Hasnain says:

I am glad this is finally out, if only because I can finally reference Mark S's famous one ampersand commit and have people believe me and not think that I'm making shit up. Great read on profilers and also TIL the code is open source.

"A seasoned performance engineer was looking through Strobelight data and discovered that by filtering on a particular std::vector function call (using the symbolized file and line number) he could identify computationally expensive array copies that happen unintentionally with the ‘auto’ keyword in C++.

The engineer turned a few knobs, adjusted his Scuba query, and happened to notice one of these copies in a particularly hot call path in one of Meta’s largest ads services. He then cracked open his code editor to investigate whether this particular vector copy was intentional… it wasn’t.

It was a simple mistake that any engineer working in C++ has made a hundred times.

So, the engineer typed an “&” after the auto keyword to indicate we want a reference instead of a copy. It was a one-character commit, which, after it was shipped to production, equated to an estimated 15,000 servers in capacity savings per year!

Go back and re-read that sentence. One ampersand! "

Posted on 2025-01-24T05:28:29+0000