Re: [PATCH v2 2/5] benchtests: Add memset zero fill benchtest

[Noah Goldstein via Libc-alpha <libc-alpha@sourceware.org>]

On Tue, Jul 20, 2021 at 2:35 AM Naohiro Tamura <naohirot@fujitsu.com> wrote:

> Memset takes 0 as the second parameter in most cases.
> However, we cannot measure the zero fill performance by bench-memset.c
> and bench-memset-large.c precisely.
> X86_64 micro-architecture has some zero-over-zero optimization, and
> AArch64 micro-architecture also has some optimization for DC ZVA
> instruction.
> This patch provides bench-memset-zerofill.c which is suitable to
> analyze the zero fill performance by zero-over-zero and zero-over-one
> test cases from 16KB(L1), through L2 and L3, to 64MB(RAM).
> ---
>  benchtests/Makefile                |   2 +-
>  benchtests/bench-memset-zerofill.c | 128 +++++++++++++++++++++++++++++
>  2 files changed, 129 insertions(+), 1 deletion(-)
>  create mode 100644 benchtests/bench-memset-zerofill.c
>
> diff --git a/benchtests/Makefile b/benchtests/Makefile
> index 1530939a8ce8..21b95c736190 100644
> --- a/benchtests/Makefile
> +++ b/benchtests/Makefile
> @@ -53,7 +53,7 @@ string-benchset := memccpy memchr memcmp memcpy memmem
> memmove \
>                    strncasecmp strncat strncmp strncpy strnlen strpbrk
> strrchr \
>                    strspn strstr strcpy_chk stpcpy_chk memrchr strsep
> strtok \
>                    strcoll memcpy-large memcpy-random memmove-large
> memset-large \
> -                  memcpy-walk memset-walk memmove-walk
> +                  memcpy-walk memset-walk memmove-walk memset-zerofill
>
>  # Build and run locale-dependent benchmarks only if we're building
> natively.
>  ifeq (no,$(cross-compiling))
> diff --git a/benchtests/bench-memset-zerofill.c
> b/benchtests/bench-memset-zerofill.c
> new file mode 100644
> index 000000000000..2579b6edd09e
> --- /dev/null
> +++ b/benchtests/bench-memset-zerofill.c
> @@ -0,0 +1,128 @@
> +/* Measure memset functions with zero fill data.
> +   Copyright (C) 2021 Free Software Foundation, Inc.
> +   This file is part of the GNU C Library.
> +
> +   The GNU C Library is free software; you can redistribute it and/or
> +   modify it under the terms of the GNU Lesser General Public
> +   License as published by the Free Software Foundation; either
> +   version 2.1 of the License, or (at your option) any later version.
> +
> +   The GNU C Library is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
> +   Lesser General Public License for more details.
> +
> +   You should have received a copy of the GNU Lesser General Public
> +   License along with the GNU C Library; if not, see
> +   <https://www.gnu.org/licenses/>.  */
> +
> +#define TEST_MAIN
> +#define TEST_NAME "memset"
> +#define START_SIZE (16 * 1024)
> +#define MIN_PAGE_SIZE (getpagesize () + 64 * 1024 * 1024)
> +#define TIMEOUT (20 * 60)
> +#include "bench-string.h"
> +
> +#include "json-lib.h"
> +
> +void *generic_memset (void *, int, size_t);
> +typedef void *(*proto_t) (void *, int, size_t);
> +
> +IMPL (MEMSET, 1)
> +IMPL (generic_memset, 0)
> +
> +static void
>
Do we want __attribute__((noinline, noclone))?

> +do_one_test (json_ctx_t *json_ctx, impl_t *impl, CHAR *s,
> +            int c1 __attribute ((unused)), int c2 __attribute ((unused)),
> +            size_t n)
> +{
> +  size_t i, iters = 16;
>

I think 16 is probably too few iterations for reliable benchmarking.
Maybe `INNER_LOOP_ITERS` which is 8192

+  timing_t start, stop, cur;
> +
> +  TIMING_NOW (start);
> +  for (i = 0; i < iters; i += 2)
> +    {
> +      CALL (impl, s, c1, n);
>
I am a bit worried that the overhead from the first call with `c1` will
distort the results.
Is it possible to implement it with a nested loop where you fill `s` with
`c1` for
`n * inner_loop_iterations` in the outer loop and in the inner loop fill
`c2` on `s + n * i`?
In that case maybe 16 for inner loop iterations and 512 for outer loop
iterations.

> +      CALL (impl, s, c2, n);
> +    }
> +  TIMING_NOW (stop);
> +
> +  TIMING_DIFF (cur, start, stop);
> +
> +  json_element_double (json_ctx, (double) cur / (double) iters);
> +}
> +
> +static void
> +do_test (json_ctx_t *json_ctx, size_t align, int c1, int c2, size_t len)
> +{
> +  align &= 63;
>
Can you make this `align &= getpagesize () - 1;`?

> +  if ((align + len) * sizeof (CHAR) > page_size)
> +    return;
> +
> +  json_element_object_begin (json_ctx);
> +  json_attr_uint (json_ctx, "length", len);
> +  json_attr_uint (json_ctx, "alignment", align);
> +  json_attr_int (json_ctx, "char1", c1);
> +  json_attr_int (json_ctx, "char2", c2);
> +  json_array_begin (json_ctx, "timings");
> +
> +  FOR_EACH_IMPL (impl, 0)
> +    {
> +      do_one_test (json_ctx, impl, (CHAR *) (buf1) + align, c1, c2, len);
> +      alloc_bufs ();
> +    }
> +
> +  json_array_end (json_ctx);
> +  json_element_object_end (json_ctx);
> +}
> +
> +int
> +test_main (void)
> +{
> +  json_ctx_t json_ctx;
> +  size_t i;
> +  int c1, c2;
> +
> +  test_init ();
> +
> +  json_init (&json_ctx, 0, stdout);
> +
> +  json_document_begin (&json_ctx);
> +  json_attr_string (&json_ctx, "timing_type", TIMING_TYPE);
> +
> +  json_attr_object_begin (&json_ctx, "functions");
> +  json_attr_object_begin (&json_ctx, TEST_NAME);
> +  json_attr_string (&json_ctx, "bench-variant", "zerofill");
> +
> +  json_array_begin (&json_ctx, "ifuncs");
> +  FOR_EACH_IMPL (impl, 0)
> +    json_element_string (&json_ctx, impl->name);
> +  json_array_end (&json_ctx);
> +
> +  json_array_begin (&json_ctx, "results");
> +
> +  c2 = 0;
> +  for (c1 = 0; c1 < 2; c1++)
> +    for (i = START_SIZE; i <= MIN_PAGE_SIZE; i <<= 1)
> +      {
> +       do_test (&json_ctx, 0, c1, c2, i);
> +       do_test (&json_ctx, 3, c1, c2, i);
> +      }
> +
> +  json_array_end (&json_ctx);
> +  json_attr_object_end (&json_ctx);
> +  json_attr_object_end (&json_ctx);
> +  json_document_end (&json_ctx);
> +
> +  return ret;
> +}
> +
> +#include <support/test-driver.c>
> +
> +#define libc_hidden_builtin_def(X)
> +#define libc_hidden_def(X)
> +#define libc_hidden_weak(X)
> +#define weak_alias(X,Y)
> +#undef MEMSET
> +#define MEMSET generic_memset
> +#include <string/memset.c>
> --
> 2.17.1
>
>