I am running Boinc 7.0.64 on a Windows 7 machine with 4 cores for many many months without any problems. I run about 10 projects.
Suddenly, only 1 core is used instead of 4.
Nothing has changed. 100% of processors can be used, etc.

I have reset all projects and only 2 projects have send new tasks.

What can I do to get all cores being used again ?

Post the first 35 lines of the message/event log that show right after a client restart. Those will tell us what hardware BOINC is seeing now and other tidbits. Saying anything before that would be speculating in the wild.

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Coelum Non Animum Mutant, Qui Trans Mare Currunt

19.jun.2013 07:29:13 | | No config file found - using defaults
19.jun.2013 07:29:13 | | Starting BOINC client version 7.0.64 for windows_x86_64
19.jun.2013 07:29:13 | | log flags: file_xfer, sched_ops, task
19.jun.2013 07:29:13 | | Libraries: libcurl/7.25.0 OpenSSL/1.0.1 zlib/1.2.6
19.jun.2013 07:29:13 | | Data directory: C:\ProgramData\Boinc
19.jun.2013 07:29:13 | | Running under account prv
19.jun.2013 07:29:13 | | Processor: 4 GenuineIntel Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz [Family 6 Model 42 Stepping 7]
19.jun.2013 07:29:13 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 cx16 sse4_1 sse4_2 popcnt aes syscall nx lm vmx smx tm2 pbe
19.jun.2013 07:29:13 | | OS: Microsoft Windows 7: Enterprise x64 Edition, Service Pack 1, (06.01.7601.00)
19.jun.2013 07:29:13 | | Memory: 3.88 GB physical, 7.86 GB virtual
19.jun.2013 07:29:13 | | Disk: 297.74 GB total, 222.21 GB free
19.jun.2013 07:29:13 | | Local time is UTC +2 hours
19.jun.2013 07:29:13 | | CUDA: NVIDIA GPU 0: NVS 4200M (driver version 266.96, CUDA version 3.20, compute capability 2.1, 483MB, 469MB available, 142 GFLOPS peak)
19.jun.2013 07:29:13 | | OpenCL: NVIDIA GPU 0: NVS 4200M (driver version 266.96, device version OpenCL 1.0 CUDA, 483MB, 469MB available, 142 GFLOPS peak)
19.jun.2013 07:29:13 | DistrRTgen | URL http://boinc.freerainbowtables.com/distrrtgen/; Computer ID 36002; resource share 200
19.jun.2013 07:29:13 | Poem@Home | URL http://boinc.fzk.de/poem/; Computer ID 149257; resource share 100
19.jun.2013 07:29:13 | Leiden Classical | URL http://boinc.gorlaeus.net/; Computer ID 90045; resource share 100
19.jun.2013 07:29:13 | Collatz Conjecture | URL http://boinc.thesonntags.com/collatz/; Computer ID 92115; resource share 100
19.jun.2013 07:29:13 | Einstein@Home | URL http://einstein.phys.uwm.edu/; Computer ID 4216186; resource share 100
19.jun.2013 07:29:13 | LHC@home 1.0 | URL http://lhcathomeclassic.cern.ch/sixtrack/; Computer ID 10142545; resource share 100
19.jun.2013 07:29:13 | Milkyway@Home | URL http://milkyway.cs.rpi.edu/milkyway/; Computer ID 325168; resource share 100
19.jun.2013 07:29:13 | ibercivis | URL http://registro.ibercivis.es/; Computer ID 265802; resource share 100
19.jun.2013 07:29:13 | SETI@home | URL http://setiathome.berkeley.edu/; Computer ID 6169925; resource share 100
19.jun.2013 07:29:13 | PrimeGrid | URL http://www.primegrid.com/; Computer ID 219482; resource share 100
19.jun.2013 07:29:13 | World Community Grid | URL http://www.worldcommunitygrid.org/; Computer ID 1715742; resource share 100
19.jun.2013 07:29:13 | DistrRTgen | General prefs: from DistrRTgen (last modified 18-Jun-2013 15:22:03)
19.jun.2013 07:29:13 | DistrRTgen | Computer location: home
19.jun.2013 07:29:13 | | General prefs: using separate prefs for home
19.jun.2013 07:29:13 | | Reading preferences override file
19.jun.2013 07:29:13 | | Preferences:
19.jun.2013 07:29:13 | | max memory usage when active: 3579.21MB
19.jun.2013 07:29:13 | | max memory usage when idle: 3579.21MB
19.jun.2013 07:29:13 | | max disk usage: 222.28GB
19.jun.2013 07:29:13 | | (to change preferences, visit a project web site or select Preferences in the Manager)
19.jun.2013 07:29:13 | | Not using a proxy
19.jun.2013 07:29:15 | Einstein@Home | Restarting task PA0057_013D1_186_3 using einsteinbinary_BRP5 version 133 (BRP4cuda32) in slot 4
19.jun.2013 07:29:15 | LHC@home 1.0 | Restarting task wlxscan2_w1cbb__46__s__64.31_59.32__2_2.5__6__10.5_1_sixvf_boinc154020_0 using sixtrack version 44401 (pni) in slot 3
19.jun.2013 07:29:15 | LHC@home 1.0 | Restarting task wlxscan2_w1cbb__46__s__64.31_59.32__2_2.5__6__4.5_1_sixvf_boinc154016_1 using sixtrack version 44401 (pni) in slot 0
19.jun.2013 07:29:15 | LHC@home 1.0 | Restarting task wlxscan2_w1cbb__46__s__64.31_59.32__2_2.5__6__25.5_1_sixvf_boinc154028_0 using sixtrack version 44401 (pni) in slot 1
19.jun.2013 07:29:15 | LHC@home 1.0 | Restarting task wlxscan2_w1cbb__45__s__64.31_59.32__9.5_10__6__13.5_1_sixvf_boinc153811_1 using sixtrack version 44401 (pni) in slot 2

