RFR(L) 8195099: Low latency hashtable for read-mostly scenarios

[Gerard Ziemski]

hi Robbin,

Still reviewing, but I have a couple of questions and some feedback I wanted to ask/share with you so far:


#1 Where does the choice of "12" for _task_size_log2  come from in:

 BucketsOperation(ConcurrentHashTable<VALUE, CONFIG, F>* cht)
   : _cht(cht), _next_to_claim(0), _task_size_log2(12),
   _stop_task(0), _size_log2(0) {}

Shouldn't "12" be a constant?


#2 Where does "8192" come from in:

 // SpinYield would be unfair here
 while (!this->trylock()) {
   if ((++i) == 8192) {

Shouldn't "8192" be a constant?


#3 How come "_first" doesn't need to be volatile to be used in Atomic::cmpxchg ?

Node* _first;
...
 if (Atomic::cmpxchg(node, &_first, expect) == expect) {


#4 Inconsistent parameter names - in:

 template <typename LOOKUP_FUNC, typename VALUE_FUNC>
 bool get_insert_lazy(Thread* thread, LOOKUP_FUNC& lookup, VALUE_FUNC& val_f,
                      bool* grow_hint = NULL) {
   return get_insert_lazy(thread, lookup, val_f, noOp, grow_hint);
 }

We use "val_f" with "_f” suffix, so we should have "lookup_f", not "lookup" (many other instances) and "found_f", not "foundf" in 

 template <typename LOOKUP_FUNC, typename FOUND_FUNC>
 bool get(Thread* thread, LOOKUP_FUNC& lookup, FOUND_FUNC& foundf,
          bool* grow_hint = NULL);


#5 I wasn't familiar with CRTP, so I read up on it, but I still don't see the CRTP in BaseConfig - which is the base class and which is derived? In particular I don't see "class Derived : public Base<Derived>" pattern here?

More to come…


btw. I edited the subject of the email slightly by adding the bug number to it, so it’s easy to search for.


cheers


> On Apr 26, 2018, at 2:38 AM, Robbin Ehn <robbin.ehn at oracle.com> wrote:
> 
> 
> Hi all, please review.
> 
> The lower latency of the new gcs have higher demands on runtime data-structure
> in terms of latency. This is a concurrent hashtable using the global-counter
> (8195099).
> 
> Webrev:
> http://cr.openjdk.java.net/~rehn/8195098/v0/webrev/
> 
> Bug:
> https://bugs.openjdk.java.net/browse/JDK-8195098
> 
> * Readers never blocks or spins.
> * Inserts do CAS from a read-side, need to re-CAS during re-size/deletion in targeted bucket or insert collision. (inserts are conceptually a reader)
> * Deletes locks the targeted bucket, all other buckets are free for operations.
> * Re-size locks one bucket at the time, all other buckets are free for operations.
> 
> It does concurrent re-size by doubling size and use one more bit from hash.
> That means a bucket in the old table contains nodes to either one of two buckets
> in the new table. Each node in the chain is sorted into one of the two buckets
> with concurrent readers. Shrinking just take two node chains and put them
> together in the corresponding bucket. To keep track of what is visible to the
> concurrent readers it's uses a global-counter, needed during re-size and for
> deletes.
> 
> A gtest is provided which passes on our platforms, we also have a prototype of the stringtable using this which passes tier 1-5 on our platforms.
> 
> Various people have pre-reviewed various parts, thanks! And a special thanks to
> Coleen for a lot of reviewing!
> 
> Thanks, Robbin