[9] RFR(S): 8059606: Enable per-method usage of CompileThresholdScaling (per-method compilation thresholds)

Zoltán Majó zoltan.majo at oracle.com
Tue Jan 20 18:46:24 UTC 2015

Hi Vladimir,

thank you for the feedback!

On 01/20/2015 06:48 PM, Vladimir Kozlov wrote:
> We usually exit with error message if a flag's value is incorrect - 
> vm_exit_during_initialization().
> We never overwrite incorrect value and hide what we did.

I was not aware of that convention. I changed the check in arguments.cpp 
so that the VM exits if the *global* CompileThresholdScaling flag has a 
negative value (instead of overwriting it).

Currently, CompileCommand=option does not accept negative values for 
flags of type double, so a *per-method* CompileThresholdScaling cannot 
have a negative value.

But the behavior of CompileCommand might change in the future, so I 
changed the

Arguments::scaled_freq_log(intx freq_log, scale)
Arguments::scaled_compile_threshold(intx threshold, double scale)

methods to return the unscaled value of parameters freq_log and 
threshold, respectively, if scale is negative.

I hope this change is fine. Flags specified using CommandCommand=option 
are always listed on the output at startup, so maybe this does not count 
as hiding overwritten flag values.

Here is the new webrev: http://cr.openjdk.java.net/~zmajo/8059606/webrev.05/

Thank you and best regards,


> Thanks,
> Vladimir
> On 1/20/15 5:29 AM, Zoltán Majó wrote:
>> Hi Vladimir,
>> thanks for the review!
>> On 01/19/2015 07:36 PM, Vladimir Kozlov wrote:
>>> What was changed in globalDefinitions.hpp? Webrev shows nothing 
>>> (sometimes it does not show spacing changes).
>> Yes, it was some whitespace I've removed around line 1150:
>> -  uintptr_t p =  1;
>> +  uintptr_t p = 1;
>> The newest webrev does not show that either. In addition to that, 
>> I've updated two comments in the newest webrev.
>>> In arguments.cpp, please, add check that CompileThresholdScaling is 
>>> not negative.
>> I added the check. If CompileThresholdScaling is negative, we set its 
>> value to 1.0 (the default value).
>> Here is the new webrev: 
>> http://cr.openjdk.java.net/~zmajo/8059606/webrev.04/
>> The webrev link is the same as the one in my latest reply to John.
>> All JPRT tests pass.
>> Thank you!
>> Best regards,
>> Zoltan
>>> Otherwise this looks good to me.
>>> Thanks,
>>> Vladimir
>>> On 1/19/15 8:56 AM, Zoltán Majó wrote:
>>>> Hi John,
>>>> On 01/16/2015 08:59 PM, John Rose wrote:
>>>>> On Jan 16, 2015, at 5:05 AM, Zoltán Majó <zoltan.majo at oracle.com> 
>>>>> wrote:
>>>>>> Here is the new webrev: 
>>>>>> http://cr.openjdk.java.net/~zmajo/8059606/webrev.02/
>>>>> Reviewed, with some comments.
>>>> thank you for your review and for the feedback! Please see detailed 
>>>> comments below.
>>>>> Overall, I like the cleanups along the way.
>>>>> The basic idea of replacing a hard-coded 'mask' with an 
>>>>> addressable variable is sound and nicely executed.
>>>>> I suppose that idea by itself is "S" small, but this really is a 
>>>>> "M" or even "L" change, as Vladimir says, especially
>>>>> since the enhanced logic is spread all around many files.
>>>> I agree that this is not a small change any more, but I would like 
>>>> to keep the subject of this discussion unchanged so
>>>> that we don't move to a different thread (unless you or Vladimir 
>>>> think it is better to change it).
>>>>> How have you regression tested this?
>>>> I used our performance infrastructure. I collected performance data 
>>>> on 6 different architectures for 41 benchmark
>>>> programs/suites. The data show that per-method compilation 
>>>> thresholds do not result in a statistically significant
>>>> change of performance. One benchmark, Footprint3-Client, degrades 
>>>> ~0.5% on the X86 Client VM, but I think that is
>>>> negligible.
>>>>> Have you verified that the compilation sequence doesn't change for 
>>>>> a non-trivial use case?  A slip in the assembly
>>>>> code (for example) might cause a comparison against a garbage mask 
>>>>> or limit that could cause compilation decisions to
>>>>> go crazy quietly.  I didn't spot any such bug, but they are hard 
>>>>> to spot and sometimes quiet.
>>>> I did extensive manual testing on all architectures targeted by the 
>>>> patch.
>>>> I checked that a target method (a method for which per-method 
>>>> compilation thresholds have been specified) is indeed
>>>> compiled sooner/later than methods for which global compilation 
>>>> thresholds are in effect. I ran tests for all
>>>> combinations of +/-TieredCompilation, +/-ProfileInterpreter, and 
>>>> +/-UseOnStackReplacement
>>>> on each architecture that we support. While working on this issue 
>>>> I've discovered and reported two interpreter-related
>>>> bugs, 8068652 and 8068505.
>>>> I cannot, unfortunately, guarantee that my changes are error-free, 
>>>> but I did my best to catch any possible error that I
>>>> can think of and that I can check.
>>>>> In the sparc assembly code (more than one file), the live range of 
>>>>> Rcounters has increased, since it is used to supply
>>>>> limits as well as to update the counter (which happens early in 
>>>>> the code).
>>>>> To make it easier to maintain the code, I suggest renaming 
>>>>> Rcounters to G3_method_counters.
>>>>> (As you can see, when a register has a logical name but has a 
>>>>> complicated live range, we add the hardware name is to
