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

Sun Oct 14 23:29:24 PDT 2012

On Oct 15, 2012, at 01:10 , David Holmes wrote:

> 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.
> 
Remember that the point of not having a boolean wasn't to enable a potential third option, but to make callsites clearer.
Alloc(size, false) is less descriptive than Alloc(size, no_throw).


> 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
>>