Dear JC,
That’s exactly modification I made ☺
Thanks a lot for making the patch!
BRs,
Lin
From: JC Beyler [mailto:jcbey...@google.com]
Sent: Wednesday, December 12, 2018 2:27 AM
To: 臧琳 <zangl...@jd.com>
Cc: Andrew Haley <a...@redhat.com>; serviceability-dev@openjdk.java.net
Subject: Re: optimize KlassInfoTable size to power of 2
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 <zangl...@jd.com<mailto:zangl...@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' <a...@redhat.com<mailto:a...@redhat.com>>;
serviceability-dev@openjdk.java.net<mailto:serviceability-dev@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:a...@redhat.com<mailto:a...@redhat.com>]
Sent: Tuesday, December 11, 2018 7:10 PM
To: 臧琳 <zangl...@jd.com<mailto:zangl...@jd.com>>;
serviceability-dev@openjdk.java.net<mailto:serviceability-dev@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
