RFR (S): 8227719: G1 pending cards estimation too conservative for cost prediction

[Kim Barrett]

> On Jul 19, 2019, at 6:42 PM, Kim Barrett <kim.barrett at oracle.com> wrote:
> 
>> On Jul 19, 2019, at 10:15 AM, Thomas Schatzl <thomas.schatzl at oracle.com> wrote:
>> New webrev at
>> 
>> http://cr.openjdk.java.net/~tschatzl/8227719/webrev.0_to_1 (diff)
>> http://cr.openjdk.java.net/~tschatzl/8227719/webrev.1 (full)
>> 
>> which removes the buffer count, and replaces the
>> "completed_buffers_num" method with what you suggested.
>> 
>> I did some very cursory perf testing with no differences.
>> 
>> Thanks,
>> Thomas
> 
> Looks good.

Oops, I realized later that I missed something.

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src/hotspot/share/gc/g1/g1DirtyCardQueue.hpp
 156     return align_up(num_entries_in_completed_buffers(), buffer_size()) / buffer_size();

align_up requires the alignment value to be a power of 2, but the
buffer size is not required to be a power of 2, so this doesn't work.
It just so happens that the default buffer_size *is* a power of 2
(G1UpdateBufferSize defaults to 256), and we probably don't have any
tests that use a non-default buffer_size.

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