Does HotSpot ever stop profiling?

[Stefan Reich]

OK, that sounds pretty much like what I had imagined. Obviously inlined
methods can appear multiple times in the final code. Thanks

On Tue, 17 Sep 2019 at 01:17, Vitaly Davidovich <vitalyd at gmail.com> wrote:

>
>
> On Mon, Sep 16, 2019 at 7:05 PM Stefan Reich <
> stefan.reich.maker.of.eye at googlemail.com> wrote:
>
>> Hey, thanks for your answer.
>>
>> So a method is normally only once C2-compiled. How does inlining fit into
>> the picture? You start from the method body and inline as deeply as
>> possible?
>>
> So saying a given method is only once compiled by C2 is slightly
> misleading.  What C2 compiles is actually a tree of methods.  I’m going to
> simplify a bit, but a method that reaches C2 compilation threshold is
> queued up for a C2 compile.  That method serves as the root of the
> compilation tree.  Callees in that method can be inlined as well, based on
> profiling info + a bunch of heuristics.  So you can end up with some leaf
> methods inljmed and compiled in multiple call graphs.  But there are also
> cases where C2 will refuse to inline a method that’s already been compiled
> if its resulting machine code size is over a (tunable) threshold - that’s
> just 1 of the different heuristics :).
>
> But the main point I wanted to convey is that once that blob is compiled
> by C2, that blob has no counters and generally sticks around (modulo
> traps/deopt or code cache pressure or some such).
>
>>
>> I always thought a method would be compiled multiple times while
>> improving its inlining. But maybe that makes no sense and you can do it all
>> in a single compilation effort.
>>
>>
>> On Tue, 17 Sep 2019 at 00:47, Vitaly Davidovich <vitalyd at gmail.com>
>> wrote:
>>
>>>
>>>
>>> On Mon, Sep 16, 2019 at 6:35 PM Stefan Reich <
>>> stefan.reich.maker.of.eye at googlemail.com> wrote:
>>>
>>>> Here's another thing I would love to have cleared up.
>>>>
>>>> Once a method is C2-compiled - does it still update its invocation
>>>> counters?
>>>
>>> Nope.  C2 compiled methods run profile-free.  There can be certain
>>> events that trigger a deopt that will toss out the C2 method (with future
>>> C2 compilations off newly collected profiling possible), but it won’t be
>>> counter based.
>>>
>>>>
>>>>
>>>> I would assume counting stops at that point since we have done all we
>>>> could
>>>> for the method. Also the invocation counting overhead is always
>>>> embarrassing when fighting some C++ guys over claims on how "slow" Java
>>>> is.
>>>> :o)
>>>>
>>>> OTOH, there might be scenarios where it makes sense to have a second
>>>> look
>>>> at "hot" methods, even if they are C2-compiled. Not sure if that
>>>> scenario
>>>> exists in the current infrastructure.
>>>
>>> That infrastructure is called a process restart :) j/k.  There’re some
>>> compiler control APIs, but they’re mostly to allow JIT testing (AFAIK).  I
>>> think it allows manually marking a C2 method for reclaim (ie not entrant),
>>> but I don’t recall the details offhand.
>>>
>>>>
>>>>
>>>> Which one is it in reality?
>>>>
>>>> --
>>>> Stefan Reich
>>>> BotCompany.de // Java-based operating systems
>>>>
>>> --
>>> Sent from my phone
>>>
>>
>>
>> --
>> Stefan Reich
>> BotCompany.de // Java-based operating systems
>>
> --
> Sent from my phone
>


-- 
Stefan Reich
BotCompany.de // Java-based operating systems