Why is the first C++ (m)allocation always 72 KB?

Why is the first C++ (m)allocation always 72 KB?(joelsiks.com)

131 points by joelsiks 78 days ago | 36 comments

pjmlp 78 days ago |

This is compiler specific and cannot be generalised as C++.

zabzonk 78 days ago | |

Well, yes, but still quite interesting, IMHO. It's not like GCC is one of the least used compilers.

pjmlp 78 days ago | | |

Yeah, but that isn't C++ in isolation, thus the tile is incorrect.

compiler-guy 78 days ago | |

And C++ library specific as well. Perhaps even more so.

surajrmal 77 days ago | |

I would also expect it to depend on whether or not you have exceptions enabled. Half the ecosystem has them disabled.

pjmlp 77 days ago | | |

I really hate that, and believe if they would have been there since the beginning, the split would not have happened as the option to disable them would not exist in first place.

It was introduced for historical compatibility reasons, it is not officially supported on the standard, but some folks really keep on using non-standard language, spitting the libraries ecosystem.

jebarker 77 days ago |

Reading this was a good reminder not to be intimidated by assumptions about complexity. (Without giving it much thought) I would have assumed that it would be hard to replace malloc for such fundamental applications as ls, but it's surprisingly simple.

syncsynchalt 77 days ago | |

There's usually an easy-ish way to override malloc/calloc/realloc/free on Unix, as it's very useful to do when debugging issues or just to collect allocation metrics.

In ELF objects (i.e. on Linux) this is usually done with the "Weak" symbol binding. This is an optional flag for symbols in ELF format that let you override a symbol by providing a competing non-weak symbol, which the linker will prefer when there is a conflict. https://en.wikipedia.org/wiki/Weak_symbol

You can see the list of Weak symbols by looking for a 'W' in the output of `nm` on linux hosts.

rramadass 77 days ago | |

Right.

Unfortunately, a lot of system level knowledge like this is not found in a single place but spread over many articles/manuals/books/etc.

However, the book Advanced C and C++ Compiling: An Engineering Guide to Compiling, Linking and Libraries using C and C++ by Milan Stevanovic brings together a lot of information which you might find interesting.

userbinator 77 days ago | |

If you started learning from the "bottom-up", you wouldn't think it's intimidating. Fortunately, it's never too late to start learning.

jebarker 77 days ago | | |

That might be true for this particular thing, but there’ll still be some other perceived barrier of complexity, e.g. maybe it’s hardware, maybe it’s math, maybe it’s some higher level application like graphics. My point was that I was reminded to not just assume something would be hard without looking into it.

ozgrakkurt 77 days ago | |

This applies to a lot of things unfortunately. There is a cult of just being afraid and scaring other people.

"You can't do it, just use a library.". "Just use this library, everyone uses it.". "Even google uses this library, do you think you are better." etc.

To add another example to this, you will read that memcpy is super mega optimized on libc and you shouldn't do it yourself etc. etc. etc.

But if you just check clickhouse [1] as an example. They implemented it, it is pretty basic and they say it works well in the comments of the code.

Also you can check musl libc code etc. and it is fairly simple.

People still would argue that you used some intrinsic so it isn't portable or you just benchmarked on one case so it won't work well overall.

Well you CAN benchmark as wide as you want inside your project and have a different memcpy code per project. This kind of thing isn't as bad as people make it out to be in my opinion.

Ofc memcpy is just an example here and it applies similarly to memory allocation, io etc.

As a negative note, imo this is one of the major reasons why most software is super crappy now. Everything uses some library -> those libraries change all the time -> more breaking -> more maintenance. Similar chain happens in terms of performance because the person that wrote that library probably doesn't even know how I am using the library.

This is also why people have endless arguments about what library/tool to use while they can be learning more and more things every day.

[1] https://github.com/ClickHouse/ClickHouse/blob/master/base/gl...

nineteen999 76 days ago | | |

100% - the number of times you will need to use a super optimized memcpy() in real life versus the benefit you can get from looking at and writing basic versions of it for different CPU's is very slim.

Then you'll have a much better idea of when to _really_ use one that depends on intrinsics, is optimized etc, and how to benchmark them ... those are the real skills.

