RFR 8055008: Clean up code that saves the previous versions of redefined classes
Coleen Phillimore
coleen.phillimore at oracle.com
Wed Sep 3 19:55:33 UTC 2014
Hi Serguei,
I'm going to cut some things...
<<cut>>
> Thank you for the explanation!
>
> There is also a potential scalability issue for class redefinitions as
> we do a search through
> all these previous_versions and their old methods in the
> mark_newly_obsolete_methods ().
> In the case of sub-sequential the same class redefinitions this search
> will become worse and worse.
> However, I'm not suggesting to fix this now. :)
I agree, it seems to take way too long to clear old methods once they
are in the CodeCache.
>
>> It's different than just saying it's emcp. It's emcp and it's
>> running also so needs a breakpoint.
>>
>> The states are really:
>>
>> is_obsolete() or !is_obsolete() same as is_emcp()
>>
>> is_running_emcp() == !is_obsolete() && method->on_stack()
>>
>> We need to distinguish the running emcp methods from the non-running
>> emcp methods.
>
> I suspect, sometimes this invariant is going to be broken:
> is_running_emcp() == !is_obsolete() && method->on_stack()
>
> When the method has been finished and the on_stack is cleared,
> the method is_running_emcp bit can remain still uncleared, right?
> Would it be more simple just to use "!is_obsolete() &&
> method->on_stack()" ?
> It must be just in a couple of places.
We only set on_stack when we do class redefinition and class unloading
with MetadataOnStackMark. After this safepoint, the bit is cleared.
We don't clear it when the method finishes.
Is running_emcp is in only 4 places, but the place where we really need
it (setting breakpoints) the "on_stack" bit isn't set because we don't
do MetadataOnStackMark at that safepoint. It's sort of an expensive
operation.
So I need is_running_emcp() to capture the last known running state.
>
>>
>> I guess we could just set breakpoints in all emcp methods whether
>> they are running or not, and not have this flag. This seemed to
>> preserve the old behavior better.
>
> I was thinking about the same but do not really have a preference.
> It is hard to estimate how big memory leak will cause these unneeded
> breakpoints.
>
It's not so much leakage, because the methods are there anyway but it
seems inefficient to do breakpoints on methods that have exited.
Setting these breakpoints looks expensive as well!
> <<cut>>
> This is nice, thanks!
> I'm looking at the new webrev version now.
Ok, let me know if there's anything else.
Coleen
>
>
> Thanks,
> Serguei
>
>
>>
>> Thanks,
>> Coleen
>>
>>> Thanks,
>>> Serguei
>>>
>>>
>>> On 9/2/14 5:29 AM, Coleen Phillimore wrote:
>>>>
>>>> Serguei, I didn't answer one of your questions.
>>>>
>>>> On 8/28/14, 5:43 PM, serguei.spitsyn at oracle.com wrote:
>>>>>> This bit is set during purging previous versions when all methods
>>>>>> have been marked on_stack() if found in various places. The bit
>>>>>> is only used for setting breakpoints.
>>>>>
>>>>> I had to ask slightly different.
>>>>> "How precise must be the control of this bit?"
>>>>> Part of this question is the question below about what happens
>>>>> when the method invocation is finished.
>>>>> I realized now that it can impact only setting breakpoints.
>>>>> Suppose, we did not clear the bit in time and then another
>>>>> breakpoint is set.
>>>>> The only bad thing is that this new breakpoint will be useless.
>>>>
>>>> Yes. We set the on_stack bit which causes setting the
>>>> is_running_emcp bit during safepoints for class redefinition and
>>>> class unloading. After the safepoint, the on_stack bit is
>>>> cleared. After the safepoint, we may also set breakpoints using
>>>> the is_running_emcp bit. If the method has exited we would set a
>>>> breakpoint in a method that is never reached. But this shouldn't
>>>> be noticeable to the programmer.
>>>>
>>>> The method's is_running_emcp bit and maybe metadata would be
>>>> cleaned up the next time we do class unloading at a safepoint.
>>>>
>>>>>
>>>>> But let me look at new webrev first to see if any update is needed
>>>>> here.
>>>>>
>>>>
>>>> Yes, please review this again and let me know if this does what I
>>>> claim it does.
>>>>
>>>> Thank you!
>>>> Coleen
>>>
>>
>
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