Reuse mashed word String

[wezm]

The profile shows a good bit of time spent allocating the mashed word string. If we instead reuse it each time we shave another 20% off the benchmark.

[sts10]

Interesting -- so it's the let that's costly?

Also, can you point me to some tips on the way you're profiling the code?

[wezm]

Interesting -- so it's the let that's costly?

It's this that's expensive:

root_word.to_owned().to_owned() + second_word;

Ignoring the double to_owned, (which might have been cloning the string twice, making the impact worse) it is doing the following:

• Make a call to the OS to allocate memory to hold the copy of root_word, let's call this mashed_word.
• Copy the contents of the root_word into the newly allocated memory.

Then the + does:

• Check if mashed_word has enough capacity to append second_word (it probably doesn't)
• Make a call to the OS to expand the original memory allocation to be large enough to hold second_word as well
• Copy second_word into the newly expanded allocation.

All of that is quite fast but it's happening in a nested loop so it's happening a lot. Calls to the OS (system calls) are slower than regular function calls as there's typically a context switch amongst other things, so avoiding these can help.

The new code will mean that mashed_word will be allocated and expanded on the first pass through the loop but then it will only make a call to the system allocator if the length of root_word + second_word is larger than any other previously seen combination. So this means that for many words the operations are only:

• Check if there's capacity for root_word in mashed_word (there probably is)
• Copy root_word into mashed_word
• Check if there's capacity for second word in mashed_word (there probably is)
• Copy second_word into mashed_word

The .clear() call sets the length of mashed_word back to 0 (empty string) but it retains the allocated memory so it can be reused next time.

Also, can you point me to some tips on the way you're profiling the code?

I have added the following to the Cargo.toml so that debug symbols are present in release builds (so that the profile contains meaningful stack traces):

[profile.release]
debug = true

And then I'm profiling with perf (on Linux). I use the following to capture the profile data:

perf record --call-graph dwarf -F99 ./target/release/csafe word_lists/eff-word-list.txt

Note, I don't wait for that to finish, just let it for for a 5–10 secs then Ctrl-C to stop.

Then this to produce a report from it:

perf report -g graph | mdemangle