memcpy performance on skylake server

[Adhemerval Zanella]

On 06/07/2021 05:17, Ji, Cheng via Libc-help wrote:
> Hello,
> 
> I found that memcpy is slower on skylake server CPUs during our
> optimization work, and I can't really explain what we got and need some
> guidance here.
> 
> The problem is that memcpy is noticeably slower than a simple for loop when
> copying large chunks of data. This genuinely sounds like an amateur mistake
> in our testing code but here's what we have tried:
> 
> * The test data is large enough: 1GB.
> * We noticed a change quite a while ago regarding skylake and AVX512:
> https://patchwork.ozlabs.org/project/glibc/patch/20170418183712.GA22211@intel.com/
> * We updated glibc from 2.17 to the latest 2.33, we did see memcpy is 5%
> faster but still slower than a simple loop.
> * We tested on multiple bare metal machines with different cpus: Xeon Gold
> 6132, Gold 6252, Silver 4114, as well as a virtual machine on google cloud,
> the result is reproducible.
> * On an older generation Xeon E5-2630 v3, memcpy is about 50% faster than
> the simple loop. On my desktop (i7-7700k) memcpy is also significantly
> faster.
> * numactl is used to ensure everything is running on a single core.
> * The code is compiled by gcc 10.3
> 
> The numbers on a Xeon Gold 6132, with glibc 2.33:
> simple_memcpy 4.18 seconds, 4.79 GiB/s 5.02 GB/s
> simple_copy 3.68 seconds, 5.44 GiB/s 5.70 GB/s
> simple_memcpy 4.18 seconds, 4.79 GiB/s 5.02 GB/s
> simple_copy 3.68 seconds, 5.44 GiB/s 5.71 GB/s
> 
> The result is worse with system provided glibc 2.17:
> simple_memcpy 4.38 seconds, 4.57 GiB/s 4.79 GB/s
> simple_copy 3.68 seconds, 5.43 GiB/s 5.70 GB/s
> simple_memcpy 4.38 seconds, 4.56 GiB/s 4.78 GB/s
> simple_copy 3.68 seconds, 5.44 GiB/s 5.70 GB/s
> 
> 
> The code to generate this result (compiled with g++ -O2 -g, run with: numactl
> --membind 0 --physcpubind 0 -- ./a.out)
> =====
> 
> #include <chrono>
> #include <cstring>
> #include <functional>
> #include <string>
> #include <vector>
> 
> class TestCase {
>     using clock_t = std::chrono::high_resolution_clock;
>     using sec_t = std::chrono::duration<double, std::ratio<1>>;
> 
> public:
>     static constexpr size_t NUM_VALUES = 128 * (1 << 20); // 128 million *
> 8 bytes = 1GiB
> 
>     void init() {
>         vals_.resize(NUM_VALUES);
>         for (size_t i = 0; i < NUM_VALUES; ++i) {
>             vals_[i] = i;
>         }
>         dest_.resize(NUM_VALUES);
>     }
> 
>     void run(std::string name, std::function<void(const int64_t *, int64_t
> *, size_t)> &&func) {
>         // ignore the result from first run
>         func(vals_.data(), dest_.data(), vals_.size());
>         constexpr size_t count = 20;
>         auto start = clock_t::now();
>         for (size_t i = 0; i < count; ++i) {
>             func(vals_.data(), dest_.data(), vals_.size());
>         }
>         auto end = clock_t::now();
>         double duration =
> std::chrono::duration_cast<sec_t>(end-start).count();
>         printf("%s %.2f seconds, %.2f GiB/s, %.2f GB/s\n", name.data(),
> duration,
>                sizeof(int64_t) * NUM_VALUES / double(1 << 30) * count /
> duration,
>                sizeof(int64_t) * NUM_VALUES / double(1e9) * count /
> duration);
>     }
> 
> private:
>     std::vector<int64_t> vals_;
>     std::vector<int64_t> dest_;
> };
> 
> void simple_memcpy(const int64_t *src, int64_t *dest, size_t n) {
>     memcpy(dest, src, n * sizeof(int64_t));
> }
> 
> void simple_copy(const int64_t *src, int64_t *dest, size_t n) {
>     for (size_t i = 0; i < n; ++i) {
>         dest[i] = src[i];
>     }
> }
> 
> int main(int, char **) {
>     TestCase c;
>     c.init();
> 
>     c.run("simple_memcpy", simple_memcpy);
>     c.run("simple_copy", simple_copy);
>     c.run("simple_memcpy", simple_memcpy);
>     c.run("simple_copy", simple_copy);
> }
> 
> =====
> 
> The assembly of simple_copy generated by gcc is very simple:
> Dump of assembler code for function _Z11simple_copyPKlPlm:
>    0x0000000000401440 <+0>:     mov    %rdx,%rcx
>    0x0000000000401443 <+3>:     test   %rdx,%rdx
>    0x0000000000401446 <+6>:     je     0x401460 <_Z11simple_copyPKlPlm+32>
>    0x0000000000401448 <+8>:     xor    %eax,%eax
>    0x000000000040144a <+10>:    nopw   0x0(%rax,%rax,1)
>    0x0000000000401450 <+16>:    mov    (%rdi,%rax,8),%rdx
>    0x0000000000401454 <+20>:    mov    %rdx,(%rsi,%rax,8)
>    0x0000000000401458 <+24>:    inc    %rax
>    0x000000000040145b <+27>:    cmp    %rax,%rcx
>    0x000000000040145e <+30>:    jne    0x401450 <_Z11simple_copyPKlPlm+16>
>    0x0000000000401460 <+32>:    retq
> 
> When compiling with -O3, gcc vectorized the loop using xmm0, the
> simple_loop is around 1% faster.

Usually differences of that magnitude falls either in noise or may be something
related to OS jitter. 

> 
> I took a brief look at the glibc source code. Though I don't have enough
> knowledge to understand it yet, I'm curious about the underlying mechanism.
> Thanks.

H.J, do you have any idea what might be happening here?