Why 70% / 51% slowdown of PARI/GP for pthread over single for 7950X / 7600X AMD CPUs?

[hermann@stamm-wilbrandt.de]

It took a while to determine pthread versus single to cause slower
computation times on faster AMD 7950X CPU over slower AMD 7600X CPU.

With same /etc/gprc, both on Ubuntu 22.04, GP 2.16.1 alpha:

________pthread	single	PARI/GP
7950X___0:34:51 0:20:33	1.70
7600X___0:36:15	0:24:04	1.51

The h:mm:ss runtimes are for 729 million evaluations of
Cayley-Menger determinant for computing volume of
tetrahedron given 6 edge distances:

? T=128;L=30;forvec (X=[[1,L],[1,L],[1,L],[1,L],[1,L],[1,L]], 
M=[0,1,1,1,1;1,0,X[1]^2,X[2]^2,X[3]^2;1,X[1]^2,0,X[4]^2,X[5]^2;1,X[2]^2,X[4]^2,0,X[6]^2;1,X[3]^2,X[5]^2,X[6]^2,0];if(matdet(M)==2*T,print(X)));
[2, 2, 2, 2, 2, 2]
cpu time = 34min, 50,094 ms, real time = 34min, 50,196 ms.
?

1) What are explanations for the massive slowdowns for pthread?

2) Should gp with pthread only be used if real multithreaded
computation is needed?


Regards,

Hermann.