Re: [PATCH v10 1/3] x86: Increase `non_temporal_threshold` to roughly `sizeof_L3 / 4`

[Carlos O'Donell <carlos@redhat.com>]

On 5/27/23 14:46, Noah Goldstein via Libc-alpha wrote:
> Current `non_temporal_threshold` set to roughly '3/4 * sizeof_L3 /
> ncores_per_socket'. This patch updates that value to roughly
> 'sizeof_L3 / 4`

LGTM. Minor typos noted, OK for you to keep my Reviewed-by if you change only those.

Reviewed-by: Carlos O'Donell <carlos@redhat.com>

> The original value (specifically dividing the `ncores_per_socket`) was
> done to limit the amount of other threads' data a `memcpy`/`memset`
> could evict.
> 
> Dividing by 'ncores_per_socket', however leads to exceedingly low
> non-temporal thresholds and leads to using non-temporal stores in
> cases where REP MOVSB is multiple times faster.
> 
> Furthermore, non-temporal stores are written directly to main memory
> so using it at a size much smaller than L3 can place soon to be
> accessed data much further away than it otherwise could be. As well,
> modern machines are able to detect streaming patterns (especially if
> REP MOVSB is used) and provide LRU hints to the memory subsystem. This
> in affect caps the total amount of eviction at 1/cache_associativity,
> far below meaningfully thrashing the entire cache.
> 
> As best I can tell, the benchmarks that lead this small threshold
> where done comparing non-temporal stores versus standard cacheable
> stores. A better comparison (linked below) is to be REP MOVSB which,
> on the measure systems, is nearly 2x faster than non-temporal stores
> at the low-end of the previous threshold, and within 10% for over
> 100MB copies (well past even the current threshold). In cases with a
> low number of threads competing for bandwidth, REP MOVSB is ~2x faster
> up to `sizeof_L3`.

DJ and I reviewed the numbers and we agree with the divisor of 2 for Skylake/Icelake
and 8 for Broadwell. We considered 4 was reasonably well balanced, and that a middle
ground in this patch serves as a good starting point.

DJ also ran STREAMS on various configurations and the changes you propose do not make
anything worse than before which was Oracle's original complaint for the tuning. In
the STREAMS case the size of the copy buffer was set following the STREAMS
recommendation i.e. much larger than L3.

All-in-all this patch should improve things.

