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Hey. ScyllaDB employee here. There are several reasons C++ was used and I don't think span ultimately matters. List from top of my head (ordered randomly): 1)

by enedil 5y ago

Hey. ScyllaDB employee here. There are several reasons C++ was used and I don't think span ultimately matters. List from top of my head (ordered randomly): 1) we use intrusive containers, so memory managing container data structures is collocated with actual data. 2) memory allocation is not tied to GC, so we don't get pauses 3) there's almost none synchronization between different threads and there are (almost) no globals. For a story why globals are a killer for performance, read https://www.p99conf.io/2021/09/28/hunting-a-numa-performance-bug/ https://www.p99conf.io/2021/09/28/hunting-a-numa-performance... 4) the previous is only possible with existence of a user space scheduler which guarantees that specific threads are pinned to a single CPU. Also, there's no need to call mmap multiple times, as Seastar (concurrency framework written with Scylla in mind) allocates whole system memory and takes advantage of overcommitting in Linux. There's no syscall at memory allocation, just some userspace work and a possible page fault. I'm not sure whether C# can do away with these problems? Let me know if you know. That being said, modern C++ is really convenient. Not anything you've seen 15 years ago in university.

pjmlp 5y ago
I can ensure you that what you would see in a university in 2022 is going to be just like 15 years ago. As for C# you can do most C++ like stuff in C# 10.
- enedil 5y ago
  As a matter of fact, I'm still at university and mine actually showed a fair bit of modern C++ (University of Warsaw here). So I don't feel ensured. As for C#, I don't claim I know stuff. I'd just like to learn something new. If you finish at an assertion like yours, sadly I don't learn anything new.
  - pjmlp 5y ago
    As proven by occasional threads on /r/cpp and including complaints from Bjarne himself on some of his talks, that is unfortunately not yet a common practice. Regarding C#, if you really want to learn how to do C++ style programming in C#, have a look at the documentation regarding C# 7.0 - 7.3, C# 8, C# 9 and C# 10 regarding readonly structs, span, stackalloc in safe code, blittable types, GC free regions, malloc/free calls, allocation free memory pipelines, in and return ref types, local references, using pattern (implementing IDispose is no longer required) Regarding classical C# (what is available until .NET Framework 4.8), you have structs, value types, manual memory management via System.Runtime.InteropServices. You can start at the free posters here, https://prodotnetmemory.com/ https://prodotnetmemory.com/
  - lossolo 5y ago
    > actually showed a fair bit of modern C++ (University of Warsaw here) You mean like C++11? So C++ standard from 11 years ago ? or C++14? C++17? Last time I checked UW was like 17 years ago so maybe things changed but back then they were like 10+ years behind industry in practical terms.
    - enedil 5y ago
      In 2019 it was in C++17. Now it's in C++20.

by enedil 5y ago

pjmlp 5y ago

I can ensure you that what you would see in a university in 2022 is going to be just like 15 years ago. As for C# you can do most C++ like stuff in C# 10.

enedil 5y ago
As a matter of fact, I'm still at university and mine actually showed a fair bit of modern C++ (University of Warsaw here). So I don't feel ensured. As for C#, I don't claim I know stuff. I'd just like to learn something new. If you finish at an assertion like yours, sadly I don't learn anything new.
- pjmlp 5y ago
  As proven by occasional threads on /r/cpp and including complaints from Bjarne himself on some of his talks, that is unfortunately not yet a common practice. Regarding C#, if you really want to learn how to do C++ style programming in C#, have a look at the documentation regarding C# 7.0 - 7.3, C# 8, C# 9 and C# 10 regarding readonly structs, span, stackalloc in safe code, blittable types, GC free regions, malloc/free calls, allocation free memory pipelines, in and return ref types, local references, using pattern (implementing IDispose is no longer required) Regarding classical C# (what is available until .NET Framework 4.8), you have structs, value types, manual memory management via System.Runtime.InteropServices. You can start at the free posters here, https://prodotnetmemory.com/ https://prodotnetmemory.com/
- lossolo 5y ago
  > actually showed a fair bit of modern C++ (University of Warsaw here) You mean like C++11? So C++ standard from 11 years ago ? or C++14? C++17? Last time I checked UW was like 17 years ago so maybe things changed but back then they were like 10+ years behind industry in practical terms.
  - enedil 5y ago
    In 2019 it was in C++17. Now it's in C++20.