BatchBatchBatch.pdf

“Batch, Batch, Batch:” What Does It Really Mean? Matthias Wloka What Is a Batch? ? Every DrawIndexedPrimitive() is a batch – Submits n number of triangles to GPU – Same render state applies to all tris in batch – SetState calls prior to Draw are part of batch ? Assuming efficient use of API – No Draw*PrimitiveUP() – DrawPrimitive() permissible if warranted – No unnecessary state changes ? Changing state means at least two batches Why Are Small Batches Bad? ? Games would rather draw 1M objects/batches of 10 tris each – versus 10 objects/batches of 1M tris each ? Lots of guesses – Changing state inefficient on GPUs (WRONG) – GPU triangle start-up costs (WRONG) – OS kernel transitions (WRONG) ? Future GPUs will make it better!? Really? ? Test app does… – Degenerate triangles (no fill cost) – 100% PostTnL cache vertices (no xform cost) – Static data (minimal AGP overhead) – ~100k tris/frame, i.e., floor(100k/x) draws – Toggles state between draw calls: (VBs, w/v/p matrix, tex-stage and alpha states) ? Timed across 1000 frames ? Theoretical maximum triangle rates! Let’s Write Code! Testing Small Batch Performance Measured Batch-Size Performance 0 10 20 30 40 50 60 70 80 90 100 10 30 50 70 90 11 0 13 0 15 0 17 0 19 0 30 0 50 0 70 0 90 0 11 00 13 00 15 00 triangles/batch m ill io n t ri an g le s/ s Athlon XP 2.7+; NVIDIA GeForce FX 5800 Athlon XP 2.7+; NVIDIA GeForce4 Ti 4600 Athlon XP 2.7+; NVIDIA GeForce3 Ti 500 Athlon XP 2.7+; NVIDIA GeForce4 MX 440 Athlon XP 2.7+; NVIDIA GeForce2 MX/MX 400 Axis scale change Optimization Opportunities 0 10 20 30 40 50 60 70 80 90 100 10 30 50 70 90 11 0 13 0 15 0 17 0 19 0 30 0 50 0 70 0 90 0 11 00 13 00 15 00 triangles/batch m ill io n t ri an g le s/ s Athlon XP 2.7+; NVIDIA GeForce FX 5800 Athlon XP 2.7+; NVIDIA GeForce4 Ti 4600 Athlon XP 2.7+; NVIDIA GeForce3 Ti 500 Athlon XP 2.7+; NVIDIA GeForce4 MX 440 Athlon XP 2.7+; NVIDIA GeForce2 MX/MX 400 40x 100x Axis scale change Measured Batch-Size Performance 0 10 20 30 40 50 60 70