> The divisor of `4` is a somewhat arbitrary value. From benchmarks it
> seems Skylake and Icelake both prefer a divisor of `2`, but older CPUs
> such as Broadwell prefer something closer to `8`. This patch is meant
> to be followed up by another one to make the divisor cpu-specific, but
> in the meantime (and for easier backporting), this patch settles on
> `4` as a middle-ground.
> 
> Benchmarks comparing non-temporal stores, REP MOVSB, and cacheable
> stores where done using:
> https://github.com/goldsteinn/memcpy-nt-benchmarks
> 
> Sheets results (also available in pdf on the github):
> https://docs.google.com/spreadsheets/d/e/2PACX-1vS183r0rW_jRX6tG_E90m9qVuFiMbRIJvi5VAE8yYOvEOIEEc3aSNuEsrFbuXw5c3nGboxMmrupZD7K/pubhtml
> Reviewed-by: DJ Delorie <dj@redhat.com>
> ---
>  sysdeps/x86/dl-cacheinfo.h | 70 +++++++++++++++++++++++---------------
>  1 file changed, 43 insertions(+), 27 deletions(-)
> 
> diff --git a/sysdeps/x86/dl-cacheinfo.h b/sysdeps/x86/dl-cacheinfo.h
> index ec88945b39..4a1a5423ff 100644
> --- a/sysdeps/x86/dl-cacheinfo.h
> +++ b/sysdeps/x86/dl-cacheinfo.h
> @@ -407,7 +407,7 @@ handle_zhaoxin (int name)
>  }
>  
>  static void
> -get_common_cache_info (long int *shared_ptr, unsigned int *threads_ptr,
> +get_common_cache_info (long int *shared_ptr, long int * shared_per_thread_ptr, unsigned int *threads_ptr,
>                  long int core)
>  {
>    unsigned int eax;
> @@ -426,6 +426,7 @@ get_common_cache_info (long int *shared_ptr, unsigned int *threads_ptr,
>    unsigned int family = cpu_features->basic.family;
>    unsigned int model = cpu_features->basic.model;
>    long int shared = *shared_ptr;
> +  long int shared_per_thread = *shared_per_thread_ptr;
>    unsigned int threads = *threads_ptr;
>    bool inclusive_cache = true;
>    bool support_count_mask = true;
> @@ -441,6 +442,7 @@ get_common_cache_info (long int *shared_ptr, unsigned int *threads_ptr,
>        /* Try L2 otherwise.  */
>        level  = 2;
>        shared = core;
> +      shared_per_thread = core;
>        threads_l2 = 0;
>        threads_l3 = -1;
>      }
> @@ -597,29 +599,28 @@ get_common_cache_info (long int *shared_ptr, unsigned int *threads_ptr,
>          }
>        else
>          {
> -intel_bug_no_cache_info:
> -          /* Assume that all logical threads share the highest cache
> -             level.  */
> -          threads
> -            = ((cpu_features->features[CPUID_INDEX_1].cpuid.ebx >> 16)
> -	       & 0xff);
> -        }
> -
> -        /* Cap usage of highest cache level to the number of supported
> -           threads.  */
> -        if (shared > 0 && threads > 0)
> -          shared /= threads;
> +	intel_bug_no_cache_info:
> +	  /* Assume that all logical threads share the highest cache
> +	     level.  */
> +	  threads = ((cpu_features->features[CPUID_INDEX_1].cpuid.ebx >> 16)
> +		     & 0xff);
> +
> +	  /* Get per-thread size of highest level cache.  */
> +	  if (shared_per_thread > 0 && threads > 0)
> +	    shared_per_thread /= threads;
> +	}
>      }
>  
>    /* Account for non-inclusive L2 and L3 caches.  */
>    if (!inclusive_cache)
>      {
>        if (threads_l2 > 0)
> -        core /= threads_l2;
> +	shared_per_thread += core / threads_l2;
>        shared += core;
>      }
>  
>    *shared_ptr = shared;
> +  *shared_per_thread_ptr = shared_per_thread;
>    *threads_ptr = threads;
>  }
>  
> @@ -629,6 +630,7 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>    /* Find out what brand of processor.  */
>    long int data = -1;
>    long int shared = -1;
> +  long int shared_per_thread = -1;
>    long int core = -1;
>    unsigned int threads = 0;
>    unsigned long int level1_icache_size = -1;
> @@ -649,6 +651,7 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>        data = handle_intel (_SC_LEVEL1_DCACHE_SIZE, cpu_features);
>        core = handle_intel (_SC_LEVEL2_CACHE_SIZE, cpu_features);
>        shared = handle_intel (_SC_LEVEL3_CACHE_SIZE, cpu_features);
> +      shared_per_thread = shared;
>  
>        level1_icache_size
>  	= handle_intel (_SC_LEVEL1_ICACHE_SIZE, cpu_features);
> @@ -672,13 +675,14 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>        level4_cache_size
>  	= handle_intel (_SC_LEVEL4_CACHE_SIZE, cpu_features);
>  
