Young generation configuration

Tony Printezis tony.printezis at sun.com
Fri Sep 11 14:17:55 PDT 2009



Paul Hohensee wrote:
> You can try out compressed pointers in 6u14.  It just won't be quite as
> fast as the version that's going into 6u18.  6u14 with compressed pointers
> will still be quite a bit faster than without.
>
> One of the gc guys may correct me, but UseAdaptiveGCBoundary allows
> the vm to ergonomically move the boundary between old and young generations,
> effectively resizing them.  I don't know if it's bit-rotted, and I seem 
> to remember
> that there wasn't much benefit.  But maybe we just didn't have a good 
> use case.
>   
Also, it's ParallelGC-only, IIRC.
> What I meant by the last paragraph was that with the tenuring threshold 
> set at
> 15 (which is what the log says), and with only 7 young gcs in the log, 
> we can't
> see at what age (or if) between 8 and 15 the survivor size goes down to 
> something
> reasonable.  If it doesn't, it might be worth it to us to revisit 
> increasing the age
> limit for 64-bit.
>   
Paul, the problem in Jeff's case is that even at age 1 he copies 1GB or 
so. So, maybe, setting a small MTT and having more CMS cycles might be 
the right option for him.

Tony
> jeff.lloyd at algorithmics.com wrote:
>   
>> Thanks for your response Paul.
>>
>> I'll take another look at the parallel collector.  
>>
>> That's a good point about the -XX:+UseCompressedOops.  We started off
>> with heaps bigger than 32G so I had left that option out.  I'll put it
>> back in and definitely try out 6u18 when it's available.
>>
>> What about the option -XX:+UseAdaptiveGCBoundary?  I don't see it
>> referenced very often.  Would it be helpful in a case like mine?
>>
>> I'm not sure I understand your last paragraph.  What is the period of
>> time that you would be interested in seeing?
>>
>> Jeff
>>
>> -----Original Message-----
>> From: Paul.Hohensee at Sun.COM [mailto:Paul.Hohensee at Sun.COM] 
>> Sent: Friday, September 11, 2009 1:23 PM
>> To: Tony Printezis
>> Cc: Jeff Lloyd; hotspot-gc-use at openjdk.java.net
>> Subject: Re: Young generation configuration
>>
>> Another alternative mentioned in Tony and Charlie's J1 slides is the 
>> parallel
>> collector.  If, as Tony says, you can make the young gen large enough to
>>
>> avoid
>> promotion, and you really do have a steady state old gen, then which old
>> gen
>> collector you use wouldn't matter much to pause times, given that young
>> gen pause times seem to be your immediate problem.
>>
>> It may be that you just need more hardware threads to collect such a big
>>
>> young
>> gen too.  You might vary the number of gc threads to see how that
>> affects
>> collection times.  If there's significant differences, then you need
>> more
>> hardware threads, i.e., a bigger machine.
>>
>> You might also try using compressed pointers via -XX:+UseCompressedOops.
>> That should cut down the total survivor size significantly, perhaps
>> enough
>> to that your current hardware threads can collect significantly faster.
>>
>> Heap size
>> will be limited to < 32gb, but you're app will probably fit.  A more 
>> efficient
>> version of compressed pointers will be available in 6u18, btw.
>>
>> I notice that none of your logs shows more than age 7 stats even though
>> the
>> tenuring threshold is 15.  It'd be nice to see if anything dies before
>> then.
>>
>> Paul
>>
>> Tony Printezis wrote:
>>   
>>     
>>> Jeff,
>>>
>>> Hi. I had a very brief look at your logs. Yes, your app does seem to 
>>> need to copy quite a lot (I don't think I've ever seen 1-2GB of data 
>>> being copied in age 1!!!). From what I've seen from the space sizes, 
