[PATCH v2] Reversing calculation of __x86_shared_non_temporal_threshold

[Patrick McGehearty]

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
---
 manual/tunables.texi    |  6 +++++-
 sysdeps/x86/cacheinfo.c | 12 +++++++-----
 2 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/manual/tunables.texi b/manual/tunables.texi
index b6bb54d..94d4fbd 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -364,7 +364,11 @@ set shared cache size in bytes for use in memory and string routines.
 
 @deftp Tunable glibc.tune.x86_non_temporal_threshold
 The @code{glibc.tune.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 b9444dd..c6767d9 100644
--- a/sysdeps/x86/cacheinfo.c
+++ b/sysdeps/x86/cacheinfo.c
@@ -778,14 +778,16 @@ intel_bug_no_cache_info:
       __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. While higher
+     single thread performance may be observed with a higher
+     threshold, having a single thread use more than it's share
+     of the cache will negatively impact 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);
 }
 
 #endif
-- 
1.8.3.1