ok guys,

using BOINC v6.10.58, i tried Claggy's suggestion and edited the <cmdline>-period_iterations_num 2 -instances_per_device 1</cmdline> line in SETI's app_info.xml file...that is, i changed instances per device from 1 to 2.

1) on the first attempt, there was no cc-Config.xml file in the BOINC data directory either disabling or enabling GPUs. no success - the tasks behaved the same as they did with scenario #1 under v6.10.58. here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692668.
2) i tried it again, this time with a "<use_all_gpus>1</use_all_gpus>" cc-config.xml file, and i got the same results. here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692667.
3) i also tried it with a "<use_all_gpus>0</use_all_gpus>" cc-config.xml file - again, same results. here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692684.

i'm gonna give it another try using BOINC v6.12.18 later tonight. i'll report back...

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ok,

using BOINC v6.12.18, i performed the same experiment as above.

1) on the first attempt, there was no cc-Config.xml file in the BOINC data directory either disabling or enabling GPUs. no success - the tasks behaved the same as they did with scenario #1 under v6.10.58 (immediate errors). here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692655.
2) i tried it again, this time with a "<use_all_gpus>1</use_all_gpus>" cc-config.xml file, and i got the same results (immediate errors). here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692620.
3) i also tried it with a "<use_all_gpus>0</use_all_gpus>" cc-config.xml file - again, same results (this time they were subsequent errors). here is the task that errored out under this configuration: http://setiathome.berkeley.edu/result.php?resultid=1836692652.

...so unfortunately changing "instances per device" from 1 to 2 in SETI's app_info.xml file did not change anything.

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Now we sure it will not work at least :)
The reason is the same - app tries to pick up non-existent device.