RFR (S): 8019902: G1: Use the average heap size rather than the minimum heap size to calculate the region size

Tony Printezis tprintezis at twitter.com
Thu Aug 29 08:22:34 PDT 2013


On 8/29/13 11:19 AM, Bengt Rutisson wrote:
> Sorry. My bad. I added the period back. Thanks for catching that! :)
Thanks Bengt, and with apologies for being a bit anal here. :-)

Tony

> Bengt
>
>>
>> Tony
>>
>> On 8/29/13 10:21 AM, Bengt Rutisson wrote:
>>> On 8/29/13 4:14 PM, Bengt Rutisson wrote:
>>>>
>>>> Tony,
>>>>
>>>> On 8/29/13 4:12 PM, Tony Printezis wrote:
>>>>> Bengt,
>>>>>
>>>>> Doesn't what I said also applies for users who do not set -Xms? 
>>>>> So, if -Xms is by default 6m, a user launches the VM with:
>>>>>
>>>>> java -Xmx32G ...
>>>>>
>>>>> and the region size is calculated to be 8m, what's the initial 
>>>>> heap size (8m, i.e. one region)?
>>>>
>>>> No, by default we calculate a reasonable -Xms. So as long as the 
>>>> policy uses that value and not the min heap size I think we are fine.
>>>
>>> Just to clarify a bit. In your example we get a -Xms of 128m:
>>>
>>> java -XX:+UseG1GC -XX:+PrintFlagsFinal -Xmx32G -version | grep 
>>> InitialHeapSize
>>>     uintx InitialHeapSize                          := 
>>> 134217728       {product}
>>>
>>>
>>> With my fix this means that we will get a region size based on 128m 
>>> + 32g / 2, which will turn out to be 8m. That gives us a minimum of 
>>> 16 regions (cool coincidence since that's what you suggested ;) ). 
>>> And it will give us a maximum of about 4000 regions.
>>>
>>> Before my fix we would get a regions size of 1m, which means about 
>>> 32000 regions.
>>>
>>> Bengt
>>>
>>>
>>>
>>>>
>>>> Bengt
>>>>
>>>>>
>>>>> Tony
>>>>>
>>>>> On 8/29/13 10:08 AM, Bengt Rutisson wrote:
>>>>>>
>>>>>> Hi Tony,
>>>>>>
>>>>>> Thanks for looking at this!
>>>>>>
>>>>>> Comments inline.
>>>>>>
>>>>>> On 8/29/13 3:03 PM, Tony Printezis wrote:
>>>>>>> Hi Bengt,
>>>>>>>
>>>>>>> Yeah, I struggled with this heuristic when I did the original 
>>>>>>> implementation of the heap region calculation. The issue only 
>>>>>>> arises when the gap between the min and max heap size is very 
>>>>>>> large. So, if someone launches the VM with:
>>>>>>>
>>>>>>> java -Xms32m -Xmx64g ...
>>>>>>>
>>>>>>> and G1 picks a region size of 8m, it'd start with only 4 regions 
>>>>>>> which will probably make performance right at the beginning will 
>>>>>>> be terrible (but I agree that it will be better as the heap 
>>>>>>> grows, compared to if an 1m region size was used).
>>>>>>
>>>>>> Agreed. And just to be clear. The main problem with the existing 
>>>>>> policy is that it by default always picks 1m regions if nothing 
>>>>>> is set on the command line. This is due to the fact that it is 
>>>>>> not based on the initial heap size (-Xms) but on the min heap 
>>>>>> size, which by default is in the order of 6m. So, those who set 
>>>>>> -Xms on the command line have experienced less of a problem. At 
>>>>>> least if they set -Xms to high enough values.
>>>>>>
>>>>>>>
>>>>>>> Can I suggest maybe an additional policy change? Use the avg to 
>>>>>>> calculate the region size, as you proposed, but potentially 
>>>>>>> adjust the min heap size based on a min region number (let's 
>>>>>>> pick a number of a hat: 16; you might want to revise this). So, 
>>>>>>> in the above example:
>>>>>>>
>>>>>>> -Xms32m -Xmx64g -> region size = 8m
>>>>>>>
>>>>>>> you'll actually adjust the min heap size 16 x 8m = 128m. This 
>>>>>>> will avoid the potentially bad behavior right at the start. Of 
>>>>>>> course, you'll start with a larger heap size than what the user 
>>>>>>> asked for. On the other hand, if someone uses a huge max they 
>>>>>>> probably expect the heap to grow. So starting with a large min 
>>>>>>> might be OK.
>>>>>>
>>>>>> I see your point, but I don't really like the fact that if 
>>>>>> someone explicitly sets -Xms on the command line we would ignore 
>>>>>> that and use a value that is four times as large. Also, there is 
>>>>>> the possibility to set the region size using G1HeapRegionSize on 
>>>>>> the command line. So, in this use case I kind of think it would 
>>>>>> be better to leave it up to the user to indicate if the heap is 
>>>>>> more likely to be 32m or 64g by setting the region size explicitly.
>>>>>>
>>>>>> Thanks,
>>>>>> Bengt
>>>>>>
>>>>>>>
>>>>>>> Tony
>>>>>>>
>>>>>>> On 8/29/13 5:26 AM, Bengt Rutisson wrote:
>>>>>>>>
>>>>>>>> Hi everyone,
>>>>>>>>
>>>>>>>> Could I have a couple of reviews of this change:
>>>>>>>>
>>>>>>>> http://cr.openjdk.java.net/~brutisso/8019902/webrev.00/
>>>>>>>>
>>>>>>>> The fact that G1 by default bases its region size on the 
>>>>>>>> minimum heap size means that out of the box the region size 
>>>>>>>> will always be 1M. This is a problem on large machines with 
>>>>>>>> lots of memory. We pick a large heap size but get a very small 
>>>>>>>> region size. The small regions are inefficient and cause a lot 
>>>>>>>> of memory footprint. Normally we aim to get around 2048 
>>>>>>>> regions, but on a machine with a lot of memory we might pick a 
>>>>>>>> default max heap size of 32G, which means that we will get 
>>>>>>>> ~32000 regions. This can lead to out of memory situations - 
>>>>>>>> especially on Solaris x86.
>>>>>>>>
>>>>>>>> This patch changes the heuristics for picking the region size 
>>>>>>>> to use the average between initial heap size (-Xms) and the 
>>>>>>>> maximum heap size (-Xmx). This means that for large heaps we 
>>>>>>>> will pick larger region sizes. In the 32G example we will now 
>>>>>>>> pick a region size of 8m which means that we will have 4000 
>>>>>>>> regions which is more reasonable.
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>> Bengt
>>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>
>

-- 
Tony Printezis | Staff Software Engineer | Twitter

@TonyPrintezis
tprintezis at twitter.com



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