RFR (M): 8060697: Improve G1 Heap Growth Heuristics

Tom Benson tom.benson at oracle.com
Tue Dec 1 21:37:15 UTC 2015

Hi Jon,
Thanks very much for the review.

On 11/30/2015 3:10 PM, Jon Masamitsu wrote:
> Tom,
> http://cr.openjdk.java.net/~tbenson/8060697/webrev/src/share/vm/gc/g1/g1CollectorPolicy.cpp.frames.html
> 1053 // Compute the ratio of just this last pause time to the entire 
> time range stored
> 1054 // in the vectors.
> 1055 _last_pause_time_ratio =
> 1056 (pause_time_ms * _recent_prev_end_times_for_all_gcs_sec->num()) / 
> interval_ms;
> _last_pause_time_ratio is the ratio of the last pause over the
> average interval in the truncated sequence?  By the latter
> I mean
> (interval_ms / _recent_prev_end_times_for_all_gcs_sec->num())
> If the truncated sequence has N sample and "interval_ms" is
> measured from the oldest sample, I'm calling interval_ms / N
> the average interval.
> Is my description correct?  Why do you prefer that to the most recent
> pause time ratio?

Yes, your description is right.  Basically I want to notice the *first* 
pause that goes over the threshold, rather than waiting for the average 
over the last 10 pauses to go over.   The reason is that this will start 
the code looking for ratios that exceed the threshold (beginning a 
"sampling window" so to speak), and I want to do that as soon as possible.

If by this: "Why do you prefer that to the most recent pause time 
ratio?" you mean "Why not just compare the last pause to the last single 
interval?", the reason is that comparing it to the entire range 'smooths 
over' some transiently-more-frequent GCs that don't really reflect a 
change in heap occupancy.  I see this happening in specjbb sometimes.  
By only comparing against the last interval, needless growth happens 
more often, resulting in higher run-to-run variability.   Another 
approach would be to raise the minimum number of threshold crossing 
pauses that are needed before triggering growth - but I don't want to 
delay that for cases where the need is real.  Thomas commented to me 
that that transient behavior is likely to be due to something we 
ultimately want to fix, anyway.   But the current approach of comparing 
against the whole recorded range seems to help alleviate the side effect 
of needless growth.

> Should the 1*M below be 1 region size?
> 1545     const size_t min_expand_bytes = 1*M;

Hmm, good question.  I didn't change that.  It could certainly be 
MIN_REGION_SIZE, which == 1*M.  I think using the actual region size 
would likely only have an effect when the heap is nearly at minimum or 
maximum sizes. In between, the math is likely to result in a size larger 
than that anyway.  I could try it.

> As the uncommitted space in the heap drops, the grow rate drops.
> 1550     size_t expand_bytes_via_pct =
> 1551       uncommitted_bytes * G1ExpandByPercentOfAvailable / 100;
> The scale_factor will increase that by up to a factor of 2, the policy
> seems to grow slowly to the maximum.  Is there a reason not to get
> to the maximum heap size quickly?

Yes, I thought about this as well.  This attribute (shrinking the growth 
increment as we approach the limit) is there in both the old and new 
code, but the new code may scale the value up.   What I considered, but 
didn't try, was to use a fixed percentage of the entire heap, once we 
have reached a certain threshold by doubling the size.   That value 
would still be scaled according to the pause ratios.   I decided not to 
pursue it for now, since the results looked acceptable and it could be 
done as a follow up.

However, it won't be hard to try, and I can do so if there's agreement 
that the rest of this approach seems reasonable.

