Tune param.h for SkylakeX

param.h defines a per-platform SWITCH_RATIO, which is used as a measure for how fine
grained the blocks for gemm need to be split up. Many platforms define this to 4.

The reality is that the gemm low level implementation for SkylakeX likes bigger blocks
due to the nature of SIMD... by tuning the SWITCH_RATIO to 32 the threading performance
improves significantly:

Before
   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10756.0   10.5      -0.5%                             18296.7      6.1      -1.7%
  64 x 64               20490.0   12.9       1.4%                             40615.0      6.5       0.0%
  65 x 65               83528.3    3.3    -210.9%                             96319.0      2.9     -83.3%
  80 x 80              101453.5    5.1    -166.3%                            128021.7      4.0     -76.6%
  96 x 96              149795.1    5.9    -143.1%                            168059.4      5.3     -47.4%
 112 x 112             191481.2    7.3    -105.8%                            204165.0      6.9     -14.6%
 128 x 128             265019.2    7.9     -99.0%                            272006.4      7.7      -5.3%

After
   Matrix          SGEMM cycles    MPC                                   DGEMM cycles      MPC
  48 x 48               10666.3   10.6       0.4%                             18236.9      6.2      -1.4%
  64 x 64               20410.1   13.0       1.8%                             39925.8      6.6       1.7%
  65 x 65               34983.0    7.9     -30.2%                             51494.6      5.4       2.0%
  80 x 80               39769.1   13.0      -4.4%                             63805.2      8.1      12.0%
  96 x 96               45169.6   19.7      26.7%                             80065.8     11.1      29.8%
 112 x 112              57026.1   24.7      38.7%                             99535.5     14.2      44.1%
 128 x 128              64789.8   32.5      51.3%                            117407.2     17.9      54.6%

With this change, threading starts to be a win already at 96x96

diff --git a/param.h b/param.h
index 49a5e85..3573fff 100644 (file)
--- a/param.h
+++ b/param.h
@@ -1626,7 +1626,7 @@ USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #define SYMV_P  8
 
-#define SWITCH_RATIO   4
+#define SWITCH_RATIO   32
 
 #ifdef ARCH_X86