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

Peter Levart peter.levart at gmail.com
Tue Nov 4 16:48:14 UTC 2014

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 
- 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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