[GNC-dev] Slow user interface in 3.x gnucash (for large files), and potential optimization
jralls at ceridwen.us
Fri Jun 15 16:47:21 EDT 2018
> On Jun 15, 2018, at 12:55 PM, Christian Stimming <christian at 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
> 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 .
>> 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.
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.
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