I see that now 4 processors are used, but the situation is not the same as before.
I had 5 projects running (on 4 cores) and they were continuous using 100% CPU

Now it is 4 projects and mot the max CPU.

Also: only 3 projects have workpackages. The rest report that they are not the highest priority

Only running 1 project:

19.jun.2013 10:10:35 | | No config file found - using defaults
19.jun.2013 10:10:35 | | Starting BOINC client version 7.0.64 for windows_x86_64
19.jun.2013 10:10:35 | | log flags: file_xfer, sched_ops, task
19.jun.2013 10:10:35 | | Libraries: libcurl/7.25.0 OpenSSL/1.0.1 zlib/1.2.6
19.jun.2013 10:10:35 | | Data directory: C:\ProgramData\Boinc
19.jun.2013 10:10:35 | | Running under account prv
19.jun.2013 10:10:35 | | Processor: 4 GenuineIntel Intel(R) Core(TM) i5-2540M CPU @ 2.60GHz [Family 6 Model 42 Stepping 7]
19.jun.2013 10:10:35 | | Processor features: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss htt tm pni ssse3 cx16 sse4_1 sse4_2 popcnt aes syscall nx lm vmx smx tm2 pbe
19.jun.2013 10:10:35 | | OS: Microsoft Windows 7: Enterprise x64 Edition, Service Pack 1, (06.01.7601.00)
19.jun.2013 10:10:35 | | Memory: 3.88 GB physical, 7.86 GB virtual
19.jun.2013 10:10:35 | | Disk: 297.74 GB total, 222.20 GB free
19.jun.2013 10:10:35 | | Local time is UTC +2 hours
19.jun.2013 10:10:35 | | CUDA: NVIDIA GPU 0: NVS 4200M (driver version 266.96, CUDA version 3.20, compute capability 2.1, 483MB, 469MB available, 142 GFLOPS peak)
19.jun.2013 10:10:35 | | OpenCL: NVIDIA GPU 0: NVS 4200M (driver version 266.96, device version OpenCL 1.0 CUDA, 483MB, 469MB available, 142 GFLOPS peak)
19.jun.2013 10:10:35 | DistrRTgen | URL http://boinc.freerainbowtables.com/distrrtgen/; Computer ID 36002; resource share 200
19.jun.2013 10:10:35 | Poem@Home | URL http://boinc.fzk.de/poem/; Computer ID 149257; resource share 100
19.jun.2013 10:10:35 | Leiden Classical | URL http://boinc.gorlaeus.net/; Computer ID 90045; resource share 100
19.jun.2013 10:10:35 | Collatz Conjecture | URL http://boinc.thesonntags.com/collatz/; Computer ID 92115; resource share 100
19.jun.2013 10:10:35 | Einstein@Home | URL http://einstein.phys.uwm.edu/; Computer ID 4216186; resource share 100
19.jun.2013 10:10:35 | LHC@home 1.0 | URL http://lhcathomeclassic.cern.ch/sixtrack/; Computer ID 10142545; resource share 100
19.jun.2013 10:10:35 | Milkyway@Home | URL http://milkyway.cs.rpi.edu/milkyway/; Computer ID 325168; resource share 100
19.jun.2013 10:10:35 | ibercivis | URL http://registro.ibercivis.es/; Computer ID 265802; resource share 100
19.jun.2013 10:10:35 | SETI@home | URL http://setiathome.berkeley.edu/; Computer ID 6169925; resource share 100
19.jun.2013 10:10:35 | PrimeGrid | URL http://www.primegrid.com/; Computer ID 219482; resource share 100
