> On Jun 15, 2018, at 12:55 PM, Christian Stimming <christ...@cstimming.de>
> wrote:
>
> Am Freitag, 15. Juni 2018, 10:05:09 schrieb John Ralls:
>>>> A very good catch indeed. But pre-constructing the string in
>>>> qofbook.cpp only saves two string constructions per invocation as the
>>>> vector still has to make its own copies. I guess that its much worse
>>>> for you because the ancient gcc on Ubuntu 14.04 (gcc4.8) doesn't do
>>>> small-string optimization.
>>>
>>> Is there any reason we cant use std::string& in the vector? Or do we
>>> think that we might lose references there?
>>
>> In this instance the string is static so it would be safe to use const
>> std::string&, but while there's an is_volatile type trait there's no
>> is_static so we can't insert a static assert to catch cases where the
>> actual std::string is on the stack. That could lead to some ugly bugs.
>
> The std containers are specified in a way so that they make only sense to
> contain the values by-value (ok, this might be formulated a bit too short,
> but
> not too long ago this was a good rule of thumb). It is up to the
> implementation to make optimizations (copy-on-write and such), but the
> interface requires the programmer to think of the stored type as "by-value".
>
> Nevertheless I think my change made good use of the std::string's already
> existing optimizations. In particular, the saving was so surprisingly large
> here that I think gcc's std::string itself implements some sort of copy-on-
> write optimization, and this means its content isn't deep-copied in these
> calls.
>
> Some numbers from the valgrind/callgrind evaluation. I got the following
> number of calls, starting from qof_query_run and skipping some uninterestings
> calls in between:
>
> - 6 qof_query_run
> - 6 g_list_sort_with_data
> - 360,000 (approx.) sort_func - due to the number of splits and txn in my
> file
> - 360,000 xaccSplitOrder
> The most expensive part in this function is this next callee:
> - 360,000 qof_book_use_split_action_for_num_field
> - 360,000 (approx.) qof_book_get_property
>
> This function in turn has approx. 1,100,000 calls to std::basic_string
> constructor and destructor. The change in my commit is this:
> Before my commit, these calls also caused 1,100,000 calls to
> std::string::_S_create and std::string::_M_destroy i.e. the by-value
> constructor and destructor. After my commit, I see a mere 22,000 calls to
> std::string::_S_create and std::string::_M_destroy.
>
>> Besides, using strings is still leaving performance on the table: A string
>> comparison is n times longer than an int comparison where n is the number
>> of consecutive leading characters that are the same in the two strings. I
>> got a huge speedup on loading a few months ago because GncGUID's
>> operator==() was doing a character-wise comparison of the ascii
>> representation instead of comparing the two int64_ts of the actual GUID.
>
> Sounds good. The above numbers seem to indicate that std::string already
> optimizes away a long comparison as long as the string memory itself is
> identical. In my opinion this is good enough of an optimization.
>
>> KVP paths aren't really a good use for std::string anyway: It's a lot of
>> weight for something that's used as a human-readable index. Even static
>> char[] is heavy for this purpose. We could get a huge boost if we use
>> something like Glib's quarks [1].
>> Want to have a go at that?
>
> Err... this would mean a major refactoring of any access to the KVP system.
> No, I don't feel motivated to work into that direction. Also, if I understnad
> the above valgrind evaluation correctly, we can already achieve almost
> optimum
> (i.e. pointer comparison) by ensuring a set of std::string constants to be
> used. I think this is the most efficient way to proceed here, and I would
> just
> push this commit after some more cleanup.
Christian,
Hardly major, particularly considering that Aaron has already completely
rewritten KVP into C++ and separately changed the access from '/'-delimited
string paths to std::vector<std::string> of the tokens.
All that's required is lifting Glib's GQuark and converting it to C++, changing
KVP frame's std::vector<std::string> to std::vector<GncQuark>, and using
g_quark_intern_from_static_string instead of static std::string.
The situation you've tested is a bit of a special case because all of the uses
of the KVP_OPTIONS keys are in one file. Any keys used in more than one file
will have different static std::strings and pointer comparison won't work.
Examples include anything accessed with qof_instance_set/get. That said, the
mallocs/deallocs are 99% or more of the performance hit. If you're willing and
have time to do it your way and have time to get it done quickly then by all
means go ahead.
Regards,
John Ralls
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