On Tue, May 5, 2020 at 4:20 PM King Kong <[email protected]> wrote:
> Thanks for your reply.
>
> Yes, I have been using arena, but I wanted to do better than batching heap
> allocation, when possible. The "scoped leasing" use case I described above
> achieve 0 heap allocation all the time. I guess my code could use
> GetArena() at runtime to decide how the "leasing" object is created and
> which set of set&release APIs to use.
>
If you want to avoid any heap allocation, you could give the arena an
initial block of memory from the stack.
If you are trying to optimize, keep in mind that most of the savings from
arenas come at deallocation time, where arena-allocated protos can simply
skip the destructor. If you are using heap-allocated protos, the destructor
has to check every message field to see if it needs to be deleted.
Arenas have given such performance benefits that we are focused on that use
case. If we loosen the checks in unsafe_arena functions, it could force us
to compromise later to keep supporting the heap-allocated case. I don't
think we want to loosen these restrictions.
> On Tuesday, May 5, 2020 at 2:25:05 PM UTC-7, Josh Haberman wrote:
>>
>> The documentation for unsafe_arena_release_foo() says:
>>
>> > This method should only be used when the parent message is on the arena
>> and the submessage is on the same arena, or an arena with equivalent
>> lifetime.
>>
>> The DCHECK() you highlighted is checking the first part of this
>> requirement.
>>
>> Stack/heap-allocated messages must always uniquely own their
>> sub-messages, which must always be on the heap. Protobuf doesn't support
>> the use case you are describing, where a message temporarily points to a
>> message it doesn't own. If we allowed that, protobuf itself would crash if
>> a message in this state was ever destroyed.
>>
>> If you are trying to avoid heap allocation overhead, the recommended
>> solution is arenas.
>>
>> On Tuesday, May 5, 2020 at 12:44:32 PM UTC-7, King Kong wrote:
>>>
>>> I am using unsafe_arena_set_allocated_* and unsafe_arena_release_* APIs
>>> to avoid
>>> unnecessary allocations when the sub-objects are only used in a function
>>> scope,
>>> unsafe_arena_release_* is always called before leaving the scope. At the
>>> beginning,
>>>
>>> I chose to always use the arena versions of unsafe APIs because the
>>> non-arena version
>>>
>>> unsafe_release does extra allocation when the object is allocated on
>>> arena, and the arena
>>>
>>> version does not check if the object is allocated in arena, based on
>>> generated code like
>>>
>>> the following:
>>>
>>>
>>> inline foo* outter1::unsafe_arena_release_foo() {
>>> foo* temp = foo_;
>>> foo_ = nullptr;
>>> return temp;
>>> }
>>>
>>>
>>> However, I later also saw the following the generated code with arena
>>> check, it means it is not okay to call for heap-allocated objects.
>>>
>>> inline void outter2::unsafe_arena_set_allocated_bar(
>>> std::string* bar) {
>>> GOOGLE_DCHECK(GetArenaNoVirtual() != nullptr);
>>> ... ...
>>> }
>>>
>>>
>>> So, why the discrepancy?
>>>
>>>
>>> The official documentation at
>>> https://developers.google.com/protocol-buffers/docs/reference/arenas says
>>> the correct set of APIs should be used
>>>
>>> according to how to the object is allocated, but having two different
>>> sets of unsafe APIs makes it harder to use, and it makes the code less
>>> robust to future changes. I am wondering if protobuf owners also realize
>>> such an issue and start to remove arena checking gradually? It that the
>>> case? I would like to hear how others think.
>>>
>>>
>>> Thanks.
>>>
>> --
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