RFR: 8234338: ZGC: Improve small heap usage

[Per Liden]

On 11/19/19 8:06 PM, Stefan Karlsson wrote:
> Hi Per,
> 
> Looks good.
> 
> I would have preferred if the per_cpu_shared_small_page() function were 
> prefixed with use_. I was again confused by that function when reading 
> this for the second time. I'll let you decide if you agree or not.

Will fix.

Thanks for reviewing, Stefan!

/Per

> 
> Thanks,
> StefanK
> 
> On 2019-11-19 19:54, Per Liden wrote:
>> When using small heaps (like less than 128M), the heap reserve tends 
>> to take up a relatively large portion of the heap. We have quite a lot 
>> of tests that explicitly set the heap size to small values (like 
>> -Xmx8M, -Xmx16M, etc). Today, these tests often fail with OOME when 
>> using ZGC. While ZGC isn't really that focused on tiny/small heaps, we 
>> still want to make testing of ZGC easy without having to adjust these 
>> tests to use a larger heap.
>>
>> There are at least two things we can do when ZGC is given a small heap:
>> 1) Dynamically scale the medium ZPage size, and even disable medium 
>> pages all together when using tiny heaps.
>> 2) Stop using per-CPU small pages for allocations, and instead switch 
>> to using a single small page.
>>
>> With this patch, ZGC can scale down to 8M.
>>
>> Bug: https://bugs.openjdk.java.net/browse/JDK-8234338
>> Webrev: http://cr.openjdk.java.net/~pliden/8234338/webrev.0
>>
>> Testing: Tier1-7 using ZGC. A new small heap test was also added.
>>
>> /Per
>