optimize KlassInfoTable size to power of 2

[臧琳]

Dear All,
     I am investigating JMap utility for large heap (~200GB) and found that
the current KlassInfoTable’s _num_buckets size(20011) may not be optimal.
     My observation is that when “jmap histo” iterating objects , the object’s klass
   is identified and then hash idx in KlassInfoTable’s buckets[] is calculated by mod
of _num_bucktes, which would issue a heavy instruction idiv on x86 platform. It
means for every object scanned, a idiv instruction is issued, which lag the performance espically when
there are large number of objects in heap. Hence if the _num_buckets can be changed to
a pow of 2, (e.g. 65536) the idiv can be replaced with a faster instruction such as shl (left bit shift),
And I have prepared a patch for this change.
  My question is that why 20011 is used now? is there any special reason? And is there
any potential problem if I change the value to 65536, or 32768?
  Thanks!

BRs,
Lin
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