From patchwork Tue Nov 13 21:40:43 2012 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Subject: [Bug, libstdc++/54075, 4.7.1] unordered_map insert still slower than 4.6.2 Date: Tue, 13 Nov 2012 11:40:43 -0000 From: =?utf-8?b?RnJhbsOnb2lzIER1bW9udCA8ZnJzLmR1bW9udEBnbWFpbC5jb20+?= X-Patchwork-Id: 198789 Message-Id: <50A2BE5B.3030809@gmail.com> To: Paolo Carlini Cc: Jonathan Wakely , "libstdc++@gcc.gnu.org" , gcc-patches Here is the proposal to remove shrinking feature from hash policy. I have also considered your remark regarding usage of lower_bound so _M_bkt_for_elements doesn't call _M_next_bkt (calling lower_bound) anymore. For 2 of the 3 calls it was only a source of redundant lower_bound invocations, in the last case I call _M_next_bkt explicitly. 2012-11-13 François Dumont * include/bits/hashtable_policy.h (_Prime_rehash_policy): Remove automatic shrink. (_Prime_rehash_policy::_M_bkt_for_elements): Do not call _M_next_bkt anymore. (_Prime_rehash_policy::_M_next_bkt): Move usage of _S_growth_factor ... (_Prime_rehash_policy::_M_need_rehash): ... here. * include/bits/hashtable.h (_Hashtable<>): Adapt. Tested under linux x86_64, normal and debug modes. Regarding performance, I have done a small evolution of the 54075.cc test proposed last time. It is now checking performance with and without cache of hash code. Result is: 54075.cc std::unordered_set 300000 Foo insertions without cache 9r 9u 0s 13765616mem 0pf 54075.cc std::unordered_set 300000 Foo insertions with cache 14r 13u 0s 18562064mem 0pf 54075.cc std::tr1::unordered_set 300000 Foo insertions without cache 9r 8u 1s 13765616mem 0pf 54075.cc std::tr1::unordered_set 300000 Foo insertions with cache 14r 13u 0s 18561952mem 0pf So the difference of performance in this case only seems to come from caching the hash code or not. In reported use case default behavior of std::unordered_set is to cache hash codes and std::tr1::unordered_set not to cache it. We should perhaps review default behavior regarding caching the hash code. Perhaps cache it if the hash functor can throw and not cache it otherwise, not easy to find out what's best to do. François On 11/09/2012 11:50 AM, Paolo Carlini wrote: > Hi again, > > + // To get previous bound we use _S_growth * 2 to avoid > ocillations in the > + // number of buckets when looping on insertion/removal of elements. > const unsigned long* __prev_bkt > - = std::lower_bound(__prime_list + 1, __next_bkt, __n / > _S_growth_factor); > + = std::lower_bound(__prime_list + 1, __next_bkt, > + __n / _S_growth_factor / _S_growth_factor); > > Looks like, here you are dividing by _S_Growth ^ 2? Is it intended? > But anyway, in my opinion the very my _M_prev_resize idea (thus rehash > shrinking, right?) is proving quite fragile and we also got negative > comments from the users about shrinking, which is new. Before pursuing > it further, I think we should double check what the other > implementations do, are they also shrinking? Because otherwise we > could, at least for the time being, remove the related bits and save > ourselves many headaches... > > Paolo. > Index: include/bits/hashtable.h =================================================================== --- include/bits/hashtable.h (revision 193484) +++ include/bits/hashtable.h (working copy) @@ -806,11 +806,6 @@ _M_rehash_policy() { _M_bucket_count = _M_rehash_policy._M_next_bkt(__bucket_hint); - - // We don't want the rehash policy to ask for the hashtable to - // shrink on the first insertion so we need to reset its - // previous resize level. - _M_rehash_policy._M_prev_resize = 0; _M_buckets = _M_allocate_buckets(_M_bucket_count); } @@ -834,16 +829,12 @@ _M_element_count(0), _M_rehash_policy() { + auto __nb_elems = __detail::__distance_fw(__f, __l); _M_bucket_count = - _M_rehash_policy._M_bkt_for_elements(__detail::__distance_fw(__f, - __l)); - if (_M_bucket_count <= __bucket_hint) - _M_bucket_count = _M_rehash_policy._M_next_bkt(__bucket_hint); + _M_rehash_policy._M_next_bkt( + std::max(_M_rehash_policy._M_bkt_for_elements(__nb_elems), + __bucket_hint)); - // We don't want the rehash policy to ask for the hashtable to - // shrink on the first insertion so we need to reset its - // previous resize level. - _M_rehash_policy._M_prev_resize = 0; _M_buckets = _M_allocate_buckets(_M_bucket_count); __try { @@ -990,6 +981,7 @@ __rehash_policy(const _RehashPolicy& __pol) { size_type __n_bkt = __pol._M_bkt_for_elements(_M_element_count); + __n_bkt = __pol._M_next_bkt(__n_bkt); if (__n_bkt != _M_bucket_count) _M_rehash(__n_bkt, _M_rehash_policy._M_state()); _M_rehash_policy = __pol; @@ -1641,19 +1633,12 @@ { const __rehash_state& __saved_state = _M_rehash_policy._M_state(); std::size_t __buckets - = _M_rehash_policy._M_bkt_for_elements(_M_element_count + 1); - if (__buckets <= __n) - __buckets = _M_rehash_policy._M_next_bkt(__n); + = std::max(_M_rehash_policy._M_bkt_for_elements(_M_element_count + 1), + __n); + __buckets = _M_rehash_policy._M_next_bkt(__buckets); if (__buckets != _M_bucket_count) - { - _M_rehash(__buckets, __saved_state); - - // We don't want the rehash policy to ask for the hashtable to shrink - // on the next insertion so we need to reset its previous resize - // level. - _M_rehash_policy._M_prev_resize = 0; - } + _M_rehash(__buckets, __saved_state); else // No rehash, restore previous state to keep a consistent state. _M_rehash_policy._M_reset(__saved_state); Index: include/bits/hashtable_policy.h =================================================================== --- include/bits/hashtable_policy.h (revision 193484) +++ include/bits/hashtable_policy.h (working copy) @@ -358,7 +358,7 @@ struct _Prime_rehash_policy { _Prime_rehash_policy(float __z = 1.0) - : _M_max_load_factor(__z), _M_prev_resize(0), _M_next_resize(0) { } + : _M_max_load_factor(__z), _M_next_resize(0) { } float max_load_factor() const noexcept @@ -380,25 +380,21 @@ _M_need_rehash(std::size_t __n_bkt, std::size_t __n_elt, std::size_t __n_ins) const; - typedef std::pair _State; + typedef std::size_t _State; _State _M_state() const - { return std::make_pair(_M_prev_resize, _M_next_resize); } + { return _M_next_resize; } void - _M_reset(const _State& __state) - { - _M_prev_resize = __state.first; - _M_next_resize = __state.second; - } + _M_reset(_State __state) + { _M_next_resize = __state; } enum { _S_n_primes = sizeof(unsigned long) != 8 ? 256 : 256 + 48 }; static const std::size_t _S_growth_factor = 2; float _M_max_load_factor; - mutable std::size_t _M_prev_resize; mutable std::size_t _M_next_resize; }; @@ -417,35 +413,28 @@ static const unsigned char __fast_bkt[12] = { 2, 2, 2, 3, 5, 5, 7, 7, 11, 11, 11, 11 }; - const std::size_t __grown_n = __n * _S_growth_factor; - if (__grown_n <= 11) + if (__n <= 11) { - _M_prev_resize = 0; _M_next_resize - = __builtin_ceil(__fast_bkt[__grown_n] + = __builtin_ceil(__fast_bkt[__n] * (long double)_M_max_load_factor); - return __fast_bkt[__grown_n]; + return __fast_bkt[__n]; } const unsigned long* __next_bkt = std::lower_bound(__prime_list + 5, __prime_list + _S_n_primes, - __grown_n); - const unsigned long* __prev_bkt - = std::lower_bound(__prime_list + 1, __next_bkt, __n / _S_growth_factor); - - _M_prev_resize - = __builtin_floor(*(__prev_bkt - 1) * (long double)_M_max_load_factor); + __n); _M_next_resize = __builtin_ceil(*__next_bkt * (long double)_M_max_load_factor); return *__next_bkt; } - // Return the smallest prime p such that alpha p >= n, where alpha + // Return the smallest integer p such that alpha p >= n, where alpha // is the load factor. inline std::size_t _Prime_rehash_policy:: _M_bkt_for_elements(std::size_t __n) const - { return _M_next_bkt(__builtin_ceil(__n / (long double)_M_max_load_factor)); } + { return __builtin_ceil(__n / (long double)_M_max_load_factor); } // Finds the smallest prime p such that alpha p > __n_elt + __n_ins. // If p > __n_bkt, return make_pair(true, p); otherwise return @@ -467,7 +456,8 @@ / (long double)_M_max_load_factor; if (__min_bkts >= __n_bkt) return std::make_pair(true, - _M_next_bkt(__builtin_floor(__min_bkts) + 1)); + _M_next_bkt(std::max(__builtin_floor(__min_bkts) + 1, + __n_bkt * _S_growth_factor))); else { _M_next_resize @@ -475,13 +465,6 @@ return std::make_pair(false, 0); } } - else if (__n_elt + __n_ins < _M_prev_resize) - { - long double __min_bkts = (__n_elt + __n_ins) - / (long double)_M_max_load_factor; - return std::make_pair(true, - _M_next_bkt(__builtin_floor(__min_bkts) + 1)); - } else return std::make_pair(false, 0); }