Re: [boost] GSOC 2015 : Project on Concurrent Hash Tables

On Fri, Mar 6, 2015 at 7:00 PM, Niall Douglas wrote:

Move construction of the map != move construction of the buckets.

Okay. I am trying to achieve move construction in the same spirit. And I am piggybacking on Niall's implementation of _rehash(). concurrent_unordered_map(concurrent_unordered_map &&old_map) BOOST_NOEXCEPT : _hasher(std::move(old_map._hasher)), _key_equal(std::move(old_map._key_equal)), _allocator(std::move(old_map._allocator)), _max_load_factor(std::move(old_map._max_load_factor)), _min_bucket_capacity(std::move(old_map._min_bucket_capacity)) { typedef decltype(_rehash_lock) rehash_lock_t; lock_guard guard(old_map._rehash_lock, adopt_lock_t()); buckets_type &old_buckets=*(old_map._buckets.load(memory_order_consume)); size_t n = old_buckets.size(); if(n) { // Lock all existing buckets for(auto &b : old_buckets) b.lock.lock(); } buckets_type *tempbuckets=new buckets_type(13); auto untempbuckets=undoer([&tempbuckets]{ delete tempbuckets; }); buckets_type *buckets=new buckets_type(n); // Swap old buckets with new buckets tempbuckets=old_map._buckets.exchange(tempbuckets, memory_order_acq_rel); // Tell all threads using old buckets to start reloading the bucket list for(auto &b : *tempbuckets) b.lock.store(2); // Simply move the old buckets into new buckets as-is for(auto obit=tempbuckets->begin(), bit=buckets->begin(); obit!=tempbuckets->end(); ++obit, ++bit) { auto &ob=*obit; auto &b=*bit; if(ob.count.load(memory_order_relaxed)) { b.items=std::move(ob.items); b.count.store(ob.count.load(memory_order_consume), memory_order_release); ob.count.store(0, memory_order_release); } } Is this the right direction?

Old buckets go into a fixed size ring buffer, and get deleted eventually.

I couldn't really figure out where the ring buffer is which you are mentioning here. What am I missing? And another question, when I use the above implementation of move constructor and later call empty() on the newly created object, I land in a segfault. I see that the empy() checks on if the lock state is 2 and continues to check the next bucket. But, I think here I have all buckets set to lock state 2. Why did the swap I performed with the tempbuckets and oldmap._buckets not help me? Any pointers will be really helpful. Thanks, Amarnath