c++ - On the use and abuse of alloca

Question

I am working on a soft-realtime event processing system. I would like to minimise as many calls in my code that have non-deterministic timing. I need to construct a message that consists of strings, numbers, timestamps and GUID's. Probably a std::vector of boost::variant's.

I have always wanted to use alloca in past code of a similar nature. However, when one looks into systems programming literature there are always massive cautions against this function call. Personally I can't think of a server class machine in the last 15 years that doesn't have virtual memory, and I know for a fact that the windows stack grows a virtual-memory page-at-a-time, so I assume Unices do as well. There is no brick wall here (anymore), the stack is just as likely to run out of space as the heap, so what gives ? Why aren't people going gaga over aloca ? I can think of many use-cases of responsible use of alloca (string processing anyone ?).

Anyhow, I decided to test the performance difference (see below) and there is a 5-fold speed difference between alloca and malloc (the test captures how I would use alloca). So, have things changed? Should we just throw caution to the wind and use alloca (wrapped in a std::allocator) whenever we can be absolutely certain of the lifetime of our objects ?

I am tired of living in fear !

Edit:

Ok so there are limits, for windows it is a link-time limit. For Unix it seems to be tunable. It seems a page-aligned memory allocator is in order :D Anyone know of a general purpose portable implementation :D ?

Code:

#include <stdlib.h>
#include <time.h>

#include <boost/date_time/posix_time/posix_time.hpp>
#include <iostream>

using namespace boost::posix_time;

int random_string_size()
{
    return ( (rand() % 1023) +1 );
}

int random_vector_size()
{
    return ( (rand() % 31) +1);
}

void alloca_test()
{
    int vec_sz = random_vector_size();

    void ** vec = (void **) alloca(vec_sz * sizeof(void *));    

    for(int i = 0 ; i < vec_sz ; i++)
    {
        vec[i] = alloca(random_string_size());     
    }
}

void malloc_test()
{
    int vec_sz = random_vector_size();

    void ** vec = (void **) malloc(vec_sz * sizeof(void *));    

    for(int i = 0 ; i < vec_sz ; i++)
    {
        vec[i] = malloc(random_string_size());     
    }

    for(int i = 0 ; i < vec_sz ; i++)
    {
        free(vec[i]); 
    }

    free(vec);
}

int main()
{
    srand( time(NULL) );
    ptime now;
    ptime after; 

    int test_repeat = 100; 
    int times = 100000;


    time_duration alloc_total;
    for(int ii=0; ii < test_repeat; ++ii)
    { 

        now = microsec_clock::local_time();
        for(int i =0 ; i < times ; ++i)
        {
            alloca_test();    
        }
        after = microsec_clock::local_time();

        alloc_total += after -now;
    }

    std::cout << "alloca_time: " << alloc_total/test_repeat << std::endl;

    time_duration malloc_total;
    for(int ii=0; ii < test_repeat; ++ii)
    {
        now = microsec_clock::local_time();
        for(int i =0 ; i < times ; ++i)
        {
            malloc_test();
        }
        after = microsec_clock::local_time();
        malloc_total += after-now;
    }

    std::cout << "malloc_time: " << malloc_total/test_repeat << std::endl;
}

output:

hassan@hassan-desktop:~/test$ ./a.out 
alloca_time: 00:00:00.056302
malloc_time: 00:00:00.260059
hassan@hassan-desktop:~/test$ ./a.out 
alloca_time: 00:00:00.056229
malloc_time: 00:00:00.256374
hassan@hassan-desktop:~/test$ ./a.out 
alloca_time: 00:00:00.056119
malloc_time: 00:00:00.265731

--Edit: Results on home machine, clang, and google perftools--

G++ without any optimization flags
alloca_time: 00:00:00.025785
malloc_time: 00:00:00.106345


G++ -O3
alloca_time: 00:00:00.021838
cmalloc_time: 00:00:00.111039


Clang no flags
alloca_time: 00:00:00.025503
malloc_time: 00:00:00.104551

Clang -O3 (alloca become magically faster)
alloca_time: 00:00:00.013028
malloc_time: 00:00:00.101729

g++ -O3 perftools
alloca_time: 00:00:00.021137
malloc_time: 00:00:00.043913

clang++ -O3 perftools (The sweet spot)
alloca_time: 00:00:00.013969
malloc_time: 00:00:00.044468

Answer 1

Well first of all, even though there is a lot of virtual memory doesn't mean your process will be allowed to fill it. On *nix there are stack size limits, whereas the heap is a lot more forgiving.

If you're only going to be allocating a few hundred / thousand bytes, sure go ahead. Anything beyond that is going to depend on what limits (ulimit) are in place on any given system, and that's just a recipe for disaster.

Why is the use of alloca() not considered good practice?

On my development box at work (Gentoo) I have a default stack size limit of 8192 kb. That's not very big, and if alloca overflows the stack then the behavior is undefined.