>>> you're doing the right thing (i.e., you're consistent with what we 
>>> talked about during the talk): you have quite large young gen and a 
>>> reasonably sized old gen. But the sheer amount of surviving objects is
>>>     
>>>       
>>   
>>     
>>> what's getting you. How much larger can you make your young gen? I
>>>     
>>>       
>> think 
>>   
>>     
>>> in this case, the larger, the better.  Maybe, you can also try 
>>> MaxTenuringThreshold=1. This goes against our general advice, but this
>>>     
>>>       
>>   
>>     
>>> might decrease the amount of objects being copied during young GCs, at
>>>     
>>>       
>>   
>>     
>>> the expense of more frequent CMS cycles...
>>>
>>> Tony
>>>
>>> jeff.lloyd at algorithmics.com wrote:
>>>   
>>>     
>>>       
>>>> Hi,
>>>>
>>>>  
>>>>
>>>> I'm new to this list and I have a few questions about tuning my young
>>>>       
>>>>         
>>   
>>     
>>>> generation gc.
>>>>
>>>>  
>>>>
>>>> I have chosen to use the CMS garbage collector because my application
>>>>       
>>>>         
>>   
>>     
>>>> is a relatively large reporting server that has a web front end and 
>>>> therefore needs to have minimal pauses. 
>>>>
>>>>  
>>>>
>>>> I am using java 1.6.0_16 64-bit on redhat 5.2 intel 8x3GHz and 64GB
>>>>       
>>>>         
>> ram.
>>   
>>     
>>>>  
>>>>
>>>> The machine is dedicated to this JVM.
>>>>
>>>>  
>>>>
>>>> My steady-state was calculated as follows:
>>>>
>>>> -          A typical number of users logged in and viewed several
>>>>       
>>>>         
>> reports
>>   
>>     
>>>> -          Stopped user actions and performed a manual full GC
>>>>
>>>> -          Look at the amount of heap used and take that number as
>>>>       
>>>>         
>> the 
>>   
>>     
>>>> steady-state memory requirement
>>>>
>>>>  
>>>>
>>>> In this case my heap usage was ~10GB.  In order to handle variance or
>>>>       
>>>>         
>>   
>>     
>>>> spikes I sized my old generation at 15-20GB.
>>>>
>>>>  
>>>>
>>>> I sized my young generation at 32-42GB and used survivor ratios of 1,
>>>>       
>>>>         
>>   
>>     
>>>> 2, 3 and 6.
>>>>
>>>>  
>>>>
>>>> My goal is to maximize throughput and minimize pauses.  I'm willing
>>>>       
>>>>         
>> to 
>>   
>>     
>>>> sacrifice ram to increase speed.
>>>>
>>>>  
>>>>
>>>> I have attached several of my many gc logs.  The file gc_48G.txt is 
>>>> just using CMS without any other tuning, and the results are much 
>>>> worse than what I have been able to accomplish with other settings.  
>>>> The best results are in the files gc_52G_20Gold_32Gyoung_2sr.txt and 
>>>> gc_57G_15Gold_42Gyoung_1sr.txt.
>>>>
>>>>  
>>>>
>>>> The problem is that some of the pauses are just too long.
>>>>
>>>>  
>>>>
>>>> Is there a way to reduce the pause time any more than I have it now?
>>>>
>>>> Am I heading in the right direction?  I ask because the default 
>>>> settings are so different than what I have been heading towards.
>>>>
>>>>  
>>>>
>>>> The best reference I have found on what good gc logs look like come 
>>>> from brief examples presented at JavaOne this year by Tony Printezis 
>>>> and Charlie Hunt.  But I don't seem to be able to get logs that 
>>>> resemble their tenuring patterns.
>>>>
>>>>  
>>>>
>>>> I think I have a lot of medium-lived objects instead of nice 
>>>> short-lived ones.
>>>>
>>>>  
>>>>
>>>> Are there any good practices for apps with objects like this?
>>>>
>>>>  
>>>>
>>>> Thanks,
>>>>
>>>> Jeff
>>>>
>>>>  
>>>>
>>>>  
>>>>
>>>>       
>>>>         
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