> Jon
> On 11/25/2015 06:06 AM, Tom Benson wrote:
>> Hi Kim,
>> Thanks very much for the review.  I've implemented all your suggestions.
>> Down
>> About this:
>>> I suspect you are introducing some implicit conversions that will cause
>>> problems for the SAP folks; see discussion of 8143215:
>> ... I think there's one, which is:
>>     expand_bytes = expand_bytes * scale_factor;
>> scale_factor is explicitly limited to being between 0.2 and 2.0, and 
>> expand_bytes is fraction of the heap size, so there's no chance of 
>> overflow.  Would you object to the static cast in this case?  How 
>> about with a comment?
>> Tom
>> On 11/24/2015 9:55 PM, Kim Barrett wrote:
>>> On Nov 23, 2015, at 10:02 PM, Tom Benson <tom.benson at oracle.com> wrote:
>>>> Hi,
>>>> Here is a proposed change to the G1 heap growth code for review. 
>>>> I've added a detailed description to the CR, but here is the short 
>>>> version: After a GC pause, the average ratio of time spent in 
>>>> recent GC pauses vs overall time is computed. If it exceeds 
>>>> GCTimeRatio, the heap is expanded by a fixed amount.  With the new 
>>>> code, some deficiencies in the ratio tracking are addressed, and 
>>>> the expansion size is scaled according to how much the desired 
>>>> ratio is, on average, exceeded by.  The target ratio itself is also 
>>>> scaled at the lowest heap sizes.
>>>> The case that triggered this was actually JDK-8132077, where the 
>>>> JVM'08 Compress benchmark saw a 40% degradation.  It was due to the 
>>>> heap being about half the size in some runs, because of the way 
>>>> heap growth worked.
>>>> I'm still collecting the final performance data for this version, 
>>>> and will attach it to the CR.  Earlier experimental versions showed 
>>>> good improvements in consistency of heap sizes.  A couple of 
>>>> benchmarks average a percentage point or two lower, while others 
>>>> improve by that much or more.  No growth percentage or scaling is 
>>>> going to be ideal for every test, but the goal was to maintain 
>>>> performance without growing too large. In fact, some tests now use 
>>>> much smaller heaps.
>>>> CR:
>>>> https://bugs.openjdk.java.net/browse/JDK-8060697
>>>> Webrev:
>>>> http://cr.openjdk.java.net/~tbenson/8060697/webrev/
>>> Generally looks good; just a few very minor things, most of which 
>>> you can
>>> take or ignore as you prefer.  I don't need a new webrev for any of 
>>> these.
>>> The comments were very helpful in understanding what's going on.
>>> I suspect you are introducing some implicit conversions that will cause
>>> problems for the SAP folks; see discussion of 8143215: gcc 4.1.2: 
>>> fix three
>>> issues breaking the build.  But the resolution for that is still being
>>> discussed, and we don't have an easy way to discover these for 
>>> ourselves, so
>>> I don't think you should worry about it here right now.
>>> ------------------------------------------------------------------------------ 
>>> src/share/vm/gc/g1/g1CollectorPolicy.cpp
>>> 1571       static double const MinScaleDownFactor = 0.2;
>>> 1572       static double const MaxScaleUpFactor = 2;
>>> 1573       static double const StartScaleDownAt = _gc_overhead_perc;
>>> 1574       static double const StartScaleUpAt = _gc_overhead_perc * 
>>> 1.5;
>>> 1575       static double const ScaleUpRange = _gc_overhead_perc * 2.0;
>>> I suggest these not be static.
>>> It doesn't really matter for the first two.
>>> But for the others, there is a hidden cost to making them static, 
>>> due to
>>> some compilers ensuring thread-safe initialization.  C++11 mandates
>>> thread-safe initialization of function scoped statics.  gcc has 
>>> implemented
>>> that starting circa gcc4.0 (if I recall correctly), controlled by a CLA
>>> (-f[no]-threadsafe-statics).  Visual Studio 2013 also includes this 
>>> feature,
>>> as part of their incremental rollout of C++11 (and later) features.  
>>> I don't
>>> know about other compilers.
>>> The cost of making them static is likely at least comparable to 
>>> computing
>>> them.  And making them static implies the _gc_overhead_perc is 
>>> effectively a
>>> constant, which appears to be true today, but who knows what will 
>>> happen
>>> tomorrow.
>>> ------------------------------------------------------------------------------ 
>>> src/share/vm/gc/g1/g1CollectorPolicy.cpp
>>> 1587         scale_factor = MAX2<double>(scale_factor, 
>>> MinScaleDownFactor);
>>> 1590         scale_factor = MIN2<double>(scale_factor, 
>>> MaxScaleUpFactor);
>>> The explicit double template parameter isn't needed here, since the
>>> arguments are already both doubles.
>>> ------------------------------------------------------------------------------ 
>>> src/share/vm/gc/g1/g1CollectorPolicy.cpp
>>> 1525     threshold = (threshold * ((double)_g1->capacity() / 
>>> (double)(_g1->max_capacity() / 2)));
>>> This might be easier to read if it used "threshold *= ...".
>>> ------------------------------------------------------------------------------ 
>>> src/share/vm/gc/g1/g1CollectorPolicy.cpp
>>> 1526     threshold = MAX2<double>(threshold, 1);
>>> The explicit double template parameter wouldn't be needed if "1" was
>>> replaced with "1.0".
>>> ------------------------------------------------------------------------------ 

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