> -      get_common_cache_info (&shared, &threads, core);
> +      get_common_cache_info (&shared, &shared_per_thread, &threads, core);
>      }
>    else if (cpu_features->basic.kind == arch_kind_zhaoxin)
>      {
>        data = handle_zhaoxin (_SC_LEVEL1_DCACHE_SIZE);
>        core = handle_zhaoxin (_SC_LEVEL2_CACHE_SIZE);
>        shared = handle_zhaoxin (_SC_LEVEL3_CACHE_SIZE);
> +      shared_per_thread = shared;
>  
>        level1_icache_size = handle_zhaoxin (_SC_LEVEL1_ICACHE_SIZE);
>        level1_icache_linesize = handle_zhaoxin (_SC_LEVEL1_ICACHE_LINESIZE);
> @@ -692,13 +696,14 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>        level3_cache_assoc = handle_zhaoxin (_SC_LEVEL3_CACHE_ASSOC);
>        level3_cache_linesize = handle_zhaoxin (_SC_LEVEL3_CACHE_LINESIZE);
>  
> -      get_common_cache_info (&shared, &threads, core);
> +      get_common_cache_info (&shared, &shared_per_thread, &threads, core);
>      }
>    else if (cpu_features->basic.kind == arch_kind_amd)
>      {
>        data = handle_amd (_SC_LEVEL1_DCACHE_SIZE);
>        core = handle_amd (_SC_LEVEL2_CACHE_SIZE);
>        shared = handle_amd (_SC_LEVEL3_CACHE_SIZE);
> +      shared_per_thread = shared;
>  
>        level1_icache_size = handle_amd (_SC_LEVEL1_ICACHE_SIZE);
>        level1_icache_linesize = handle_amd (_SC_LEVEL1_ICACHE_LINESIZE);
> @@ -715,6 +720,9 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>        if (shared <= 0)
>          /* No shared L3 cache.  All we have is the L2 cache.  */
>  	shared = core;
> +
> +      if (shared_per_thread <= 0)
> +	shared_per_thread = shared;
>      }
>  
>    cpu_features->level1_icache_size = level1_icache_size;
> @@ -730,17 +738,25 @@ dl_init_cacheinfo (struct cpu_features *cpu_features)
>    cpu_features->level3_cache_linesize = level3_cache_linesize;
>    cpu_features->level4_cache_size = level4_cache_size;
>  
> -  /* The default setting for the non_temporal threshold is 3/4 of one
> -     thread's share of the chip's cache. For most Intel and AMD processors
> -     with an initial release date between 2017 and 2020, a thread's typical
> -     share of the cache is from 500 KBytes to 2 MBytes. Using the 3/4
> -     threshold leaves 125 KBytes to 500 KBytes of the thread's data
> -     in cache after a maximum temporal copy, which will maintain
> -     in cache a reasonable portion of the thread's stack and other
> -     active data. If the threshold is set higher than one thread's
> -     share of the cache, it has a substantial risk of negatively
> -     impacting the performance of other threads running on the chip. */
> -  unsigned long int non_temporal_threshold = shared * 3 / 4;
> +  /* The default setting for the non_temporal threshold is 1/4 of size
> +     of the chip's cache. For most Intel and AMD processors with an
> +     initial release date between 2017 and 2023, a thread's typical
> +     share of the cache is from 18-64MB. Using the 1/4 L3 is meant to
> +     estimate the point where non-temporal stores begin outcompeting

s/outcompeting/out-competing/g

> +     REP MOVSB. As well the point where the fact that non-temporal
> +     stores are forced back to main memory would already occurred to the
> +     majority of the lines in the copy. Note, concerns about the
> +     entire L3 cache being evicted by the copy are mostly alleviated
> +     by the fact that modern HW detects streaming patterns and
> +     provides proper LRU hints so that the maximum thrashing
> +     capped at 1/associativity. */
> +  unsigned long int non_temporal_threshold = shared / 4;
> +  /* If no ERMS, we use the per-thread L3 chunking. Normal cacheable stores run
> +     a higher risk of actually thrashing the cache as they don't have a HW LRU
> +     hint. As well, there performance in highly parallel situations is

s/there/their/g

> +     noticeably worse.  */
> +  if (!CPU_FEATURE_USABLE_P (cpu_features, ERMS))
> +    non_temporal_threshold = shared_per_thread * 3 / 4;
>    /* SIZE_MAX >> 4 because memmove-vec-unaligned-erms right-shifts the value of
>       'x86_non_temporal_threshold' by `LOG_4X_MEMCPY_THRESH` (4) and it is best
>       if that operation cannot overflow. Minimum of 0x4040 (16448) because the

-- 
Cheers,
Carlos.