RFR(L): JDK-8032463 VirtualDispatch test timeout with DeoptimizeALot

Christian Thalinger christian.thalinger at oracle.com
Fri May 9 02:25:18 UTC 2014

+   nmethod* nm = (nmethod*)caller_frame.cb();
+   assert(nm != NULL && nm->is_nmethod(), "Sanity check");
There is CodeBlob::as_nmethod_or_null().

The C1 code looks good.

On May 8, 2014, at 3:28 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com> wrote:

> Looks good.
> Thanks,
> Vladimir
> On 5/8/14 12:30 PM, Igor Veresov wrote:
>> Alright, here is the update:
>> http://cr.openjdk.java.net/~iveresov/8032463/webrev.02/
>> The change is that I added a trap counter in the MDO that is used to
>> scale up the number of stack walks that are necessary for the tenured
>> method to revert back to the standard flushing policy. This required
>> some changes in c1_Runtime.cpp and deoptimization.cpp to handle these
>> deopts and allocate MDOs (in C1). The counter increment is guarded by
>> the make_not_entrant() call that guarantees that the increment happens
>> only once per method state change.
>> Also fixed the comments in sweeper.cpp and indent in globals.hpp that I
>> forgot to do the last time.
>> Thanks!
>> igor
>> On May 7, 2014, at 2:27 PM, Vladimir Kozlov <vladimir.kozlov at oracle.com
>> <mailto:vladimir.kozlov at oracle.com>> wrote:
>>> On 5/7/14 12:37 AM, Igor Veresov wrote:
>>>> Vladimir,
>>>> Thanks for the review!
>>>> Please find the reply inline.
>>>> igor
>>>> On May 6, 2014, at 5:05 PM, Vladimir Kozlov
>>>> <vladimir.kozlov at oracle.com <mailto:vladimir.kozlov at oracle.com>> wrote:
>>>>> I think we need to let SAP know about this change (counter code in
>>>>> C2) since it may affect their tiered compilation. It use the same
>>>>> Deoptimization::Reason_age.
>>>> Oh, I didn’t know that. I’ll introduce another constant.
>>>>> Can you separate C1 trap_request newa code in a separate RFE and
>>>>> push it first?
>>>> Do you think this is necessary? It so happens that the DeoptimizeStub
>>>> was not used before this by any code. So the aging scheme is the only
>>>> user of it so far.
>>> I am fine to keep it in this fix but I can't review it. Ask Christian
>>> to look.
>>>>> Can you add the field to MDO and not to MethodCounters? We doing
>>>>> this only for methods which were compiled already and have MDO.
>>>>> MethodCounters small size is important.
>>>> It was in MDO originally, unfortunately I have to make it work for
>>>> pure C1 and we usually don’t allocate MDOs in that configuration.
>>>> Although, we need to only allocate MDOs for methods with aging, a set
>>>> of which may be would be smallish..  But MDOs are kind of big.. Would
>>>> that be better?
>>> I forgot about pure C1.
>>> Chris Plummer pointed me, for RTM changes, MethodCounters are
>>> allocated for all executed methods which may not be compiled. About
>>> 30% methods are executed.
>>>>> Why the change in methodData.hpp and nmethod.cpp? '||' should be
>>>>> '&&' in nmethod::inc_decompile_count(), I think.
>>>> You’re right, for pure C1 that won’t work anyways, because there are
>>>> no MDOs. I’ll remove these for now. I probably need another decompile
>>>> counter to put a cap on the number of recompiles, see the idea below.
>>>>> In globals.hpp flags description indention is off.
>>>> Fixed.
>>> You moved it to opposite direction :) . Indention of all comments are
>>> the same in the file. They are not aligned to a flag's declaration line.
>>>>> sweeper.cpp:
>>>>> // The stack-scanning low-cost detection may not see the method was
>>>>> used (which can happen for
>>>>> // previous MinPassesBeforeFlush sweeps. Reset the counter. Stay in
>>>>> the existing
>>> You did not change above comments as I suggested.
>>>>> Why do you need to repeat? For small leaf nmethods (with no
>>>>> safepoint, for example) it could trigger continues recompilation:
>>>>> +             if (time_since_reset > MinPassesBeforeFlush * 2) {
>>>>> +               // It's been long enough, we still haven't seen it
>>>>> on stack.
>>>>> +               // Try to flush it, but enable counters the next time.
