如何在C ++中有效地管理内存/时间？

Question

The case:

I wrote a minimax algorithm in Java, then I ported the code in C++, mantaining the same classes and methods. Then I valgrinded the C++ code until all memory leaks were fixed by putting delete functions in destructors (when possible). When all leaks were fixed, I tested the algorithm on a tic-tac-toe game, which code was then ported identically in C++. I was sure that the C++ version would have been more performant, however I was surprised seeing that 100 (not random) instances of the game were solved in 125 seconds by C++ version while the Java version solved them in 30 seconds! Then, using System Monitor I checked the memory usage of the two programs: the Java code memory usage at the end of the test increased roughly 20% while with the C++ version the memory only increased of 5%.

Then, at my eyes, it's clear that this delete policy is memory saving but kills time performance, which is not what I want, at least in this case. Is there another deletion policy to better design softwares which requires small runtime but allow high memory usage?

Answer 1

malloc and delete have to do more work as

• memory is not allocates in a multi-threaded way
• memory is not allocated continuously in memory, but for regions of free memory.
• delete is performed in the current thread.
• for 64-bit applications you may find the memory alignment is 16 bytes instead of 8 bytes, resulting in more padding per allocation.

Possible solutions in C++

• don't use the heap so much, allocating on the stack is much faster than Java's or C++'s heap, and it is multi-threaded.
• allocate blocks of objects at once if possible eg an array of objects is one allocation in C++ instead of N+1 in Java.
• use a multi-threaded allocator. C++ supports multiple allocators.
• use an arena allocation pattern. If you have a large data structure with lots of nodes, you can allocate blocks of N node at a time and when you free such nodes, build a linked list of free node. You need to do this with the nodes themselves. In this approach the whole data structure/arena is deallocated at once.

In short, if you do a literal translation of Java code to C++, it may well be slower as this doesn't take advantage of all the optimisations C++ allows but Java doesn't.

BTW Java 6+ uses 32-bit references for up to 32 GB of memory. If you build a 64-bit C++ application, see if using 32-bit pointers/references/indexes is an option for you.

Answer 2

If you are constantly building a tree (with a lot of small dynamically allocated nodes) and tearing it down, you are in the most favorable case for garbage collection. You might want to use the Boehm collector with C++. (IIRC, Boehm's benchmark, designed to show that garbage collection can be faster than manual allocation, does something very similar to this: builds large trees, then tears them down.)

Alternatively, you could look into some sort of memory pool. This is not a beginner's technique in C++, but it isn't that difficult either. Or, rather than deleting the nodes, you put them onto a free list, to recycle later. (When allocating, you look first to the free list, and only call new if it is empty.) Or combine the two.