prometheus-cpp VS abseil-cpp

Rust does have some very cool mechanisms for safety, including in the presence of concurrency.

But this thread is generating blowback from someone saying: “slow down there with the hand-rolled atomic operations, you can hand-roll your multi-threaded locking strategy and it’ll be way safer at a modest cost!”

So, probably not the target audience for Rust ;)

I use a lot of C++ still because there are libraries I want and I have a significant investment in existing code, but I’d love to get to something more modern.

Hand-rolled atomics and load/store relaxation in application code make even seasoned C++ hackers a bit nervous: we saw this shit from business logic hackers at FB all the time and my colleague coined the term “aggressively intermediate” for the style.

I don’t mean to pick on the author of a quite good library (and it is quite good), but I ran across this the other day:

https://github.com/jupp0r/prometheus-cpp/blob/master/core/sr...

It’s correct (I think, very easy to be wrong about this sort of thing), but what are we measuring here where we can’t delegate that CAS into pthread? Branch mispredictions?

Either threads are fighting over whatever cache line that’s on (exclusive -> invalid -> exclusive -> invalid), or not. If they are, I’ve just deprived the scheduler of the opportunity to wake me up when the other 59 threads are done. If they’re not, I’ve maybe saved like one line in my L1.

And in something like a metrics library, you could be wrong for a very long time before someone pinned it down.

Some metric collecting tool, for example, Prometheus and its client library for C++

You can use https://github.com/jupp0r/prometheus-cpp (your custom data) in combination with https://grafana.com/ that has plugins for things like cpu usage.

Grafana and Prometheus is what I’d use if you’re deploying this in Kubernetes. I haven’t used the C++ library, but generally you can just add a Prometheus client library (e.g. https://github.com/jupp0r/prometheus-cpp ) which makes it quite easy to instrument your application and expose metrics via HTTP. Then you need to configure Prometheus to scrape metrics from your application.

Yeah, it's nice! And Abseil does it, IFF you use LLVM libc++.

https://github.com/abseil/abseil-cpp/blob/master/absl/string...

The standard adopted it as resize_and_overwrite. Which I think is a little clunky.

This may be confusing to those familiar with Google's libraries. The baseline is the Go BTree, which I personally never heard of until just now, not the C++ absl::btree_set. The benchmarks aren't directly comparable, but the C++ version also comes with good microbenchmark coverage.

https://github.com/google/btree

https://github.com/abseil/abseil-cpp/blob/master/absl/contai...

An implementation of call_once that accommodates callbacks that throw: https://github.com/abseil/abseil-cpp/blob/master/absl/base/c...

I wouldn't say it's that cryptic. It's just a few bitwise rotations/shifts/xor operations.

You can see hashing optimizations as well https://www.deepmind.com/blog/alphadev-discovers-faster-sort..., https://github.com/abseil/abseil-cpp/commit/74eee2aff683cc7d...

I was one of the members who reviewed expertly what has been done both in sorting and hashing. Overall it's more about assembly, finding missed compiler optimizations and balancing between correctness and distribution (in hashing in particular).

It was not revolutionary in a sense it hasn't found completely new approaches but converged to something incomprehensible for humans but relatively good for performance which proves the point that optimal programs are very inhuman.

Note that for instructions in sorting, removing them does not always lead to better performance, for example, instructions can run in parallel and the effect can be less profound. Benchmarks can lie and compiler could do something differently when recompiling the sort3 function which was changed. There was some evidence that the effect can come from the other side.

For hashing it was even funnier, very small strings up to 64 bit already used 3 instructions like add some constant -> multiply 64x64 -> xor upper/lower. For bigger ones the question becomes more complicated, that's why 9-16 was a better spot and it simplified from 2 multiplications to just one and a rotation. Distribution on real workloads was good, it almost passed smhasher and we decided it was good enough to try out in prod. We did not rollback as you can see from abseil :)

But even given all that, it was fascinating to watch how this system was searching and was able to find particular programs can be further simplified. Kudos to everyone involved, it's a great incremental change that can bring more results in the future.

Check out https://abseil.io. It offers absl::optional, which is a backport of std::optional.