[9] RFR(L) 8013267 : move MemberNameTable from native code to Java heap, use to intern MemberNames

[Peter Levart]

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.chase at 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.levart at gmail.com> wrote:
>>
>>> On 11/04/2014 04:19 PM, David Chase wrote:
>>>> On 2014-11-04, at 5:07 AM, Peter Levart <peter.levart at 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

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