and in case your wondering if you change that app_config.xml to the following it will run like a ST task. just remember that when the yafu-x64-13401.exe is running ( that is on a 64 bit windows host) it will still try and use all of the cores, but that is for very short times, the avg_ncups and --nthreads part control the number of copies of the ecm and gnfs that it will start..

<!-- YAFU -->
<app_config>
<!-- YAFU -->
<!-- YAFU -->
<app_version>
<app_name>yafu</app_name>
<plan_class>mt</plan_class>
<avg_ncpus>1</avg_ncpus>
<cmdline>--nthreads 1</cmdline>
</app_version>
</app_config>