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Date: Thu, 22 Jan 2015 15:21:57 +0000
From: Bruce Richardson <bruce.richardson@intel.com>
To: Linhaifeng <haifeng.lin@huawei.com>
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Subject: Re: [dpdk-dev] some questions about rte_memcpy
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On Thu, Jan 22, 2015 at 08:53:13PM +0800, Linhaifeng wrote:
>
>
> On 2015/1/22 19:34, Bruce Richardson wrote:
> > On Thu, Jan 22, 2015 at 07:23:49PM +0900, Tetsuya Mukawa wrote:
> >> On 2015/01/22 16:35, Matthew Hall wrote:
> >>> On Thu, Jan 22, 2015 at 01:32:04PM +0800, Linhaifeng wrote:
> >>>> Do you mean if call rte_memcpy before rte_eal_init() would crash?why?
> >>> 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.
> >>
> >> 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.
> >>
> >
> > 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
> >
> >> Tetsuya
> >>
> >>> Matthew.
> >>
> >>
> >
> >
>
> 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]);
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
> 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
>