http://llvm.org/bugs/show_bug.cgi?id=21275
Bug ID: 21275
Summary: Performance of unordered_multimap::insert much slower
than GCC 4.8.2
Product: libc++
Version: 3.4
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: All Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected]
Classification: Unclassified
The performance of unordered_multimap::insert is very slow in LLVM libc++
when used on large data sets that have degenerate non-uniform key distribution.
// g++ -std=c++11 -O3 umm.cpp -o umm && time ./umm
// 13.5s for Apple LLVM version 6.0 (clang-600.0.51) (based on LLVM 3.5svn).
// 0.016s for GCC version 4.8.2.
#include <unordered_map>
int main() {
std::unordered_multimap<int,int> m;
for (int x = 0; x < 100000; ++x)
m.insert(std::pair<int,int>(x%4, x));
return 0;
}
I recognize that a another ticket was filed on this issue and marked as
WONTFIX.
But even using max_load_factor() does not help LLVM libc++ get anywhere near
the performance of GCC.
One cannot always know in advance what the key distribution of the data set
will
be - it can be uniform or degenerate. Data sets can be multi-gigabyte and would
not be conducive to multiple passes to determine key distribution. As a
workaround I used #ifdefs with an ordered std::multimap on Mac which is only
twice as slow as GCC 4.8.2 libstdc++'s unordered_multimap for this scenario,
which is not ideal.
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