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

[Vladimir Kozlov]

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
>