Rendering in Blender on a machine with multiple GPUs
So here’s the premise: For scenes that take around a minute or less to render, performance is actually worse if you render on all of the cards with a single instance of Blender. This is because (AFAIK) there’s a bit of additional time necessary to collect the render results from each card and stitch them together. That time is a fixed short duration, so it’s negligible on larger/longer render jobs. However, on shorter render jobs, the ‘stitch time’ has a much more significant impact.
I ran into this with a machine I render on that has 4 Quadro K6000s in it. To render animations, I ended up writing a few little scripts to facilitate launching 4 separate instances of Blender, each one tied to one GPU. Overall rendertime was much shorter with that setup than one instance of Blender using all 4 GPUs.
The setup works basically like this… I have a the following Python script (it can be anywhere on your hard drive, so long as you remember the path to it). I call it
import sys import bpy #XXX Assumes sys.argv is the CUDA device argv = sys.argv[sys.argv.index("--") + 1:] if len(argv) > 0: dev = int(argv) C = bpy.context cycles_prefs = C.user_preferences.addons['cycles'].preferences C.scene.render.use_overwrite = False C.scene.render.use_placeholder = True cycles_prefs.compute_device_type = "CUDA" for device in cycles_prefs.devices: device.use = False cycles_prefs.devices[dev].use = True
This script takes a single compute device (an integer number) as an input argument (
argv). The script disables all compute devices except for the one specified. By doing that, only one GPU is used for this running instance of Blender. The
use_placeholder settings are also critical to have in there. Otherwise, all of my Blender instances will be rendering the same frames.
batch_cycles.py script created and accessible, I just need a script to launch 4 instances of Blender. As I’m in Linux, I did it in bash. However, you could just as easily write a Windows
BAT script or even a Python script. The key is that you need a mechanism that lauches Blender and lets that instance run in the background while you launch another instance of Blender. My bash script looks like this:
#!/bin/bash if [ $# = 0 ]; then echo "You forgot to mention the .blend file you want to render" else for i in `seq 0 3`; do blender -b $1 --python /path/to/batch_cycles.py -a -- $i & done fi
In short, this script launches 4 instances of Blender (
seq 0 3), and each instance uses a different CUDA device. The ampersand (
&) at the end is what lets each instance of Blender run in the background. I’m not sure what the correct flag is in Windows. It does exist, though. I’d looked it up at one point, but have since forgotten it.
And that’s basically how I do it. Now, I know that this could certainly stand to be refined. In the meantime, however, this certainly does the job for me.