Hacker News Reader

Comment by jzwinck | Hacker News Reader

This is missing something though: "new" and malloc() are just like managed memory: convenient, but sometimes (and often unpredictably) slow.

Some people eschew Java and the JVM due to the spectre of garbage collection ruining their quasi-realtime system. But others embrace the JVM, organizing their programs to avoid garbage collection during critical times (which could last for hours or days).

With C and C++ it is the same, but backward: people flock to these languages for the predictable performance. Yet once there, they discover they must avoid the default allocator, for it is a source of unpredictable delays. Pooled allocators are created, and the end result is much the same as for the JVM folks.

View on HN

The funny thing is that most of STL gives no realtime guarantees either, even if you use a custom realtime allocator with it. E.g. vector.push_back takes O(n) time sometimes. Running a chain of destructors caused by reference count dropping to 0 can be unpredictable as well.

I once had to re-implement the pieces of the STL I wanted in a real-time embedded system, for precisely this reason (well, that plus the fact that STL's use of allocators is broken, and I wanted to be able to allocate from fixed-size pools to avoid non-O(1) trips through an allocator at run-time).

But you can be sure of when those "sometimes" will be with e.g. std::vector::reserve

http://en.cppreference.com/w/cpp/container/vector/reserve

If you want realtime guarantees, you have a lot more unpredictable problems, like CPUs having cache.

Yep, but exactly the same applies to Java. Just preallocate all your objects statically and you're never going to have GC problems :P

If you don't want the occasional O(n) copy on growth, use an stl list with a custom allocator.

reserve can be very easily misused. For example, let's assume a function append_10_items_to_vector. Since we're adding 10 items, we may get a better performance if we call reserve() before 10 push_back()s. Right?

Not always. If we have a loop that calls our append_10_items_to_vector N times (on the same vector), then your reserve() just transformed the loop into O(N^2) code. Without reserve, it would be O(N).

As a rule of thumb, use reserve only if you are really sure that nobody's ever going to add anything more to your vector in the future.

Reply on news.ycombinator.com