I have added two GPUs, but BOINC only uses one

By default BOINC only uses the best GPU if more than one of the same brand is found. The best being decided by (decreasing priority):

Nvidia

• compute capability
  
• software version
  
• available memory
  
• speed
  

AMD

• double precision support
• local RAM
• speed

It can happen that same brand and same model GPUs don't all get used at the same time, that's then usually due to a mismatch in one of the above criteria.

BOINC can be instructed to use all GPUs it finds, if they aren't used by default. This is done via the client configuration file, cc_config.xml which by default lives in the BOINC data directory.

Note

You can edit the cc_config.xml file with a standard ASCII/text-editor such as Notepad, there is no need to use an XML editor.

If you have a cc_config.xml file already find the line <use_all_gpus>0</use_all_gpus> and change it to <use_all_gpus>1</use_all_gpus>.

If you don't have a cc_config.xml file yet, you can manually make one with a standard ASCII/text-editor such as Notepad and add into it:

<cc_config>
    <log_flags>
    </log_flags>
    <options>
        <use_all_gpus>1</use_all_gpus>
    </options>
<cc_config>

Note

Do not use Wordpad or any text editor from Office or Open/Libre Office as these add invisible characters that BOINC cannot read and will choke on.

Note

Save the cc_config.xml file as:

File name: cc_config.xml

Save as type: All Files (*.*)

Encoding: ANSI

These make sure the file gets the correct extension and all the right characters that BOINC can read.

After saving this file, exit & restart the BOINC client for changes to take effect.
We need to restart the client, as GPU detection decisions are only made at BOINC startup.