From af992e7abdc9049714da76cae1e5e18bc4838fb8 Mon Sep 17 00:00:00 2001

From: Noah Goldstein <goldstein.w.n@gmail.com>

Date: Wed, 7 Jun 2023 13:18:01 -0500

Subject: [PATCH] x86: Increase `non_temporal_threshold` to roughly `sizeof_L3

 / 4`

Content-type: text/plain; charset=UTF-8

Current `non_temporal_threshold` set to roughly '3/4 * sizeof_L3 /

ncores_per_socket'. This patch updates that value to roughly

'sizeof_L3 / 4`

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`.

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>

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

---

 sysdeps/x86/dl-cacheinfo.h | 70 +++++++++++++++++++++++---------------

 1 file changed, 43 insertions(+), 27 deletions(-)

[DJ - ported to C8S]

diff -rup a/sysdeps/x86/cacheinfo.h b/sysdeps/x86/cacheinfo.h

--- a/sysdeps/x86/cacheinfo.h	2023-08-08 11:54:09.969791421 -0400

+++ b/sysdeps/x86/cacheinfo.h	2023-08-08 13:44:55.185333601 -0400

@@ -46,7 +46,7 @@ long int __x86_rep_movsb_threshold attri

 long int __x86_rep_stosb_threshold attribute_hidden = 2048;

 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;

@@ -65,6 +65,7 @@ get_common_cache_info (long int *shared_

   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;

@@ -80,6 +81,7 @@ get_common_cache_info (long int *shared_

       /* Try L2 otherwise.  */

       level  = 2;

       shared = core;

+      shared_per_thread = core;

       threads_l2 = 0;

       threads_l3 = -1;

     }

@@ -236,29 +238,28 @@ get_common_cache_info (long int *shared_

         }

       else

         {

-intel_bug_no_cache_info:

-          /* Assume that all logical threads share the highest cache

-             level.  */

-          threads

-            = ((cpu_features->features[COMMON_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[COMMON_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;

 }

@@ -272,6 +273,7 @@ init_cacheinfo (void)

   int max_cpuid_ex;

   long int data = -1;

   long int shared = -1;

+  long int shared_per_thread = -1;

   long int core;

   unsigned int threads = 0;

   const struct cpu_features *cpu_features = __get_cpu_features ();

@@ -287,22 +289,25 @@ init_cacheinfo (void)

       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;

-      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;

-      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);

       long int core = handle_amd (_SC_LEVEL2_CACHE_SIZE);

       shared = handle_amd (_SC_LEVEL3_CACHE_SIZE);

+      shared_per_thread = shared;

       /* Get maximum extended function. */

       __cpuid (0x80000000, max_cpuid_ex, ebx, ecx, edx);

@@ -352,6 +357,9 @@ init_cacheinfo (void)

 	      shared += core;

             }

 	}

+

+      if (shared_per_thread <= 0)

+	shared_per_thread = shared;

     }

   if (cpu_features->data_cache_size != 0)

@@ -380,20 +388,30 @@ init_cacheinfo (void)

       __x86_shared_cache_size = shared;

     }

-  /* 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. */

+  /* 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 out-competing

+     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, their performance in highly parallel situations is

+     noticeably worse.  */

+  if (!CPU_FEATURE_USABLE_P (cpu_features, ERMS))

+    non_temporal_threshold = shared_per_thread * 3 / 4;

+

   __x86_shared_non_temporal_threshold

     = (cpu_features->non_temporal_threshold != 0

        ? cpu_features->non_temporal_threshold

-       : __x86_shared_cache_size * 3 / 4);

+       : non_temporal_threshold);

   /* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8.  */

   unsigned int minimum_rep_movsb_threshold;

Only in b/sysdeps/x86: cacheinfo.h~