optimize KlassInfoTable size to power of 2

[JC Beyler]

Hi all,

I was playing with this yesterday because I was curious to see the
different code generation patterns so I had the webrev ready to go:

Webrev: http://cr.openjdk.java.net/~jcbeyler/8215228/webrev.00/
Bug: https://bugs.openjdk.java.net/browse/JDK-8215228

@Lin: check but I'd imagine this is what you had originally done but you
also changed the size

Thanks,
Jc




On Tue, Dec 11, 2018 at 3:44 AM 臧琳 <zanglin5 at jd.com> wrote:

>
> 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>
> EAC8 43EB D3EF DB98 CC77 2FAD A5CD 6035 332F A671
>


-- 

Thanks,
Jc
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