19.jun.2013 10:10:35 | World Community Grid | URL http://www.worldcommunitygrid.org/; Computer ID 1715742; resource share 100
19.jun.2013 10:10:35 | DistrRTgen | General prefs: from DistrRTgen (last modified 18-Jun-2013 15:22:03)
19.jun.2013 10:10:35 | DistrRTgen | Computer location: home
19.jun.2013 10:10:35 | | General prefs: using separate prefs for home
19.jun.2013 10:10:35 | | Reading preferences override file
19.jun.2013 10:10:35 | | Preferences:
19.jun.2013 10:10:35 | | max memory usage when active: 3579.21MB
19.jun.2013 10:10:35 | | max memory usage when idle: 3579.21MB
19.jun.2013 10:10:35 | | max disk usage: 222.35GB
19.jun.2013 10:10:35 | | (to change preferences, visit a project web site or select Preferences in the Manager)
19.jun.2013 10:10:35 | | Not using a proxy
19.jun.2013 10:10:36 | DistrRTgen | Restarting task 22186457 mysqlsha1_mixalpha-numeric#1-9_16_755000x250000 - 227362500000_0 using distrrtgen version 348 (cuda23) in slot 2

Only running 1 project

Are there any other tasks ready to start or waiting to run?

Gruß,
Gundolf

Yes, plenty

I re-installed Boinc Manager 7.0.64, but still the same.
Only 1 project running and not maxing the CPU core.

There are many other project WUs waiting to be processed

Noticed some of the files in the Boinc folder were weeks old.
Stopped all WUs
De-installed Boinc
Removed all files from all Boinc folders
Installed Boinc again
Added all my projects.
At this moment, all seems to be back to old (correct) situation.
5 projects running at 100% CPU.

Keeping my fingers crossed

Yes, plenty

Which are?

For anyone else watching they were all Cuda tasks, with the exception of two tasks being Milkyway@home Multithreaded tasks:

http://boinc.fzk.de/poem/show_host_detail.php?hostid=149257

http://boinc.gorlaeus.net/show_host_detail.php?hostid=90045

http://boinc.thesonntags.com/collatz/show_host_detail.php?hostid=92115

http://einstein.phys.uwm.edu/show_host_detail.php?hostid=4216186

http://lhcathomeclassic.cern.ch/sixtrack/show_host_detail.php?hostid=10142545

http://milkyway.cs.rpi.edu/milkyway/show_host_detail.php?hostid=325168

http://registro.ibercivis.es/show_host_detail.php?hostid=265802

http://setiathome.berkeley.edu/show_host_detail.php?hostid=6169925

Since this host only has one GPU they were only ever going to run on that GPU, and not on any of the CPU cores.

Claggy

I'm not sure about that.
I had multiple tasks running at the same time from the same projects for years and it suddenly stopped.

As mentioned, i have killed all WUs, removed Boinc completely, installed Boinc fresh again, installed all projects again and now I have multiple projects running at the same time again.

No idea what went wrong, but it works again

No idea what went wrong, but it works again

You don't have any Milkywayathome Multithreaded tasks now.

Claggy