>>>>> the logical name, to make it easier to spot interfering uses, when 
>>>>> manually editing code.)
>>>> Thanks for the suggestion, I've changed the register's name.
>>>>> If scale==0.0 is a valid input checked specially in compileBroker, 
>>>>> perhaps the effect of a zero should be documented?
>>>>> Suggest adding to globals.hpp:
>>>>>     "; values greater than one delay counter overflow; zero forces 
>>>>> counter overflow immediately"
>>>>>     "; can be set as a per-method  option."
>>>> If CompileThresholdScaling is set to 0.0, all methods are interpreted.
>>>> The reason is that CompileThreshold==0 has been historically 
>>>> equivalent to setting -Xint to true. Setting
>>>> CompileThresholdScaling to 0.0 scales down CompileThreshold to 0 
>>>> and we wanted to keep VM behavior consistent when we've
>>>> added support for the global CompileThresholdScaling flag in 
>>>> JDK-8059604.
>>>> If you think it would be good if we changed the meaning of 
>>>> CompileThreshold==0, please let me know. I'll file an RFE for
>>>> it and change it. As CompileThreshold is a product flag, we will 
>>>> need CCC approval for that change.
>>>> As you've suggested, I added a comment to globals.hpp that 
>>>> precisely describes the behavior of CompileThresholdScaling.
>>>>> Question:  What if both a global scale and a method option for 
>>>>> scale are both set?  Is the global one ignored?  Do
>>>>> they multiply?  It's worth specifying it explicitly (and checking 
>>>>> that the logic DTRT).
>>>> Global and per-method values multiply. That behavior is now 
>>>> described in the comment in globals.hpp.
>>>>> Question:  How are the log values (like Tier0InvokeNotifyFreqLog 
>>>>> or the result of get_scaled_freq_log) constrained to
>>>>> fit in a reasonable range?  (I would suppose that range is 0..31 
>>>>> or less.)  Should we have range clipping code in the
>>>>> scaler functions?
>>>> Currently InvocationCounter::number_of_count_bits=29 bits are 
>>>> reserved for counting. As a result, the value of the log2
>>>> of the notification frequency can be at most 30. I updated the 
>>>> source code accordingly.
>>>>> It would give notably simpler code, in MethodData::init, to use a 
>>>>> branch-free setup of tier_0_invoke_notify_freq_log
>>>>> etc.  Set scale = 1.0 and then update it conditionally. 
>>>>> Special-case scale=1.0 in get_scaled_freq_log to taste.
>>>> I did that.
>>>>> Same comment about less-branchy code for methodCounters.hpp.  
>>>>> (It's better to have a one-way branch that sets up
>>>>> 'scale' than a two-way branch with duplicate setups of one or more 
>>>>> variables.)
>>>> I changed the code according to your suggestions.
>>>>> In MethodCounters, I think the conditional scaling of 
>>>>> _interpreter_backward_branch_limit is going to confuse someone,
>>>>> at some point.  It should be scaled, unconditionally, like its 
>>>>> sibling variables.  (That would remove another somewhat
>>>>> verbose initialization branch!)
>>>> The value of the *global* interpreter backward branch limit 
>>>> (InvocationCounter::InterpreterBackwardBranchLimit) is
>>>> computed based on the value of CompileThreshold (in 
>>>> InvocationCounter::reinitialize()). The backward branch limit is
>>>> assigned a different value depending on whether interpreter 
>>>> profiling is enabled or not.
>>>> The logic of the *per-method* interpreter backward branch limit 
>>>> (MethodCounters::_interpreter_backward_branch_limit) is
>>>> intended to be identical to that of the global branch limit. 
>>>> Therefore, I'm afraid that we have to keep the if-then-else
>>>> construct initializing _interpreter_backward_branch_limit in 
>>>> methodCounters.hpp. I hope that I understood right what
>>>> you've previously suggested.
>>>>> Small style nit:  The noise-word "get_" is discouraged in the 
>>>>> style doc:
>>>>>>   • Getter accessor names are noun phrases, with no "get_" noise 
>>>>>> word. Boolean getters can also begin with "is_" or
>>>>>> "has_".
>>>>>> https://wiki.openjdk.java.net/display/HotSpot/StyleGuide
>>>>> Arguments.cpp follows this rule partially (in older code maybe?). 
>>>>> It would be better to decrease counter-examples to
>>>>> the rule instead of increase them.
>>>>> Bigger style nit:  Since the functions are not getting a preset 
>>>>> value (from the arguments) but rather normalizing a
>>>>> provided argument value, I suggest naming them 
>>>>> "scale_compile_threshold" (i.e., a verb phrase instead of a noun
>>>>> phrase).  Again from the style doc:
>>>>>>   • Other method names are verb phrases, as if commands to the 
>>>>>> receiver.
>>>> I did not know that, thanks for pointing it out to me. I changed
>>>> get_scaled_compile_threshold -> scaled_compiled_threshold
>>>> get_scaled_freq_log -> scaled_freq_log.
>>>>> Since you are providing overloads of the scaling functions, the 
>>>>> header file should either contain inline code for the
>>>>> convenience methods, or else document how the optional argument 
>>>>> ('scale') defaults.  I'd prefer inline code, since it
>>>>> is simple.  It's as much text to document with a comment as just 
>>>>> to supply the inline overload.
>>>> I inlined the convenience methods into the header file.
>>>> Here is the new webrev: 
>>>> http://cr.openjdk.java.net/~zmajo/8059606/webrev.03/
>>>>> As I said before, nice work!
>>>> Thank you!
>>>> Best regards,
>>>> Zoltan
>>>>> — John

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