanks,
> Vladimir
> On 10/13/14 7:55 AM, Albert Noll wrote:
>> Hi Vladimir,
>> thanks for the feedback. Here is the updated webrev:
>> http://cr.openjdk.java.net/~anoll/8046809/webrev.04/
>> Best,
>> Albert
>> On 10/10/2014 06:57 PM, Vladimir Kozlov wrote:
>>> On 10/10/14 7:30 AM, Albert Noll wrote:
>>>> Tobias, Vladimir, Dean, Nils, thanks for looking at the patch and 
>>>> for your feedback.
>>>> @Tobias
>>>> I have adapted your suggestions.
>>>> @Vladimir
>>>> I tried to add a condition to 
>>>> SafepointSynchronize::is_cleanup_needed() that returns 'true' if 
>>>> the code cache has less
>>>> than 10% free space. Unfortunately, that does not fix the bug. A 
>>>> safepoint interval of 1000ms seems to be too coarse
>>>> to free space in the code cache fast enough.
>>> Okay, thank you for trying.
>>>> It seems that the concept of critical memory allocations 
>>>> (allocations that must succeed for the VM to be able to
>>>> continue to execute) is broken. For example, we need to compile 
>>>> method handle intrinsics (otherwise we get a
>>>> java.lang.VirtualMachineError: out of space in CodeCache for method 
>>>> handle intrinsic). However, method handle intrinsic
>>>> are not critical allocations. For this reason, 
>>>> test/compiler/startup/SmallCodeCacheStartup.java fails with a 32-bit
>>>> client
>>>> version. We will get a new nightly bug soon... The current patch 
>>>> fixes this.
>>>> I want to keep the removal of critical allocations in this patch, 
>>>> since aggressive sweeping (enabled by the VM
>>>> operation that forces stack scanning) replaces the concept of 
>>>> critical allocations in the code cache. I think
>>>> these two changes belong together. If you still want me to, I will 
>>>> do a separate change for critical allocation
>>>> removal.
>>> Okay, I am fine with this removal.
>>>> @Dean
>>>> I removed the corresponding code. It was a fragment that I missed 
>>>> to delete.
>>>> @Nils
>>>> CodeCacheMinimumFreeSpace did not guarantee that the VM can 
>>>> continue to execute. In the failing test
>>>> that is reported in the bug, 500K was not enough to generate all 
>>>> adapters. The test can be changed such
>>>> that a CodeCacheMinimumFreeSpace size of 1m, 2m, 3m, etc. is too 
>>>> small too. What value should we choose?
>>>> Also, as noted above, method handle intrinsic need to be compiled 
>>>> to be able to continue execution.
>>>> Method handle intrinsic are currently not critical, so we must make 
>>>> them critical allocations. As a consequence,
>>>> we must re-evaluate the default size of CodeCacheMinimumFreeSpace.
>>>> The current approach enables very aggressive sweeping, if the code 
>>>> cache is 90% full. It is very likely that code
>>>> will be flushed from the code cache in the next 5-10 invocations of 
>>>> CodeCache::allocate(). In a sense, the remaining
>>>> 10% can be considered as a 'critical region' that is used as a 
>>>> 'buffer' until we free space in the code cache. This bug
>>>> proves that the proposed strategy solves the problem better than 
>>>> CodeCacheMinimumFreeSpace.
>>>> Maybe we should provide the user with control over this threshold, 
>>>> i.e., replace CodeCacheMinimumFreeSpace
>>>> with a different command line flag that allows the user to specify 
>>>> the percentage (currently 90%) at which aggressive
>>> Yes, it should be flag. I think it should be percentage.
>>> Thanks,
>>> Vladimir
>>>> sweeping starts. We could also use a fixed-size. I don't think that 
>>>> having two thresholds (the threshold where we start
>>>> aggressive sweeping AND CodeCacheMinimumFreeSpace) is necessary.
>>>> What do you think?
>>>> The performance runs show that waking up the sweeper thread for 
>>>> every allocation has a negative impact
>>>> on startup time. To fix this, in the current patch, the sweeper 
>>>> thread is only woken up, if the code cache is
>>>> more than 10% occupied. I will issue a new performance run and 
>>>> compare it against b34 (which includes
>>>> the segmented code cache).
>>>> Here is the new webrev:
>>>> http://cr.openjdk.java.net/~anoll/8046809/webrev.03/
>>>> Best,
>>>> Albert
>>>> On 10/10/2014 11:01 AM, Nils Eliasson wrote:
>>>>> Hi, Albert
>>>>> Overall a very welcome change to move the sweeper into a separate 
>>>>> thread.
>>>>> On 2014-10-09 10:24, Albert Noll wrote:
>>>>>> The patch also removes CodeCacheMinimumFreeSpace and 'critical' 
>>>>>> code cache allocations. Due to a bug in
>>>>>> heap.cpp, the CodeCacheMinimumFreeSpace was in fact not reserved 
>>>>>> for 'critical' allocations. The following
>>>>>> lines produce an underflow if heap_unallocated_capacity() < 
>>>>>> CodeCacheMinimumFreeSpace:
>>>>>> segments_to_size(number_of_segments) > 
>>>>>> (heap_unallocated_capacity() - CodeCacheMinimumFreeSpace)
>>>>>> Since the critical part in the code cache was never used for its 
>>>>>> intended purpose and we did not have problems
>>>>>> due to that, we can remove it.
>>>>> Are you sure? The reasons for the CodeCacheMinimumFreeSpace and 
>>>>> critical allocations where problems with code cache
>>>>> fragmentation in long running applications, where small 
>>>>> compilations would starve out the adaptors and cause VM
>>>>> shutdown. You won't see this other than in the really long running 
>>>>> tests. It might be broken - but then we should open
>>>>> a bug and fix it.  (And in the long run we should handle the 
>>>>> fragmentation with relocating code.)
>>>>> Regards,
>>>>> //Nils

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