C++ memcopy from mapped file too slow

Question

I would like to ask if anybody sees a bottle bottleneck in my code or any way to optimize it. I am thinking about if my code has a fault somewhere or if I need to choose a completely new approach.

I have memory-mapped a file, and I need to read doubles from this memory-mapped file. I need to do this around 100.000 times as fast as possible.

I was expecting that it would be quite fast in Release mode, but that is not the case. The first time I do it, it takes over 5 seconds. The next time it takes around 200 ms. This is a bit faster (I guess it has to do with the way Windows handles a memory-mapped file), but it is still too slow.

void clsMapping::FeedJoinFeaturesFromMap(vector<double> &uJoinFeatures,int uHPIndex)
{
    int iBytePos=this->Content()[uHPIndex];
    int iByteCount=16*sizeof(double);

    uJoinFeatures.resize(16);
    memcpy(&uJoinFeatures[0], &((char*)(m_pVoiceData))[iBytePos],iByteCount);
}

Does anybody see a way to improve my code? I hardcoded the iByteCountCount, but that did not really change anything.

Thank you for your ideas.

Answer 1

You're reading 12.5MB of data from the file. That's not so much, but it's still not trivial.

The difference between your first and second run is probably due to file caching - the second time you want to read the file, the data is already in memory so less I/O is required.

However, 5 seconds for reading 12.5MB of data is still a lot. The only reason I can find for this is that your doubles are scattered all over the file, requiring Windows read a lot more than 12.5MB to memory.

You can avoid memory mapping altogether. If the data is stored in order in the file (not consecutive, but in order - you can read the data without seeking back), you can try avoiding the memory mapped file altogether, and just seek your way to the right place.

I doubt this will help much. Other things you can do is reorder your file, if it's at all possible, or place it on an SSD.