G1GC, Java8u40ea, Metaspace questions

[Jon Masamitsu]

Wolfgang,

Can you run you test program with UseParallelGC and see if you
see the same growth in the Metaspace?

Jon

On 2/19/2015 2:37 AM, Wolfgang Pedot wrote:
> Hi
>
> Am 18.02.2015 05:12, schrieb Yu Zhang:
>> Hi,
>>
>> After discussing with the developers, here is the explanation:
>> >shows a slow but continuous increase of the "commited" value for 
>> Metaspace even though "used" after a collect is quite stable and 
>> "commited" is shrunk after every collect.
>>
>> This might be due to fragmentation in metadata space.  Even though 
>> the total used is stable, the memory is fragmented so it has to 
>> allocate more.  If you have Flight Recorder(a commercial feature), 
>> you can see this information in JFR.
>
> Unfortunately I do not have JFR available, however I did notice that 
> the number of classes that fit into Metaspace before the collect also 
> kept increasing so it cant all be fragmentation.
>
>> >I have found out that I need to set Min and MaxMetaspaceFreeRatio 
>> really really low (like <=10%) to keep the high-water-mark in check 
>> and I dont quite see why in the numbers.
>>
>> This might be related to how the high water mark is calculated. There 
>> are some ways to improve this but not implemented yet.
>
> The more I test the more I come to the conclusion that there is 
> something strange about that calculation (or my understanding of it). 
> I am currently running a very long test on 64bit linux (>9million 
> classes loaded so far) and there is a definite upwards trend of both 
> the HWM and used-after-GC. Class unloading does not seem to be as 
> successful when a concurrent cycle is triggered by Metaspace threshold 
> compared to a Full-GC and that gap is growing over time. I guess thats 
> because the heap of the test-JVM is rather large and empty and since 
> G1 is allowed to waste 5% if it that means some classloaders stay 
> alive longer although that should stabilize at some point. Increasing 
> the usage in oldGen from ~0.5GB to ~1GB made class unloading "better" 
> for one collect.
>
> The high-point of Metaspace usage slowly increased from ~400MB to 
> ~600MB during the first part of the test and used-after-GC started at 
> 200MB and crept up to around 400MB. I decided to throw in a manual 
> FullGC during the test to see what happens and that brought the usage 
> down to ~110MB as it should be so there is no real class-leak. Since 
> MaxMetaspaceFreeRatio is configured to 30% (Min is 10%) I would have 
> expected the HWM to go down after that collect but appearantly it did 
> not, it stayed at ~600MB for the next collect and then went up another 
> ~50MB.
> I dont really see the HWM-value so when I write "HWM" I am referring 
> to the highest point of Metaspace-usage before a collect is initiated.
>
> I have attached a screenshot of jconsole showing the part with the 
> FullGC (~9:40), I reconnected to the JVM at ~8:55 so it does not cover 
> the whole uptime.
>
> Is this expected or explainable behaviour? The way I see it the high 
> points of that curve may keep on climbing until they hit the limit and 
> after that its FullGCs all the time.
>
> regards
> Wolfgang
>