RFR(S): 8209162: Page size selection does not always select optimal page size

[Stefan Johansson]

On 2020-04-21 13:22, Thomas Schatzl wrote:
> Hi,
> 
> On 21.04.20 13:18, Stefan Johansson wrote:
>> Hi Ivan,
>>
>> I'm not very familiar with this code, but one observation. This is a 
>> bit similar to what I looked at recently for G1. There the region size 
>> was different for a given heap size depending on if Xms was set or 
>> not. Here we can run into a situation where the page size differ 
>> depending on Xms, and this also feels a bit strange. Would there be a 
>> problem to simplify this to a single line and always just care about min:
>> const size_t page_sz = os::page_size_for_region_aligned(MinHeapSize, 4);
>>
>> I'm trying to understand when this would be a problem, but can't think 
>> of anything straight away.
>>
> 
>    the RFE is a suggestion to maximize page sizes when Xms == Xmx. I.e. 
> if you do that, you do not care about uncommitting memory, so there is 
> no need for being able to uncommit. In this case it is favorable to use 
> maximum page size for throughput.

I agree with that, and only looking at MinHeapSize will achieve that, 
since for a fixed heap it is the same as MaxHeapSize. I'm just trying to 
come up with a case where the min-calculation leads to using a larger 
page size than we want so that we need to do the max/8 calculation. 
Something like:
-Xmx10m -Xms8m => 4k pages because 10/8 is below 2mb, while 8/4 == 2mb. 
Why is this good? I guess it might be good if -Xmx was 9m, but in such 
case, wouldn't some other alignment step in and prevent us from using 
2mb pages?

Stefan

> 
> Thanks,
>    Thomas
>