[MOAB-dev] Discussion on return type of MOAB routines
Tim Tautges (ANL)
tautges at mcs.anl.gov
Fri Jan 24 09:07:04 CST 2014
On 01/23/2014 10:14 PM, Paul Wilson wrote:
> Hi Tim,
>
> On 01/23/2014 11:05 AM, Tim Tautges (ANL) wrote:
>> The trouble is then where do you leave the extra information to be picked up if the caller needs it. We want to avoid
>> things like a last_error string on the instance, since that's not thread safe.
> I'm imagining that you effectively repeat the same query that produced the information in the first place, but through a
> different interface that was setup to collect the interesting information.
>
> This may be too expensive in general, but I thought I'd throw the idea out there.
>
> Do you have a good sense of how common this would be? You offered one use case, but are there many places in MOAB that
> offer more information to the calling function?
>
Not that many, but it's one of those things where the application determines what's "normal".
And for applications, if we pass back only an ErrorCode, plus maybe a stack trace gets printed under certain conditions,
what about those cases where the application needs more information, e.g. which handle turned out to be invalid or
didn't have a given tag? Then we're stuck back with a member of the Core class, which we don't want because it's not
thread safe. Is it enough to just make that thread local?
- tim
> Paul
>>
>> - tim
>>
>> On 01/23/2014 07:54 AM, Paul Wilson wrote:
>>> Hi all,
>>>
>>> Just catching up on this thread...
>>>
>>> On 01/22/2014 03:24 PM, Wu, Danqing wrote:
>>>> Below is the summary of the discussions on return type so far. I notice that how we handle special "non-success"
>>>> condition (like TAG_NOT_FOUND) will affect our decision. Any decision has its pros and cons. Maybe we need more
>>>> opinions from other people.
>>>>
>>>> As for existing ErrorCode return type, Vijay and I prefer to use it. Iulian also voted to keep it simple (enum/int)
>>>> 1) This does not break any existing user code nor does it need to change existing method signatures
>>>> 2) According to Jed, PETSc returns error code because it is simple and portable between languages
>>>> 3) Benchmarking tests show that it has least overhead for function return
>>>> 4) No need to store any context explicitly, including error messages that are already printed out in the stack trace
>>>>
>>>> Tim has some other reasons to return a class object instead of a plain enum type
>>>> 5) The overhead might be acceptable for MOAB
>>>> 6) For non-success conditions, we might need extra information hat can only come from the lower-level functions.
>>> Since the calling function knows what it asked, couldn't this be addressed by a combination of:
>>> * return one of a rich set of int/enum return codes
>>> * API for querying the necessary information that is missing in a simple error code
>>>
>>> Cons:
>>> * for cases in which there is necessary information, this would be slower than the other options proposed because of
>>> another function call
>>>
>>> Pros:
>>> * the calling function gets to decide whether it cares about (and hence bothers to query) the additional information
>>> * there can be a richer interface for that additional information other than parsing a string
>>> * a sufficiently rich enough set of error codes, may reduce the number of times that a calling function cares about that
>>> info.
>>>
>>> Using Tim's tag_get_handle example (mismatched properties to tag_get_handle), if the return code was
>>> MOAB_TAG_EXISTS_WITH_DIFFERENT_PROPERTIES (not a great choice, but illustrative), then there may be many cases where the
>>> calling function just wants to handle this and move on, rather than query something like tag_get_props().
>>>
>>> Or am I missing something?
>>>
>>> Paul
>>>
>>>> Regarding 6), Vijay and I have similar opinion:
>>>> Is that extra information in the returned class object really necessary? An upper level caller, based on some context,
>>>> usually knows how to handle a non-error condition. It can choose to treat is as an expected error, and set it. It can
>>>> choose to handle it and then return success. It can also keep it unhanded, and return this condition code unchanged to
>>>> its callers. Of course, the lowest routines, like tag_get_handle(), will never set it as an error (because it does not
>>>> know, totally decided by higher level callers), and just returns this code as is. In this case, an ErrorCode still
>>>> would suffice. Using an object to store any context explicitly is probably not necessary here.
>>>>
>>>> BTW, according to Jed, PETSc never uses error handlers/return codes for non-error conditions. It defines what should
>>>> happen in that case and have a flag return if the caller needs to know.
>>>
>>>>
>>>> Regarding returning a class object, there are three different options:
>>>> 1) Use ostringstream as a member. This has more overhead according to benchmarking tests. The size of the object is
>>>> 360 bytes.
>>>> 2) Use std::string as a member. This is slightly better than a). The size of the object is 16 bytes.
