字符串向量与字符串指针向量的STL排序性能比较

Question

I tried to compare the performance of STL sort on vector of strings and vector of pointers to strings.

I expected the pointers version to outperform, but the actual results for 5 million randomly generated strings are

vector of strings : 12.06 seconds
vector of pointers to strings : 16.75 seconds

What explains this behavior? I expected swapping pointers to strings should be faster than swapping string objects.

The 5 million strings were generated by converting random integers.
Compiled with (gcc 4.9.3): g++ -std=c++11 -Wall
CPU: Xeon X5650

// sort vector of strings
 int main(int argc, char *argv[])
    {
      const int numElements=5000000;
      srand(time(NULL));
      vector<string> vec(numElements);

      for (int i = 0; i < numElements; i++)
            vec[i] = std::to_string(rand() % numElements);

      unsigned before = clock();

      sort(vec.begin(), vec.end());

      cout<< "Time to sort: " << clock() - before << endl;

       for (int i = 0; i < numElements; i++)
         cout << vec[i] << endl;

      return 0;
    }



// sort vector of pointers to strings
    bool comparePtrToString (string *s1, string *s2)
    {
      return (*s1 < *s2);
    }

    int main(int argc, char *argv[])
    {
      const int numElements=5000000;
      srand(time(NULL));
      vector<string *> vec(numElements);

      for (int i = 0; i < numElements; i++)
            vec[i] = new string( to_string(rand() % numElements));

      unsigned before = clock();

      sort(vec.begin(), vec.end(), comparePtrToString);

      cout<< "Time to sort: " << clock() - before << endl;

       for (int i = 0; i < numElements; i++)
         cout << *vec[i] << endl;

      return 0;
    }

Answer 1

This is because all the operations that sort performs on strings is moves and swaps. Both move and swap for an std::string are constant time operations, meaning that they only involve changing some pointers.

Therefore, for both sorts moving of the data has the same performance overhead. However, in case of pointers to strings you pay some extra cost to dereference the pointers on each comparison, which causes it to be noticeably slower.

Answer 2

In the first case the internal pointers to representations of the strings are swapped and not the complete data copied.

You should not expect any benefit from the implementation with pointers, which in fact is slower, since the pointers have to be dereferenced additionally, to perform the comparison.

Answer 3

What explains this behavior? I expected swapping pointers to strings should be faster than swapping string objects.

There's various things going on here which could impact performance.

1. Swapping is relatively cheap both ways. Swapping strings tends to always be a shallow operation (just swapping PODs like pointers and integrals) for large strings and possibly deep for small strings (but still quite cheap -- implementation-dependent). So swapping strings tends to be pretty cheap overall, and typically not much more expensive than simply swapping pointers to them*.

   [ sizeof(string) is certanly bigger than sizeof(string*) , but it's not an astronomical difference basically as the operation still occurs in constant-time, and quite a bit cheaper in this context when the string fields already have to be fetched into a faster form of memory for the comparator, giving us temporal locality with respect to its fields.]

2. String contents must be accessed anyway both ways. Even the pointer version of your comparator has to examine the string contents (including the fields designating size and capacity ). As a result, we end up paying the memory cost of fetching the data for the string contents regardless. Naturally if you just sorted the strings by pointer address (ex: without using a comparator) instead of a lexicographical comparison of the string contents, the performance edge should shift towards the pointer version since that would reduce the amount of data accessed considerably while improving spatial locality (more pointers can fit in a cache line than strings, eg).

3. The pointer version is scattering (or at least increasing the stride of) the string fields in memory. For the pointer version, you're allocating each string on the free store (in addition to the string contents which may or may not be allocated on the free store). That can disperse the memory and reduce locality of reference, so you're potentially incurring a greater cost in the comparator that way with increased cache misses. Even if a sequential allocation of this sort results in a very contiguous set of pages being allocated (ideal scenario), the stride to get from one string's fields to the next would tend to get at least a little larger because of the allocation metadata/alignment overhead (not all allocators require metadata to be stored directly in a chunk, but typically they will at least add some small overhead to the chunk size).

   It might be simpler to attribute this to the cost of dereferencing the pointer but it's not so much the cost of the mov/load instruction doing the memory addressing that's expensive (in this relative context) as loading from slower/bigger forms of memory that aren't already cached/paged to faster, smaller memory. Allocating each string individually on the free store will typically increase this cost whether it's due to a loss of contiguity or a larger constant stride between each string entry (in an ideal case).

   Even at a basic level without trying too hard to diagnose what's happening at the memory level, this increases the total size of the data that the machine has to look at (string contents/fields + pointer address) in addition to reduced locality/larger or variable strides (typically if you increase the amount of data accessed, it has to at least have improved locality to have a good chance of being beneficial). You might start to see more comparable times if you just sorted pointers to strings that were allocated contiguously (not in terms of the string contents which we have no control over, but just contiguous in terms of the adjacent string objects themselves -- effectively pointers to strings stored in an array). Then you'd get back the spatial locality at least for the string fields in addition to packing the data associated more tightly within a contiguous space.

Swapping smaller data types like indices or pointers can sometimes offer a benefit but they typically need to avoid examining the original contents of the data they refer to or provide a significantly cheaper swap/move behavior (in this case string is already cheap and becomes cheaper in this context considering temporal locality) or both.

Answer 4

Well, a std::string is typically about 3-4 times as big as a std::string* .
So just straight-up swapping two of the former shuffles that much more memory around.

But that is dwarfed by the following effects:

1. Locality of reference. You need to follow one more pointer to a random position to read the string.
2. More memory-usage: A pointer plus bookkeeping per allocation of each std::string .

Both put extra demand on caching, and the former cannot even be prefetched.

Answer 5

Swaping containers change just container's content, in string case is the pointer to first character of string, not whole string.

In case vectors of pointers of strings you performed one additional step - casting pointers