[glibc/ibm/2.32/master] Reversing calculation of __x86_shared_non_temporal_threshold

[glibc/ibm/2.32/master] Reversing calculation of __x86_shared_non_temporal_threshold

[Tulio Magno Quites Machado Filho]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=e61a8fd8fadbf1a8cef997a0f921575cb2905ea2

commit e61a8fd8fadbf1a8cef997a0f921575cb2905ea2
Author: Patrick McGehearty <patrick.mcgehearty@oracle.com>
Date:   Mon Sep 28 20:11:28 2020 +0000

    Reversing calculation of __x86_shared_non_temporal_threshold
    
    The __x86_shared_non_temporal_threshold determines when memcpy on x86
    uses non_temporal stores to avoid pushing other data out of the last
    level cache.
    
    This patch proposes to revert the calculation change made by H.J. Lu's
    patch of June 2, 2017.
    
    H.J. Lu's patch selected a threshold suitable for a single thread
    getting maximum performance. It was tuned using the single threaded
    large memcpy micro benchmark on an 8 core processor. The last change
    changes the threshold from using 3/4 of one thread's share of the
    cache to using 3/4 of the entire cache of a multi-threaded system
    before switching to non-temporal stores. Multi-threaded systems with
    more than a few threads are server-class and typically have many
    active threads. If one thread consumes 3/4 of the available cache for
    all threads, it will cause other active threads to have data removed
    from the cache. Two examples show the range of the effect. John
    McCalpin's widely parallel Stream benchmark, which runs in parallel
    and fetches data sequentially, saw a 20% slowdown with this patch on
    an internal system test of 128 threads. This regression was discovered
    when comparing OL8 performance to OL7.  An example that compares
    normal stores to non-temporal stores may be found at
    https://vgatherps.github.io/2018-09-02-nontemporal/.  A simple test
    shows performance loss of 400 to 500% due to a failure to use
    nontemporal stores. These performance losses are most likely to occur
    when the system load is heaviest and good performance is critical.
    
    The tunable x86_non_temporal_threshold can be used to override the
    default for the knowledgable user who really wants maximum cache
    allocation to a single thread in a multi-threaded system.
    The manual entry for the tunable has been expanded to provide
    more information about its purpose.
    
            modified: sysdeps/x86/cacheinfo.c
            modified: manual/tunables.texi
    
    (cherry picked from commit d3c57027470b78dba79c6d931e4e409b1fecfc80)

Diff:
---
 manual/tunables.texi    |  6 +++++-
 sysdeps/x86/cacheinfo.c | 16 +++++++++++-----
 2 files changed, 16 insertions(+), 6 deletions(-)

diff --git a/manual/tunables.texi b/manual/tunables.texi
index 23ef0d40e7..d72d7a5ec0 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -432,7 +432,11 @@ set shared cache size in bytes for use in memory and string routines.
 
 @deftp Tunable glibc.cpu.x86_non_temporal_threshold
 The @code{glibc.cpu.x86_non_temporal_threshold} tunable allows the user
-to set threshold in bytes for non temporal store.
+to set threshold in bytes for non temporal store. Non temporal stores
+give a hint to the hardware to move data directly to memory without
+displacing other data from the cache. This tunable is used by some
+platforms to determine when to use non temporal stores in operations
+like memmove and memcpy.
 
 This tunable is specific to i386 and x86-64.
 @end deftp
diff --git a/sysdeps/x86/cacheinfo.c b/sysdeps/x86/cacheinfo.c
index 217c21c34f..dadec5d58f 100644
--- a/sysdeps/x86/cacheinfo.c
+++ b/sysdeps/x86/cacheinfo.c
@@ -854,14 +854,20 @@ init_cacheinfo (void)
       __x86_shared_cache_size = shared;
     }
 
-  /* The large memcpy micro benchmark in glibc shows that 6 times of
-     shared cache size is the approximate value above which non-temporal
-     store becomes faster on a 8-core processor.  This is the 3/4 of the
-     total shared 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. */
   __x86_shared_non_temporal_threshold
     = (cpu_features->non_temporal_threshold != 0
        ? cpu_features->non_temporal_threshold
-       : __x86_shared_cache_size * threads * 3 / 4);
+       : __x86_shared_cache_size * 3 / 4);
 
   /* NB: The REP MOVSB threshold must be greater than VEC_SIZE * 8.  */
   unsigned int minimum_rep_movsb_threshold;