Xcode 9.0 - Simulator - Known Issue https://developer.apple.com/library/content/releasenotes/DeveloperTools/RN-Xcode/Chapters/Introduction.html#//apple_ref/doc/uid/TP40001051-CH1-SW1017
When running several simulators simultaneously you may exceed the maximum number of processes or maximum open file limits. If this happens attempts to start new processes or open more files will fail. You can recover by shutting down some simulators or closing open programs. macOS 10.13 automatically scales these limits based on available system memory. On earlier releases these limits are fixed values. These limits can be increased by using launchctl. For more information, choose Help > Simulator Help and navigate to Troubleshoot Simulator > Insufficient resources. (31179087)
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