RFR(S) 8246019 PerfClassTraceTime slows down VM start-up

Thu Jun 18 11:38:50 UTC 2020

On 2020-06-17 05:19, Ioi Lam wrote:
> 
> 
> On 6/16/20 6:20 PM, David Holmes wrote:
>> Hi Ioi,
>>
>> On 17/06/2020 6:14 am, Ioi Lam wrote:
>>> https://bugs.openjdk.java.net/browse/JDK-8246019
>>> http://cr.openjdk.java.net/~iklam/jdk16/8246019-avoid-PerfClassTraceTime.v01/ 
>>>
>>>
>>> PerfClassTraceTime is (a rarely used feature) for measuring the time 
>>> spent during class linking and initialization.
>>
>> "A special command jcmd <process id/main class> PerfCounter.print 
>> prints all performance counters in the process."
>>
>> How do you know this is a "rarely used feature"?
> Hi David,
> 
> Sure, the counter will be dumped, but by "rarely used" -- I mean no one 
> will find this particular counter useful, and no one will be actively 
> looking at it.
> 
> I changed two parts of the code -- class init and class linking.
> 
> For class initialization, the counter may be useful for people who want 
> to know how much time is spent in their <clinit> functions, and my patch 
> doesn't change that. It only avoids using the counter when a class has 
> no <clinit>, i.e., we know that the counter counts nothing (except for a 
> logging statement).
> 
> =====
> 
> For class linking, no user code is executed, so it only measures VM 
> code. If it's useful for anyone, that would be VM engineers like me who 
> are trying to optimize the speed of class loading. However, due to the 
> overhead of the counter vs what it's trying to measure, the results are 
> pretty meaningless.
> 
> Note that I've not disabled the counter altogether. Instead, I disable 
> it only when linking a CDS shared class, and we know that very little is 
> happening for this class (e.g., no verification).
> 
> I think the class linking timer might have been useful 15 years ago when 
> it was introduced, or it might be useful today when CDS is disabled. But 
> with CDS enabled, we are paying a constant price that seems to benefit 
> no one.
> 
> I think we should short-circuit it when it seems appropriate. If this 
> indeed causes problems for our users, it's easy to re-enable it. That's 
> better than just keeping this forever just because we're afraid to touch 
> anything.

I think this seems like well-rounded approach overall, but this assumes
that we're mostly measuring the overhead of measurement here. I don't
doubt that's the case for the scenarios you're excluding here and now,
but it's hard to guarantee this property hold in the future.

Perhaps a diagnostic flag to enable timing unconditionally would be
appropriate? With such a flag we could verify that the time deltas of
running some applications with and without the flag roughly matches the
time delta in reported linking time. If they diverge, we might need to
adjust the conditions.

> 
>>
>> I find it hard to evaluate whether this short-circuiting of the time 
>> tracing is reasonable or not. Obviously any monitoring mechanism 
>> should impose minimal overhead compared to what is being measured, and 
>> these timers fall short in that regard. But if these stats become 
>> meaningless then they may as well be removed.
>>
>> I think the serviceability folk (cc'd) need to evaluate this in the 
>> context of the M&M tools.

As a complement (or even alternative) there might be ways we can reduce
time-to-measure overheads. E.g, JFR added
FastUnorderedElapsedCounterSource (share/utilities/ticks.hpp) which uses
rdtsc if available (x86 - fallback to os::elapsed_counter otherwise).

This might be a reasonable alternative for the Perf* timers, which
should be short-running events on a single thread.

/Claes

>>
>>> However, it's quite expensive and it needs to start and stop a bunch 
>>> of timers. With CDS, it's quite often for the overhead of the timer 
>>> itself to be much more than the time it's trying to measure, giving 
>>> unreliable measurement.
>>>
>>> In this patch, when it's clear that the init and linking will be very 
>>> quick, I disable the timer and count only the number of invocations. 
>>> This shows a small improvement in start-up
>>
>> I'm curious if you tried to forcing EagerInitialization to be true to 
>> see how that improves the baseline. I've always noticed eager_init in 
>> the code, but hadn't realized it is disabled by default.
>>
> 
> I think it cannot be done by default, as it will violate the JLS. A 
> class can be initialized only when it's touched by bytecodes.
> 
> It can also backfire as we may load many classes without initializing 
> them. E.g., during bytecode verification, we load many classes and just 
> check that one is a supertype of another.
> 
> Thanks
> - Ioi
> 
>> Cheers,
>> David
>> -----
>>
>>> Results of " perf stat -r 100 bin/java -Xshare:on 
>>> -XX:SharedArchiveFile=jdk2.jsa -Xint -version "
>>>
>>> 59623970 59341935 (-282035)   -----  41.774  41.591 ( -0.183) -
>>> 59623495 59331646 (-291849)   -----  41.696  41.165 ( -0.531) --
>>> 59627148 59329526 (-297622)   -----  41.249  41.094 ( -0.155) -
>>> 59612439 59340760 (-271679)   ----   41.773  40.657 ( -1.116) -----
>>> 59626438 59335681 (-290757)   -----  41.683  40.901 ( -0.782) ----
>>> 59618436 59338953 (-279483)   -----  41.861  41.249 ( -0.612) ---
>>> 59608782 59340173 (-268609)   ----   41.198  41.508 (  0.310) +
>>> 59614612 59325177 (-289435)   -----  41.397  41.738 (  0.341) ++
>>> 59615905 59344006 (-271899)   ----   41.921  40.969 ( -0.952) ----
>>> 59635867 59333147 (-302720)   -----  41.491  40.836 ( -0.655) ---
>>> ================================================
>>> 59620708 59336100 (-284608)   -----  41.604  41.169 ( -0.434) --
>>> instruction delta =      -284608    -0.4774%
>>> time        delta =       -0.434 ms -1.0435%
>>>
>>> The number of PerfClassTraceTime's used is reduced from 564 to 116 
>>> (so we have an overhead of about 715 instructions per use, yikes!).
> 