Code Review Request: 6819085 G1: use larger and/or user settable region size

[Tony Printezis]

Vikram,

Hi. With testing we have done so far with 1G-4G heaps, 1m region size 
seemed to work well. However, We have been recently experimenting with 
24g-32g heaps and for that we've seen that larger region sizes are 
beneficial (in order to cut down the number of regions). Which is one of 
the reasons why I worked on this CR. Also note that, after the region 
size calculation you showed, I do bound the region size by 
MIN_REGION_SIZE and MAX_REGION_SIZE. So, the minimum region size, 
currently, will be 1m and in some cases ,we might use a larger one (up 
to currently 32m).

Tony

Vikram A wrote:
> hi Tony,
>
> Just thinking out loud here.
>
> You had once outlined  that we get most benefit with 1MB regions based
> on your testing.
>
>  in arguments.cpp
>
> +   uintx region_size = G1HeapRegionSize;
> +   if (FLAG_IS_DEFAULT(G1HeapRegionSize)) {
> +     // We base the automatic calculation on the min heap size. This
> +     // can be problematic if the spread between min and max is quite
> +     // wide, imagine -Xms128m -Xmx32g. But, if we decided it based on
> +     // the max size, the region size might be way too small for the
> +     // min size. Either way, some users might have to set the region
> +     // size manually for some -Xms / -Xmx combos.
> +     region_size = min_heap_size() / TARGET_REGION_NUMBER;
> +   }
>
> with this for many small application it seems to me that the region
> size is disturbed.
>
> eg. in the above doc. you mention
> -Xms128, consider the heap to be  and TARGET_REGION_NUMBER is defined as 2048.
> So, total heap will be
> 128 * 1024(K) = 131072K
> now dividing 131072K/2048 = 64K for each region.
>
> If this inpterpretation is right and there is a reason for doing it
> this way, aren't we losing the adv we gained with 1024K region.
> this seems like a very small region size and would induce frequent GC,
> although could be with small pause time.
>
> I would assume that many enterprise would run their java with larger
> heaps than this, but considering this is also for every one, should
> the 1024K region size be useful to keep by default while allowing this
> fix to modify it for advanced user.
>
> Like I said just my thoughts.
>
> regards,
> Vikram.
>
> On Fri, Jul 31, 2009 at 2:51 AM, Tony Printezis
> <Antonios.Printezis at sun.com> wrote:
>   
>> http://cr.openjdk.java.net/~tonyp/6819085/webrev.0/
>>
>> The idea is to allow the user to set the region size with a parameter (-XX:G1HeapRegionSize=N).
>>
>> Tony
>>
>> --
>> ----------------------------------------------------------------------
>> | Tony Printezis, Staff Engineer    | Sun Microsystems Inc.          |
>> |                                   | MS BUR02-311                   |
>> | e-mail: tony.printezis at sun.com    | 35 Network Drive               |
>> | office: +1 781 442 0998 (x20998)  | Burlington, MA01803-0902, USA  |
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>>
>>     

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| Tony Printezis, Staff Engineer   | Sun Microsystems Inc.          |
|                                  | MS UBUR02-311                  |
| e-mail: tony.printezis at sun.com   | 35 Network Drive               |
| office: +1 781 442 0998 (x20998) | Burlington, MA 01803-2756, USA |
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