On 02/13/2015 03:54 AM, David Holmes wrote:
Hi Tom,
If you are potentially messing with the (identity) hash of all Java
objects in the 32-bit case then this needs a broader discussion eg on
core-libs-dev (cc'd) as this will impact end-user code the most!
The rest seems okay but I'm still mulling over it. :)
Thanks,
David H.
Hi,
As I understand, this will make identity hashCode have 2^24 instead of
2^25 distinct values on 32 bit architectures, right? This will mostly
affect java.util.IdentityHashMap performance (and any use of objects
that don't override hashCode in other hashCode-based Maps). IHM has a
maximum capacity of 2^29 (key, value) slots. Performance will start to
degrade sooner - at sizes > 2^24 / 1.5 (~10M) instead of 2^25 / 1.5
(~20M) entries.
IHM has the following hashCode -> array slot index mapping function:
/**
* Returns index for Object x.
*/
private static int hash(Object x, int length) {
int h = System.identityHashCode(x);
// Multiply by -127, and left-shift to use least bit as part of
hash
return ((h << 1) - (h << 8)) & (length - 1);
}
Left-shift is added because keys are located at even indexes and
associated values are at odd indexes in the same array. So the function
to map hashCode to ordinal key index is actually:
(h - (h << 7)) & (capacity - 1)
where capacity is a power of two <= 2^29, which means that it is
necessary that 24 hash bits from Object header be mapped to lower 24
bits of Object.hashCode(). Object.hashCode() in range 0..2^24-1 should
still be enough to address the whole range of 2^29 capacity table given
the above mapping function.
So the question is, how frequent are IdentityHashMap(s) with > 10M
entries or any other HashMaps with keys that don't override
Object.hashCode().
Here's an JMH (http://openjdk.java.net/projects/code-tools/jmh/)
micro-benchmark you can use to measure the impact of change on
IdentityHashMap:
http://cr.openjdk.java.net/~plevart/misc/IHMBench/IHMBench.java
by default it creates an IdentityHashMap with size of 2^24 entries. This
is where the performance difference is expected to start to be different
between 24bit vs. 25bit hash codes. You can also try to use larger (up
to 28) 'log2size' parameter, but you might want to increate -Xmx too in
this case.
Regards, Peter
On 13/02/2015 6:14 AM, Tom Benson wrote:
Hi,
I need reviewers and a commit sponsor for changes for bug 6764713, which
will increase the size of the age field in an object header from 4 bits
to 5. This will allow a maximum MaxTenuringThreshold of 31, though the
default will remain at the current value of 15.
This includes the same change to the 32-bit version, which would close
bug 6719225 as well. In that case, the hash field in the header is
affected, losing one bit (25 bits -> 24), so I have asked for review
from hotspot-runtime-dev as well as gc-dev.
Webrev: http://cr.openjdk.java.net/~jprovino/6764713/webrev.00
JBS bug: https://bugs.openjdk.java.net/browse/JDK-6764713
Testing: JPRT and reference server performance tests
Notes:
Contrary to what earlier notes in the JBS entry said, this does not
require stronger alignment for the JavaThread structure for when biased
locking stores that pointer in the header. The JavaThread* was already
being aligned 1 power of 2 more strongly than it needed to be, so there
was an unused bit that could be stolen.
In the 32-bit version, it does require taking one bit from the hash
field, which goes from 25 to 24 bits. This is something I'd especially
like feedback on. Running reference server performance tests, I saw no
impact from this change. We *could* make this change 64-bit-only, and
leave the age field at 4 bits for the 32-bit version. If we did so, we
could also decrease the alignment required for the JavaThread* to 512
from the current 1024.
The comment changes imply that the bits available for the JavaThread*
have been reduced by 1, and that the alignment is now stronger, but
neither is true. The comments have been corrected to match the
alignment that was already enforced.
Three tests needed to be corrected to match the new limits. These check
the maximum valid values, what value represents NeverTenure, and so on.
Thanks,
Tom
