copy_file is around 6 times slower than it was in 1.70

[barendgehrels]

Upgrading Boost from 1.70 to 1.75 we discovered that the performance of copy_file is quite decreased.

Especially if you copy many small files this is noticeable.

Here are my measurements (3 runs of both versions):

107500 - copy /home/gehrels/current_boost/boost time 42933 ms
107500 - copy /home/gehrels/current_boost/boost time 43256 ms
107500 - copy /home/gehrels/current_boost/boost time 42684 ms

107000 - copy /home/gehrels/current_boost/boost time 6646 ms
107000 - copy /home/gehrels/current_boost/boost time 6844 ms
107000 - copy /home/gehrels/current_boost/boost time 6574 ms

(I clean the file cache before each run using echo 3 | sudo tee /proc/sys/vm/drop_caches)

Attached the minimal example, creating these measurements, copying the "boost" folder from Boost (15344 files on my system)

use_boost_fs.cpp.zip

[Lastique]

What OS are you running? If Linux, what is your kernel version?

[barendgehrels]

I'm running on Linux nl1lxl-110718 4.15.0-142-generic #146-Ubuntu SMP Tue Apr 13 01:11:19 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux

But the problem also occurs on computers of my colleagues and for our test suites.

And, another observation, if I comment these lines in operations.cpp:

//#if defined(BOOST_FILESYSTEM_HAS_FDATASYNC)
//  err = ::fdatasync(outfile.fd);
//#else
//  err = ::fsync(outfile.fd);
//#endif

the degradation is gone. So apparently it's the syncing taking time.

[Lastique]

It is expected then. The comment above that call explains why syncing is performed.

[barendgehrels]

Sure, I read it. But the std library (C++17) doesn't do this. Shouldn't they be compatible?

I think that the syncing, if any, should be optional. At any rate, there should not be a regression of 6 times which cannot be avoided somehow.

[barendgehrels]

Thanks for the fix!