CppCon 2016: Nicholas Ormrod “The strange details of std::string at Facebook"

Side note: FB also developed a string library to replace std::string ( [LINK] ). Here's a video from explaining the motivation behind the project ( [VIDEO] ).

I'm not going to take the responsibility whether it's correct or not - here's the talk I learned it from: [VIDEO]

I think this is the talk you're referring to. If not, it's still definitely worth a watch: [VIDEO]

You're all massively underestimating the memory requirements of a parsed data structure, which is very entertaining to watch. There's this rule-of-thumb that most programmers are wrong about performance costs by at least a couple of orders of magnitude, and it's on full display here. In most languages, dynamically allocated variable-sized objects have a minimum overhead of two pointer-sized fields: a pointer and a length. On a 64-bit platform, that's 16 bytes before you actually start having any data. Next, depending on the allocator used, the requested data size is probably rounded up, either to the next power of 2, or there may be a minimum allocation size of 1 pointer. That's "safe" on machines that can't do misaligned (byte-granularity) referencing directly from memory. Last but not least, most heap-based allocators have some "bookkeeping" overheads. Similarly, interpreted languages also have their internal "object" metadata overheads. Typically this is 1 or 2 extra pointers (+8 or +16 bytes). Assuming 24-48 bytes for all variable-length values is actually a pretty safe bet! There are exceptions: Unusually, C skips the 'length' value for strings by using null-termination, so C strings are often just 16 bytes (8 for the pointer, and 8 for the allocated object on the heap, assuming some sort of small-object optimisation is going on). The approach in C++ is to use "small string optimisation" where the string values are inlined into the std::string structure itself. This work

Can you have the runtime run a job every second, to trim thread stack depths and return memory to the kernel (whatever was meant by [VIDEO] ) while holding a FFI mutex (or a sharded one), and have every C function call first lock the mutex and expand the stack to the necessary depth?

At least Linux does this, and it is not a security hole because it only reuses pages with memory of the current process (as in the worst case you end up reading data from some other part of your program, but not other programs). And yes, it does happen in practice, most famously it has been mentioned in the "The strange details of std::string at Facebook" talk at CppCon 2016 [VIDEO]

The FAANGS certainly have people doing this kind of work. For example, Andrei Alexandrescu has a bunch of keynote talks at CPPCon where he talks about 1-4% speed optimisations he makes to the std library at Facebook. [LINK] I also like the talk by Nicholas Ormrod “The strange details of std::string at Facebook": [VIDEO]

Yes, there are really cool packing techniques. See this talk for example: [VIDEO] I don't include the null-terminator because I use this type in my own environment where I never use null-terminated strings so there is no need for it.

It was from C++con 2016 - Facebook take on small strings [VIDEO] I believe it is about a page that was conditionally returned to the kernel.

I don't believe they are allocating 128 bytes, or accessing out of bounds memory. He explicitly states 128byte filename allocates 129 bytes. [VIDEO]

You'd be surprised how it actually is worth the effort, even just a 1% improvement. If you have the time, this is a great talk to listen to: [VIDEO] For a little toy ray tracer, it is pretty measly. But for a larger corporation (with a professional project) a 4% speed improvement can mean MASSIVE cost savings. Some of these tiny improvements can also have a cascading effect. Imagining finding a +4%, a +2% somewhere else, +3% in neighboring code, and a bunch of +1%s here and there. Eventually you'll have built up something that is 15-20% faster. Down the road you'll come across those optimizations which can yield the big results too (e.g. the +25%).