When running my application the very first cuda_malloc takes 40 seconds which is due to the initialization of the GPU. When I build in debug mode this reduces to 5 seconds and when I run the same code on a Fermi device, it takes far less than a second (not even worth measuring in my case).
Now the funny thing is that if I compile for this specific architecture, using the flag sm35 instead of sm20, it becomes fast again. As I should not use any new sm35 features just yet, how can I compile for sm20 and not have this huge delay? Also I am curious what is causing this delay? Is the machine code recompiled on the fly into sm35 code?
Ps. I run on windows but a colleague of mine encountered the same problem, probably on windows. The device is a Kepler, driver version 320.
Yes, the machine code is recompiled on the fly. This is called the JIT-compile step, and it will occur any time the machine code does not match the device that is being used (and assuming valid PTX code exists in the executable.)
You can learn more about JIT-compile here. Note the discussion of the cache which should alleviate the issue after the first run.
If you specify compilation for both sm_20 and sm_35, you can build a binary/executable that will run quickly on both types of devices, and you will also get notification if you are using a sm_35 feature that is not supported on sm_20 (during the compile process).