AVX + Triangle8 slow

[theigors]

Hi All

Unfortunately I've no machine with AVX and can't debug. Users tell me that with AVX render time is in 3 times slower (bvh4 + triangle8) are used. If I set bvh4 + triangle4 then render time with AVX is approx same as SSSE3. Any hint/advice is very appreciated

Thanks 

[SvenW_Intel]

Are you using Embree as is, or did you extract the ray traversal kernels to your application? If the latter is the case, you have to be carefull that the __mm256_zeroupper() intrinsic is active at the beginning and end of the Embree traversal kernel. Otherwise there will be a performance penalty, if the user code it NOT compiled with AVX enabled.

Further, the bvh4.triangle8 will anyway only give you a small performance benefit (if any) over using the bvh4.triangle4. Thus the best workaround is to simply use the bvh4.triangle4.

[theigors]

Hi, Sven

>> Are you using Embree as is, or did you extract the ray traversal kernels to your application? If the latter is the case, you have to be carefull that the __mm256_zeroupper() intrinsic is active at the beginning and end of the Embree traversal kernel. Otherwise there will be a performance penalty, if the user code it NOT compiled with AVX enabled.<<

Yes, I've extracted kernels (btw it was much easier than I expected). I've built with __AVX__ (and __SSE4_2__), so __mm256_zeroupper() should be active. I can use Triangle8 with SSSE3 etc - no time penalty but also no speedup. I'll debug on user's side (it needs time) and let you know.

>>Further, the bvh4.triangle8 will anyway only give you a small performance benefit (if any) over using the bvh4.triangle4. Thus the best workaround is to simply use the bvh4.triangle4.<<

Ops! It's really surprising, I expected like 1.5 times speedup. If possible tell me why so? 

Thx for your help

[theigors]

Hi, Sven

1) Yes, I've extracted rt cores and built with __AVX__ (and __SSE4_2__), so __mm256_zeroupper() should be active. If I use Triangle8 with SSSE3 etc - no time penalty, approx same speed as for Triangle4. I'll debug on user's side (it needs time) and let you know.

2) It's a surprise/unexpected for me that Triangle8 is not significantly faster! If possible - explain why

Thx for your help