optimize KlassInfoTable size to power of 2

[臧琳]

Oh, to make it clear, when testing with 65536, I made the change to use hash & 65535 
instead of hash % 65536.


BRs,
Lin



-----Original Message-----
From: 臧琳 
Sent: Tuesday, December 11, 2018 7:36 PM
To: 'Andrew Haley' <aph at redhat.com>; serviceability-dev at openjdk.java.net
Subject: RE: optimize KlassInfoTable size to power of 2

>There is something that I do not understand. You said that GCC didn't 
>know that _num_buckets was constant. In  that case, how did GCC know not to use a divide instruction when you tried 65536?

The original code doesn't use _num_buckets directly when calculating the hash idx, it use a non-constant variable _size, a member of KlassInfoTable, which is assigned to _num_buckets. I guess it is the reason GCC didn't do the constant propagation in this case. And _size is not changed during the life of KlassInfoTable, it only set to zero at de-construction.

In my experiment, I use _num_buckets directly instead of _size to calculate hash idx. So GCC provides the optimizations.

I think your suggestion of using 65537 is valuable, And I agree that it is not easy to prove that non-prime for hash is better. So how about make the change to use constant _number_buckets directly for calculating hash idx, and leave the prime not changed. In my case, speed up JMap histo helps at avoid getting killed by a timer, especially when heap is large at ~200GB.

Do you think it is reasonable to have a patch? 

Thanks,
Lin

-----Original Message-----
From: Andrew Haley [mailto:aph at redhat.com]
Sent: Tuesday, December 11, 2018 7:10 PM
To: 臧琳 <zanglin5 at jd.com>; serviceability-dev at openjdk.java.net
Subject: Re: optimize KlassInfoTable size to power of 2

On 12/11/18 11:05 AM, Andrew Haley wrote:
> unsigned mod_m(unsigned n) {
>   unsigned tmp = n % 65536;
>   tmp -= n / 65536;
>   if (tmp >= 65537) // overflow
>     tmp += 65537;
>   return tmp;
> }

NB: this assumes that unsigned int is uint64_t.

--
Andrew Haley
Java Platform Lead Engineer
Red Hat UK Ltd. <https://www.redhat.com>
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