some questions about rte_memcpy

#define rte_memcpy(dst, src, n)              \
	((__builtin_constant_p(n)) ?          \
	memcpy((dst), (src), (n)) :          \
	rte_memcpy_func((dst), (src), (n)))

Why call memcpy when n is constant variable?
Can i change them to the follow codes?

#define rte_memcpy(dst, src, n)              \
    {   \
        int num = n;    \
	rte_memcpy_func((dst), (src), (num)))   \
    }

Regards,
Haifeng

One theory. Many DPDK functions crash if they are called before rte_eal_init() is called. So perhaps this could be a cause, since that won't have been called when working on a constant?

Matthew.

Hi, Matthew

Thank you for your response.

Do you mean if call rte_memcpy before rte_eal_init() would crash?why?

Regards,
Haifeng

No guarantee. But a theory. It might use some things from the EAL init to figure out which version of the accelerated algorithm to use.

Matthew.

This selection is done at compile-time.
And if the size is constant, I guess DPDK assumes memcpy is replaced by inline __builtin_memcpy.
I haven't checked the performance of builtin memcpy, but probably much faster.

Tetsuya

Yes, that assumption is correct. A couple of years ago we discovered that for constant size values, the compiler would generate much faster code for us using a regular memcpy than rte_memcpy, hence the macro.

/Bruce

Hi,Bruce

I test it,most results like you said use constant may be faster,but sometimes not.

linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 16 9999999
rte_memcpy(constant) used:279893712 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277818600 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 16 9999999
rte_memcpy(constant) used:279264328 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277667116 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 16 9999999
rte_memcpy(constant) used:279491832 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277622772 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 32 9999999
rte_memcpy(constant) used:279402156 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277738464 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 32 9999999
rte_memcpy(constant) used:279305172 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277483004 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 32 9999999
rte_memcpy(constant) used:279784124 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:277605332 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 48 9999999 rte_memcpy(constant) used:322817260 rte_memcpy(variable) used:350333864 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 48 9999999 rte_memcpy(constant) used:322840748 rte_memcpy(variable) used:350297868 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 48 9999999 rte_memcpy(constant) used:322488240 rte_memcpy(variable) used:350348652 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 64 9999999 rte_memcpy(constant) used:322021428 rte_memcpy(variable) used:350416440 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 64 9999999 rte_memcpy(constant) used:321370900 rte_memcpy(variable) used:350355796 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 64 9999999 rte_memcpy(constant) used:322704552 rte_memcpy(variable) used:349900832 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 128 9999999 rte_memcpy(constant) used:422705828 rte_memcpy(variable) used:425493328 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 128 9999999
rte_memcpy(constant) used:422421840 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:413691412 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 128 9999999
rte_memcpy(constant) used:425233088 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:421136724 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 256 9999999
rte_memcpy(constant) used:901014608 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:900997388 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 256 9999999
rte_memcpy(constant) used:900803308 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:900794076 linux-mnSyvH:/mnt/sdb/linhf/test # ./rte_memcpy_test 256 9999999
rte_memcpy(constant) used:901842436 @@@@@@@@@@@@@@ not faster
rte_memcpy(variable) used:901218984 linux-mnSyvH:/mnt/sdb/linhf/test #

here is my test codes:

#include <stdio.h>
#include <rte_memcpy.h>
#include <rte_cycles.h>

int main(int narg, char** args)

{
        int i;
        char buf[1024];
        uint64_t start, end;

        if (narg < 3) {
                printf("usage:./rte_memcpy_test size times\n");
                return 0;
        }

        size_t size_v = atoi(args[1]);
        const size_t size_c = atoi(args[1]);
        int times = atoi(args[2]);

        start = rte_rdtsc();
        for(i = 0; i < times; i++) {
                rte_memcpy(buf, buf, size_c);
        }
        end = rte_rdtsc();
        printf("rte_memcpy(constant) used:%llu\n", end - start);

        start = rte_rdtsc();
        for (i = 0; i < times; i++) {
                rte_memcpy(buf, buf, size_v);
        }
        end = rte_rdtsc();
        printf("rte_memcpy(variable) used:%llu\n", end - start);

        return 0;
}

Regards,
Haifeng

This (size_c) is a run-time constant, not a compile-time constant. To trigger the memcpy optimizations inside the compiler, the size value must be constant at compile time.

Regards,
/Bruce

Hi, Bruce

You are right. When use compile-time constant memcpy is faster.Thank you for all.

Here is my test result:

rte_memcpy(constant) size:8 time:876
rte_memcpy(variable) size:8 time:2824
rte_memcpy(constant) size:16 time:868
rte_memcpy(variable) size:16 time:4436
rte_memcpy(constant) size:32 time:856
rte_memcpy(variable) size:32 time:3264
rte_memcpy(constant) size:48 time:872
rte_memcpy(variable) size:48 time:3972
rte_memcpy(constant) size:64 time:856
rte_memcpy(variable) size:64 time:3644
rte_memcpy(constant) size:128 time:868
rte_memcpy(variable) size:128 time:4720
rte_memcpy(constant) size:256 time:868
rte_memcpy(variable) size:256 time:9624

Here is my test program(Who know how to use a loop to test 'constant memcpy'?):

#include <stdio.h>
#include <rte_memcpy.h>
#include <rte_cycles.h>

int main(int narg, char** args)

{
        int i,t;
        char buf[256];
        int tests[7] = {8,16,32,48,64,128,256};
        char buf8[8],buf16[16],buf32[32],buf48[48],buf64[64],buf128[128],buf256[256];
        uint64_t start, end;
        int times = 9999999;
        uint64_t result_c[7];

        if (narg < 2) {
                printf("usage:./rte_memcpy_test times\n");
                return -1;
        }

        times = atoi(args[1]);

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf8, buf8, sizeof buf8);
        }
        end = rte_rdtsc();
        result_c[0] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf16, buf16, sizeof buf16);
        }
        end = rte_rdtsc();
        result_c[1] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf32, buf32, sizeof buf32);
        }
        end = rte_rdtsc();
        result_c[2] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf48, buf48, sizeof buf48);
        }
        end = rte_rdtsc();
        result_c[3] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf64, buf64, sizeof buf64);
        }
        end = rte_rdtsc();
        result_c[4] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf128, buf128, sizeof buf128);
        }
        end = rte_rdtsc();
        result_c[5] = end - start;

        start = rte_rdtsc();
        for(t = 0; t < times; t++) {
                rte_memcpy(buf256, buf256, sizeof buf256);
        }
        end = rte_rdtsc();
        result_c[6] = end - start;

        for (i = 0; i < (sizeof tests / sizeof tests[0]); i++) {
                start = rte_rdtsc();
                for(t = 0; t < times; t++) {
                        rte_memcpy(buf, buf, tests[i]);
                }
                end = rte_rdtsc();
                printf("rte_memcpy(constant) size:%d time:%llu\n", tests[i], result_c[i]);
                printf("rte_memcpy(variable) size:%d time:%llu\n", tests[i], end - start);
        }

        return 0;
}

Regards,
Haifeng