Porting/LLVMpipe

Thread Count

LLVMpipe is limited to 16 threads. The only easily findable justification for this limit is in a commit from 2013, where it was increased from 8 because a user reported on a mailing list that 16 was faster for them. Given that POWER9 systems often have much higher thread counts than this, this limit may be suboptimal for POWER9.

luke-jr anecdotally reports that bumping the limit to 128 threads noticeably improved performance in 3D games, e.g. Jedi Academy became playable. However, it also had the side effect that most GUI applications spawned 128 LLVMpipe threads, which was annoying in gdb/top. Luke worked around this issue by setting the environment variable LP_NUM_THREADS=2 globally, and overriding for 3D applications that needed more threads. Luke's patch is:

diff -ur mesa-17.3.9.orig/src/gallium/drivers/llvmpipe/lp_limits.h mesa-17.3.9/src/gallium/drivers/llvmpipe/lp_limits.h
--- mesa-17.3.9.orig/src/gallium/drivers/llvmpipe/lp_limits.h	2018-04-18 04:44:00.000000000 -0400
+++ mesa-17.3.9/src/gallium/drivers/llvmpipe/lp_limits.h	2018-05-02 05:20:57.586000000 -0400
@@ -61,7 +61,7 @@
 #define LP_MAX_WIDTH  (1 << (LP_MAX_TEXTURE_LEVELS - 1))
 
 
-#define LP_MAX_THREADS 16
+#define LP_MAX_THREADS 128
 
 
 /**
Only in mesa-17.3.9/src/gallium/drivers/llvmpipe: lp_limits.h~

Vector Optimizations

As of 2022 June 10, grepping main branch src/gallium/drivers/llvmpipe/ for altivec yields only 2 files (lp_rast_tri.c and lp_setup_tri.c, both of which are POWER8 LE), while grepping for PIPE_ARCH_SSE yields 11 files. This seems to suggest that a lot of POWER vector optimizations are missing from LLVMpipe. POWER9 vector optimizations (the LLVM power9-vector feature), and POWER8 BE optimizations, appear to be completely absent.