>>>> 3) Make the class a POD type, like C style struct, without constructors. Then char* is used as a member. A global heap
>>>> space should be allocated to store the string. The issue is how to make this thread safe.
>>>> For now, it is more likely that we will use either 2) or 3)
>>>>
>>>> Best
>>>> Danqing
>>>> ________________________________________
>>>> From: Tautges, Timothy J.
>>>> Sent: Wednesday, January 22, 2014 3:36 PM
>>>> To: Vijay S. Mahadevan
>>>> Cc: Jed Brown; Wu, Danqing; moab-dev at mcs.anl.gov
>>>> Subject: Re: [MOAB-dev] Discussion on return type of MOAB routines
>>>>
>>>> On 01/22/2014 01:34 PM, Vijay S. Mahadevan wrote:
>>>>>> It's entirely dependent on the application, same API call, same return,
>>>>>> different handling, and in some cases the application is a tool inside the
>>>>>> library.
>>>>> If the calling function knows how to handle the error based on the
>>>>> context i.e., fail with an error message or return with an error code
>>>>> to be handled by upper level routines, an ErrorCode still would
>>>>> suffice. Using an object to store any context explicitly is not be
>>>>> necessary here IMO.
>>>>>
>>>> But it's information from lower-level functions that will go missing. Maybe that's not enough information to really
>>>> worry about, esp. if we have a stack trace, but that's the reason I'm saying we need this string.
>>>>
>>>> - tim
>>>>
>>>>> If you just want the control to augment the printed error message (in
>>>>> fatal failure cases), use macros like SETERR that take a stream. Since
>>>>> the program is going to exit due to the failure, each calling routine
>>>>> can print its message and return appropriately without worrying about
>>>>> the actual cost of storage of strings etc. For successful exits,
>>>>> string remains null. The const char* version in my timing results will
>>>>> be appropriate here.
>>>>>
>>>>> Vijay
>>>>>
>>>>> On Wed, Jan 22, 2014 at 1:26 PM, Tim Tautges (ANL) <tautges at mcs.anl.gov> wrote:
>>>>>>
>>>>>> On 01/22/2014 01:21 PM, Jed Brown wrote:
>>>>>>> "Tim Tautges (ANL)" <tautges at mcs.anl.gov> writes:
>>>>>>>> This depends on how often it happens. In all cases, IMO, a success
>>>>>>>> condition should result in nothing more than a
>>>>>>>> POD-sized object (and I should add, no virtual table).
>>>>>>>
>>>>>>> vtables are just pointers like any other in terms of performance.
>>>>>>>
>>>>>> Sure but we still don't want that extra expense.
>>>>>>
>>>>>>
>>>>>>>> If it's a non-success condition that is "expected" (normal condition
>>>>>>>> that's handled by the calling code), IMO this shouldn't print anything
>>>>>>>> by default. If it's a non-success that's also unexpected, printing is
>>>>>>>> fine (including stack trace).
>>>>>>>
>>>>>>> I don't believe in expected failures. I find keeping expected and error
>>>>>>> conditions in-band leads to confusing control flow, but YMMV.
>>>>>>>
>>>>>> It's entirely dependent on the application, same API call, same return,
>>>>>> different handling, and in some cases the application is a tool inside the
>>>>>> library.
>>>>>>
>>>>>> - tim
>>>>>>
>>>>>>
>>>>>> --
>>>>>> ================================================================
>>>>>> "You will keep in perfect peace him whose mind is
>>>>>> steadfast, because he trusts in you." Isaiah 26:3
>>>>>>
>>>>>> Tim Tautges Argonne National Laboratory
>>>>>> (tautges at mcs.anl.gov) (telecommuting from UW-Madison)
>>>>>> phone (gvoice): (608) 354-1459 1500 Engineering Dr.
>>>>>> fax: (608) 263-4499 Madison, WI 53706
>>>>>>
>>>> --
>>>> ================================================================
>>>> "You will keep in perfect peace him whose mind is
>>>> steadfast, because he trusts in you." Isaiah 26:3
>>>>
>>>> Tim Tautges Argonne National Laboratory
>>>> (tautges at mcs.anl.gov) (telecommuting from UW-Madison)
>>>> phone (gvoice): (608) 354-1459 1500 Engineering Dr.
>>>> fax: (608) 263-4499 Madison, WI 53706
>>>>
>>>>
>>>
>>
>
--
================================================================
"You will keep in perfect peace him whose mind is
steadfast, because he trusts in you." Isaiah 26:3
Tim Tautges Argonne National Laboratory
(tautges at mcs.anl.gov) (telecommuting from UW-Madison)
phone (gvoice): (608) 354-1459 1500 Engineering Dr.
fax: (608) 263-4499 Madison, WI 53706
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