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Comment by idoubtit | Hacker News Reader

My experiments confirms your analyze.

On a SATA hard drive using ext4, I copied a directory with a large hierarchy of file and 216MB overall. After 2 warm-ups, I launch this simple benchmark.

    time cp -r moodle X
    13.76s s (0.06s u + 2.44s k) 18% CPU

    time fcp-0.1.0-x86_64-unknown-linux-gnu moodle X
    46.12s s (0.15s u + 2.47s k) 3% CPU

I emptied linux cache before each run with `echo 1 > /proc/sys/vm/drop_caches`, but this does not remove the HDD buffering. I suspect that fcp would be worse in real life conditions (no warm-up).

This does not mean the fcp is bad or not performant. But it some cases, it can be 3x slower.

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What do you mean by "After 2 warm-ups"? I've seen this expression in the context of benchmarks before and it puzzles me.

Does it mean you are doing this operation enough times to trick the kernel to cache everything in RAM? Or you run it 3 times and chose the fastest timings?

I'm not sure about the specific context here, but benchmark warm-ups are often used to make sure that the program's run time is nothing but that, and doesn't include things like setting up an environment or compiling to a cache.

I think there are sometimes arguments to be made that, for the user, that is run time. When using benchmarks to compare programs like this, though, I think isolating the "actual" (i.e. useful) program execution is productive.

First run: fcp binary has to be read from disk, so it's slow(ish) to start.

Second run: fcp binary is cached by the OS in RAM so it starts up faster.

Third run: just to make sure :)

This is just one of the things of a type of cache that can be warmed up.

Since you've already started, can you do a benchmark of a pair of copies started at same time?

One copy operation is the little files and deep hierarchy. The other copy operation is just a big file. Both operations start at same time and use the same tool.

I'm curious what cp and fcp do to the cache as they operate.

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