Performance issues on Windows

[alexbsys]

ghc::filesystem is slower than std::filesystem on Windows when working with directories that contain many files.
I tested it in directory which contains about 16600 files.
Enumerating files via ghc::filesystem takes about 50-60 seconds on my PC (NTFS, HDD).
Enumerating files via std::filesystem used from example code takes 2.5-5 seconds (same directory).

  fs::path object_path = fs::u8path(start_path);

  for (auto &entry_path : fs::directory_iterator(object_path)) {
    fs::file_status entry_status = fs::status(entry_path);
    // some simple operations like push names to list
  }

I think the problem corresponds to non-optimized iterator operators and 'status' operations.
Please look on call stack on image. Simple iterator increment operation requires call for 'status', which processed all data gathering, strings assigning, etc. So, for single pass for one file, 'status' operation called so many times with gathering a lot of not necessary information.

Maybe caching of file names, extensions, root paths, etc. instead calculation on each call, will make library faster.

[gulrak]

Yeah, that is more than I expected. There is an overhead in ghc::filesystem::path on Windows because of it using the generic representation as internal representation instead of the native one, but I guess this might be more the result of the directory_iterator / directory_entry workings.

I'll do some tests and try to optimize it. I'm quite busy currently, but I plan to work on this and #70, the other Windows issue the next days, hopefully tomorrow.

[gulrak]

I found at least one of the reasons going through the source with the call stack as hint, the evaluation of the permissions "executable" part is quite expensive and most of the time permissions of a directory_entry are not used.

I think I'll defer the X-Part of the flags until permissions are actually used, and additionally try to optimize their evaluation.

I'm on a trip right now (browsing code on the phone), but I think a test of that should be on a branch sometime tomorrow. Not sure about the impact yet.

[gulrak]

Sorry, there is some delay with the availability of a branch on this.

My test was with recursive_directory_iterator as I had no large enough single directory, and there where differences between the results of iterating a huge tree with ghc::filesystem and MS std::filesystem in the number of regular files and the sum of their sizes, so I took quite some time to analyze this to find a possible hidden bug, and it seems I found an issue in the std::filesystem implementation that I'm going to report over there.

I hope to give it another shot after work today, but I wanted to report back why there is no branch yet.

[alexbsys]

ok, thanks for information! I'm ready for test

[gulrak]

My work is on branch: feature-73-performance-optimization

Okay, let me start by saying, I was not able to reproduce the huge gap from your measurements. My tests where on my Dev-Laptop, with an SSD (I have no Windows system with HDD available), and I'm using VisualStudio 2019, 16.7.4 and my baseline measurements are:

Recursive iteration over C:\Windows (nested directories, 324681 entries, 247597 files) with additional fs::status() call, as in the given code:

Implementation	Time	Relative
std::filesystem	47.2s	100%
ghc::filesystem	51.5s	+9%
ghc::filesystem with reduced path overhead	49.7s	+5%

Not that impressive, still useful, with the difference of internal implementation in mind. I want to point out, that the additional fs::status(entry_path) call is the mail culprit of time taken. The fs::directory_entry from the iterator already has a status, so using fs::file_status entry_status = entry_path.status() inside the loop instead leads to:

Implementation	Time	Relative
std::filesystem	10.5s	100%
ghc::filesystem	13.1s	+25%
ghc::filesystem with reduced path overhead	11.8s	+12%

So my optimizations halved the overhead of the different internal representations, I'm happy with that.

I then also created a single test directory with 20k files, to have a comparison without the impact of many directory_iterator creations during the work of the recursive_directory_iterator used on the C:\Windows folder:

Implementation	Time
std::filesystem with fs::status(entry_path)	1480ms
ghc::filesystem with fs::status(entry_path)	1570ms
ghc::filesystem with fs::status(entry_path) and path optimizations	1500ms
std::filesystem with entry_path.status()	47ms
ghc::filesystem with entry_path.status()	66ms
ghc::filesystem with entry_path.status() and path optimizations	44ms

So in the best case, no additional status call, just flat iteration, the optimizations make it faster than std::filesystem on average. I guess there is some non-optimal code in there as well, as it should be faster with native storage of the path.

I hope this helps your performance issues as well, even if I couldn't replicates your numbers.

[gulrak]

Released with v1.3.6