Hi,
On 11/13/2012 10:40 PM, François Dumont wrote:
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 <fdum...@gcc.gnu.org>
* 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.
Thanks. First blush the patch looks good but please give us a few days
to analyze the details of it, we don't want to make mistakes for 4.8.
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.
Ah good. I think we finally have nailed the core performance issue. And,
as it turns out, I'm a bit confused about the logic we have in place now
for the defaults: can you please summarize what we are doing and which
are the trade offs (leaving out the technicalities having to do with the
final types)? I think the most interesting are three:
1- std::hash<int>
2- std::hash<std::string>
3- user_defined_hash<xxx> which cannot throw
In the first we should normally not cache; in the second, from a
performance point of view (from the exception safety point of view we
could do both, because std::hash<std::string> doesn't throw anyway) it
would be better to cache; the third case is rather tricky, because, like
the case of std::string, from the exception safety point of view we
could do both, thus it's purely a performance issue. Do I understand
correctly that currently we handle 2- and 3- above in the same way, thus
we cache? It seems to me that whereas that kind of default makes a lot
of sense for std::string, doesn't necessarily make sense for everything
else, and it seems to me that such kind of default makes a suboptimal
use of the knowledge we have via __is_noexcept_hash that the functor
doesn't throw. That seems instead a sort of user-hint to not cache!
Given the unfortunate situation that the user has no way to explicitly
pick a behavior when instantiating the container, we can imagine that he
can anyway provide a strong if indirect hint by decorating or not with
noexcept the call operator. We could even document that as part of our
implementation defined behavior. How does it sound? Do we have a way to
figure out what other implementations are doing? Outside std::hash, it
should be pretty easy to instantiate with a special functor which
internally keeps a counter... if we have evidence that the other best
implementations don't cache for 3- we should definitely do the same.
To summarize my intuitions are (again, leaving out the final technicalities)
a- std::hash specializations for scalar types -> no cache
b- std::hash specialization for for std::string (or maybe
everything else, for simplicity) -> cache
c- user defined functor -> cache or not basing on __is_noexcept_hash
Jon?
Thanks!
Paolo.