Joker_vD 78 days ago |

Huh. Why is this emergency pool not statically allocated? Is it possible to tune the size of this pool on libc++ startup somehow? Because otherwise it absolutely should've been statically allocated.

joelsiks 78 days ago | |

I did mention it briefly in the post, but you can opt-in for a fixed-size statically allocated buffer by configuring libstdc++ with --enable-libstdcxx-static-eh-pool. Also, you can opt-out of the pool entirely by configuring the number of objects in the pool to zero with the environment variable GLIBCXX_TUNABLES=glibcxx.eh_pool.obj_count=0.

ninkendo 78 days ago | | |

I wonder why it’s opt-in. Maybe it’s part of the whole “you only pay for what you use” ethos, i.e. you shouldn’t have to pay the cost for a static emergency pool if you don’t even use dynamic memory allocation.

throwaway2037 78 days ago |

I would like the see the source code for libmymalloc.so, however, I don't see anything in the blog post. Nor do I see anything in his GitHub profile: https://github.com/jsikstro

Also, I cannot find his email address anywhere (to ask him to share it on GitHub).

Am I missing something?

joelsiks 78 days ago | |

The exact implementation of mymalloc isn't relevant to the post. I have an old allocator published at https://github.com/joelsiks/jsmalloc that I did as part of my Master's thesis, which uses a similar debug-logging mechanism that is described in the post.

nly 78 days ago | |

dlsym() with the RTLD_NEXT flag basically:

https://catonmat.net/simple-ld-preload-tutorial-part-two

There's actually a better way to hook GNUs malloc:

https://www.man7.org/linux/man-pages/man3/malloc_hook.3.html

This is better because you can disable the hook inside the callback, and therefore use malloc within your malloc hook (no recursion)

But you can't use this mechanism before main()

Joker_vD 78 days ago | | |

    The use of these hook functions is not safe in multithreaded
    programs, and they are now deprecated.  From glibc 2.24 onwards,
    the __malloc_initialize_hook variable has been removed from the
    API, and from glibc 2.34 onwards, all the hook variables have been
    removed from the API.  Programmers should instead preempt calls to
    the relevant functions by defining and exporting malloc(), free(),
    realloc(), and calloc().

Panzerschrek 77 days ago |

What happens if I turn exceptions off using -fno-exceptions compiler flag? Does C++ runtime still allocates this block of memory?

znpy 78 days ago |

> TLDR; The C++ standard library sets up exception handling infrastructure early on, allocating memory for an “emergency pool” to be able to allocate memory for exceptions in case malloc ever runs out of memory.

Reminds me of Perl's $^M: https://perldoc.perl.org/variables/$%5EM

In Perl you can "hand-manage" that. This line would allocate a 64K buffer for use in an emergency:

    $^M = 'a' x (1 << 16);

anonymousiam 77 days ago |

So basically, before any of the code even runs, this environment begins by gobbling up more than the total RAM that most of my first computers had (SYM-1, IAMSAI-8080, Ferguson Big Board, Kaypro II, and CCS S-100 Z-80). All of these systems were 8-bit, with various RAM sizes from 8KB to 64KB. That was the maximum RAM available, and it was shared by the OS and the applications.

surajrmal 77 days ago | |

What's the purpose of making such a comparison? The implication is that we're being wasteful, but I'm not certain that's the point you're trying to make.

anonymousiam 77 days ago | | |

That was close to my point. RAM prices are up 400% over last year. I know that 72KB seems like a pittance today, but any waste is bad -- especially in embedded environments.

I'm not really even saying this is wasteful. Maybe a static allocation would make more sense for critical exception handling code...

aliveintucson 78 days ago |

I think you should read up on what "always" means.

cendyne 77 days ago |

This was a fun little share. Thanks for writing it up!

The use of these hook functions is not safe in multithreaded programs, and they are now deprecated. From glibc 2.24 onwards, the __malloc_initialize_hook variable has been removed from the API, and from glibc 2.34 onwards, all the hook variables have been removed from the API. Programmers should instead preempt calls to the relevant functions by defining and exporting malloc(), free(), realloc(), and calloc().