Hi David,
I played a little with the idea of having a hash table instead of packed
sorted array for interning. Using ConcurrentHashMap would present quite
some memory overhead. A more compact representation is possible in the
form of a linear-scan hash table where elements of array are MemberNames
themselves:
http://cr.openjdk.java.net/~plevart/misc/MemberName.intern/jdk.06.diff/
This is a drop-in replacement for MemberName on top of your jdk.06
patch. If you have some time, you can run this with your performance
tests to see if it presents any difference. If not, then perhaps this
interning is not so performance critical after all.
Regards, Peter
On 11/07/2014 10:14 PM, David Chase wrote:
New webrev:
bug: https://bugs.openjdk.java.net/browse/JDK-8013267
webrevs:
http://cr.openjdk.java.net/~drchase/8013267/jdk.06/
http://cr.openjdk.java.net/~drchase/8013267/hotspot.06/
Changes since last:
1) refactored to put ClassData under java.lang.invoke.MemberName
2) split the data structure into two parts; handshake with JVM uses a linked
list,
which makes for a simpler backout-if-race, and Java side continues to use the
simple sorted array. This should allow easier use of (for example) fancier
data structures (like ConcurrentHashMap) if this later proves necessary.
3) Cleaned up symbol references in the new hotspot code to go through vmSymbols.
4) renamed oldCapacity to oldSize
5) ran two different benchmarks and saw no change in performance.
a) nashorn ScriptTest (see https://bugs.openjdk.java.net/browse/JDK-8014288 )
b) JMH microbenchmarks
(see bug comments for details)
And it continues to pass the previously-failing tests, as well as the new test
which has been added to hotspot/test/compiler/jsr292 .
David
On 2014-11-04, at 3:54 PM, David Chase <david.r.ch...@oracle.com> wrote:
I’m working on the initial benchmarking, and so far this arrangement (with
synchronization
and binary search for lookup, lots of barriers and linear cost insertion) has
not yet been any
slower.
I am nonetheless tempted by the 2-tables solution, because I think the simpler
JVM-side
interface that it allows is desirable.
David
On 2014-11-04, at 11:48 AM, Peter Levart <peter.lev...@gmail.com> wrote:
On 11/04/2014 04:19 PM, David Chase wrote:
On 2014-11-04, at 5:07 AM, Peter Levart <peter.lev...@gmail.com> wrote:
Are you thinking of an IdentityHashMap type of hash table (no linked-list of
elements for same bucket, just search for 1st free slot on insert)? The problem
would be how to pre-size the array. Count declared members?
It can’t be an identityHashMap, because we are interning member names.
I know it can't be IdentityHashMap - I just wondered if you were thinking of an
IdentityHashMap-like data structure in contrast to standard HashMap-like. Not
in terms of equality/hashCode used, but in terms of internal data structure.
IdentityHashMap is just an array of elements (well pairs of them - key, value
are placed in two consecutive array slots). Lookup searches for element
linearly in the array starting from hashCode based index to the element if
found or 1st empty array slot. It's very easy to implement if the only
operations are get() and put() and could be used for interning and as a shared
structure for VM to scan, but array has to be sized to at least 3/2 the number
of elements for performance to not degrade.
In spite of my grumbling about benchmarking, I’m inclined to do that and try a
couple of experiments.
One possibility would be to use two data structures, one for interning, the
other for communication with the VM.
Because there’s no lookup in the VM data stucture it can just be an array that
gets elements appended,
and the synchronization dance is much simpler.
For interning, maybe I use a ConcurrentHashMap, and I try the following idiom:
mn = resolve(args)
// deal with any errors
mn’ = chm.get(mn)
if (mn’ != null) return mn’ // hoped-for-common-case
synchronized (something) {
mn’ = chm.get(mn)
if (mn’ != null) return mn’
txn_class = mn.getDeclaringClass()
while (true) {
redef_count = txn_class.redefCount()
mn = resolve(args)
shared_array.add(mn);
// barrier, because we are a paranoid
if (redef_count = redef_count.redefCount()) {
chm.add(mn); // safe to publish to other Java threads.
return mn;
}
shared_array.drop_last(); // Try again
}
}
(Idiom gets slightly revised for the one or two other intern use cases, but
this is the basic idea).
Yes, that's similar to what I suggested by using a linked-list of MemberName(s) instead of the
"shared_array" (easier to reason about ordering of writes) and a sorted array of
MemberName(s) instead of the "chm" in your scheme above. ConcurrentHashMap would
certainly be the most performant solution in terms of lookup/insertion-time and concurrent
throughput, but it will use more heap than a simple packed array of MemberNames. CHM is much better
now in JDK8 though regarding heap use.
A combination of the two approaches is also possible:
- instead of maintaining a "shared_array" of MemberName(s), have them form a
linked-list (you trade a slot in array for 'next' pointer in MemberName)
- use ConcurrentHashMap for interning.
Regards, Peter
David
And another way to view this is that we’re now quibbling about performance,
when we still
have an existing correctness problem that this patch solves, so maybe we should
just get this
done and then file an RFE.
Perhaps, yes. But note that questions about JMM and ordering of writes to array
elements are about correctness, not performance.
Regards, Peter
David
