RFR: 8000617: It should be possible to allocate memory without the VM dying.

Sun Oct 14 16:10:16 PDT 2012

On 13/10/2012 1:40 AM, Nils Loodin wrote:
> So in general we have two competing wishes here:
>
> 1. Use std::nothrow, in operator new() and enum in the allocation methods. (basically webrev 03)
> Pros: more c++ standard-ish
> cons: more boiler plate, not descriptive of what actually happens, two different mechanisms.
>
>
> 2. Use enums everywhere (basically a bugfree version of 04):
> pros: more accurate description, cleaner implementation
> cons: it's an instance of "not invented here"-syndrom, ie, we're making up our own stuff.

I'm less supportive of using an Enum over a boolean now. A two-valued 
enum is a boolean. And we are treating it as a boolean any time we use 
if-else rather than a switch-style selector - we couldn't add a third 
option without revisiting every place it is currently checked. And I 
just don't see there being any third option.

I never liked std::nothrow because we never threw in the first place, 
but if it had instead been std::return_null I would have been fine with 
it. But given we use custom multi-arg variants of operator new I don't 
see that the "familiarity"/"standard" argument really has that much 
weight. If anything using a standard feature in a non-standard 
environment is likely to be more confusing to people who might naturally 
expect that if they see a no_throw variant then the other variant will 
in fact throw, not abort the VM!

I have some specific comments on specific changes but there doesn't seem 
much point going to that level of discussion yet. Except I will mention 
that in thread.cpp the pre-existing throw_excpt argument to allocate 
seems to really mean "exit_out_of_memory" and should be renamed.

David
-----

> So.. discuss! John, what do you think having seen the different implementations? It would be nice to be able to get in any version of it so I can develop the feature that will depend on it... :)
>
> Regards,
> Nisse
>
> On Oct 12, 2012, at 17:03 , Coleen Phillimore<coleen.phillimore at oracle.com>  wrote:
>
>>
>> I still prefer webrev 03 to webrev 04.   std::nothrow as a parameter means something to any C++ developer and AllocationStrategy::OOM only means something to a hotspot developer.  I think we should use the standard C++ apis when possible.  The duplication of operator new is not significant.   Plus if someone adds new (std::nothrow) call or you forgot to clean up one, it think it will overload to the std::operator new() which could lead to a subtle bug.
>>
>> Coleen
>>
>> On 10/12/2012 10:40 AM, Nils Loodin wrote:
>>>
>>>
>>>
>>> 12 okt 2012 kl. 16:25 skrev Keith McGuigan<keith.mcguigan at oracle.com>:
>>>
>>>> May as well lift the allocation fail strategy up into Thread::allocate() as well, replacing the boolean parameter there.
>>>>
>>> Ah, good idea.
>>>
>>>> I think you're replacements of new(std::nothrow) XXX() wit new (EXIT_OOM) XXX() are all reversed: if it was nothrow you want to replace it with RETURN_NULL.
>>>>
>>>> -
>>> Argh, darn, you're right of course.
>>>
>>> Will fix.
>>>
>>>> - Keith
>>>>
>>>> On Oct 12, 2012, at 9:14 AM, Nils Loodin wrote:
>>>>
>>>>> Hey guys!
>>>>>
>>>>> Thanks for yet another round of informative code reviews!
>>>>> So I got rid of all the instances of std::nothrow throughout the code and replaced them with a new shiny and descriptive enum.
>>>>>
>>>>> Was able to fold together a few specialized operator new() because of it, with more shared code as a result.
>>>>>
>>>>> What do you think now?
>>>>> http://cr.openjdk.java.net/~nloodin/8000617/webrev.04/
>>>>>
>>>>> Have a nice weekend!
>>>>>
>>>>> Regards,
>>>>> Nils Loodin
>>>>>
>>>>>
>>>>> On 10/12/2012 06:47 AM, David Holmes wrote:
>>>>>> On 12/10/2012 2:21 PM, David Holmes wrote:
>>>>>>> Hi Nils,
>>>>>>>
>>>>>>> There are a number of existing bugs/RFEs in this area - not sure we
>>>>>>> needed yet another.
>>>>>>>
>>>>>>> On 11/10/2012 10:55 PM, Nils Loodin wrote:
>>>>>>>> Hey guys.
>>>>>>>>
>>>>>>>> Your comments on this issue are great! So here's another version that
>>>>>>>> uses an enum instead of std::nothrow_t trickery!
>>>>>>>>
>>>>>>>> http://cr.openjdk.java.net/~nloodin/8000617/webrev.03/
>>>>>>> I dislike the std::nothrow usage that NMT introduced (there aren't even
>>>>>> Apologies it wasn't NMT that introduced this - it is a bit older than
>>>>>> that: April 2011. See
>>>>>> https://jbs.oracle.com/bugs/browse/JDK-7036747
>>>>>>
>>>>>> and some comments in
>>>>>>
>>>>>> https://jbs.oracle.com/bugs/browse/JDK-4719004
>>>>>>
>>>>>> Till then we had avoided any use of C++ std:: stuff - particularly
>>>>>> anything related to the exception mechanism, which we don't use at all.
>>>>>>
>>>>>>> any exceptions involved!) but introducing a completely different
>>>>>>> mechanism seems counter-productive. I prefer the "alloc fail strategy"
>>>>>>> approach but would like to see this solved holistically, replacing
>>>>>>> std::nothrow. Given there exist bigger RFE's to have the VM handle all
>>>>>>> OOM situations gracefully rather than just aborting, I'd rather not see
>>>>>>> just another point-patch put in place.
>>>>>> The bigger problem is probably still too big to really handle - so
>>>>>> either copy what we already introduced for CHeapObj to ResourceObj for
>>>>>> consistency, or replace both with something better.
>>>>>>
>>>>>> Cheers,
>>>>>> David
>>>>>> -----
>>>>>>
>>>>>>> For the record I found the original proposal extremely confusing:
>>>>>>> nothrow_constant vs throw_constant.
>>>>>>>
>>>>>>> Sorry.
>>>>>>>
>>>>>>> David
>>>>>>> -----
>>>>>>>
>>>>>>>> Regards,
>>>>>>>> Nils Loodin
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> On 10/11/2012 01:21 PM, Dmitry Samersoff wrote:
>>>>>>>>> Keith,
>>>>>>>>>
>>>>>>>>> Personally, I would prefer explicit AllocWithoutThrow(...) function.
>>>>>>>>>
>>>>>>>>> -Dmitry
>>>>>>>>>
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> On 2012-10-11 03:40, Keith McGuigan wrote:
>>>>>>>>>> Personally, I don't have strong feelings on how this is implemented,
>>>>>>>>>> other than it should be done in a way that's maintainable going
>>>>>>>>>> forward
>>>>>>>>>> and easily understandable by future generations of hotspot developers.
>>>>>>>>>> With this in mind, the only potential solution that I don't like is
>>>>>>>>>> using a boolean with naked true/false values as discriminators.
>>>>>>>>>>
>>>>>>>>>> Using some sort of "failure mode" parameter is the natural way to do
>>>>>>>>>> this, whether it be enums, std::nothrow_t, or whatever. Since
>>>>>>>>>> std::nothrow_t already has a type and one value, and is already
>>>>>>>>>> present
>>>>>>>>>> in the few places we're interested in, it seemed easy to simple just
>>>>>>>>>> add
>>>>>>>>>> a new value to use. However if this ends up being confusing because
>>>>>>>>>> this is not the normal use of std::notype_t, then fine, we can do
>>>>>>>>>> something else.
>>>>>>>>>>
>>>>>>>>>> --
>>>>>>>>>> - Keith
>