>>>>> +               mc->reset_nmethod_age();
>>>> That’s a good question. Before this change we would’ve recompiled
>>>> warm methods continuously (given some code cache pressure), so the
>>>> number of such cases is probably small. The options here with the
>>>> code aging are:
>>>> - don’t ever again flush a method that we’ve seen deopt once;
>>>> - allow the flusher machinery to get rid of it if it becomes idle again.
>>>> Currently I decided to stick with the latter but also give the method
>>>> twice the standard time initially to be noticed by the stack-walker.
>>>> You are right that there is a problem for small methods. Technically
>>>> there is always one safepoint in the epilog though, right? I’m good
>>>> with either option. Vladimir, what are your thoughts on this? Albert,
>>>> if you skimming through this, what do you think?
>>>> Here is an idea... We could count the number of decompiles for this
>>>> particular reason and use it to scale the number of stack-walks we
>>>> have to do before we flush the method again. For example we can wait
>>>> MinPassesBeforeFlush * number_of_aging_deopts() stack-walks.
>>>> Essentially this makes it unflushable in the limit - a combination of
>>>> the two options above.
>>>> I’ll need to allocate an MDO or add another counter to methodCounters
>>>> for that though, or pack a small value info method_age itself, at the
>>>> expense of a slightly bigger code. And there is an MT problem, as
>>>> usual, with counting the number of deopts.
>>> As we discussed the check will be scaled by number of recompilation.
>>> Thanks,
>>> Vladimir
>>>> Here is the updated (with the changes addressed so far) webrev:
>>>> http://cr.openjdk.java.net/~iveresov/8032463/webrev.01/
>>>> Thanks,
>>>> igor
>>>>> Thanks,
>>>>> Vladimir
>>>>> On 5/5/14 7:44 PM, Igor Veresov wrote:
>>>>>> The current implementation of the code cache flusher tries to
>>>>>> determine the working set of methods by doing periodic stack walks
>>>>>> during safepoints. Methods that are seen on the stack are
>>>>>> considered to be used by the program and are excluded from the set
>>>>>> of flush candidates. Such sampling is a great zero-overhead
>>>>>> approach to collect the working set of super-hot methods without
>>>>>> instrumentation. However it doesn’t work that good for flat
>>>>>> profiles, which the test in the bug report happens to expose
>>>>>> (thousands of methods calls sequentially one after another in the
>>>>>> loop). The solution I ended up implementing uses conservative
>>>>>> on-demand instrumentation to obtain information about the method
>>>>>> usage. The algorithms is as follows:
>>>>>> 1. Initially a method is compiled as usual (no instrumentation, no
>>>>>> overhead).
>>>>>> 2. When the sampling (stack-walking) scheme fails to detect
>>>>>> activity we flush the method (again, as usual). However before the
>>>>>> flush we set the age counter (an int added to MethodCounters) to a
>>>>>> value that instructs the compilers to instrument the code.
>>>>>> 3. If we ever request to compile this method again the aging code
>>>>>> is inserted, which decrements the counter.
>>>>>> 4. The value of the counter is then used by the sweeper to
>>>>>> determine is the method is in fact used.
>>>>>> 5. If the counter reaches zero before the sweeper is able examine
>>>>>> and reset the counter we deopt and recompile the method without the
>>>>>> counters, basically switching it back to use the sampling scheme again.
>>>>>> Webrev: http://cr.openjdk.java.net/~iveresov/8032463/webrev.00/
>>>>>> Testing:
>>>>>> - jprt,
>>>>>> - aurora-perf No performance effects with the default code cache,
>>>>>> since we have enough headroom and the flusher is not exercised that
>>>>>> much. No statistically significant effects after warmup (for spec
>>>>>> benchmarks) in stress mode (-XX:+StressCodeAging) either, that is
>>>>>> if all methods are compiled initially with counters deopt and
>>>>>> switch to sampling later after 100000 invocations.
>>>>>> igor

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