2 of 4 cores slow down significantly when I turn main monitor off
 

I am seeing some strange behavior when I turn my main monitor off - Prime95 threads on cores 2 and 4 slow down - per iteration times increase by 50% or more. The threads on cores 1 and 3 remain unchanged. Sometimes I have to stop the workers on 2 and 4 and restart them in order to make them resume at "normal" speeds. This is with v. 27.7 build 2 on Win7 Home Premium 64-bit, Core i7 3770S processor. Anyone have any ideas what could be causing this and how to prevent it from happening? I have Power Options set for the computer to never sleep - that's the only thing I could think of to check.

Prime95 is at the lowest possible priority. I would guess that there are some processes which start when your computer is in "idle" state, that is main monitor turned off. So first thing I would do is to go into Task Manager and look which program/process is using your cycles, maybe an anti-virus program, maybe automatic upgrade.

Additional to what Ake said, The "S" processors may have the particularly fancy habit to reduce the power (they are low power versions of the "K" processors) - and therefore the clock - when parts of the chips are not used. This is (partially, beside of lower default clock, etc) how they get the lower TDP. If you are using HD4000 integrated graphic to drive that monitor, you may want to look for this cause too. I may talking rubbish here (this mean take it with a big dose of skepticism) but I saw something similar long ago with a 2600S compared with a 2600K, that time I didn't care about, but now I remember it.

[QUOTE=aketilander;309067]Prime95 is at the lowest possible priority. I would guess that there are some processes which start when your computer is in "idle" state, that is main monitor turned off. So first thing I would do is to go into Task Manager and look which program/process is using your cycles, maybe an anti-virus program, maybe automatic upgrade.[/QUOTE]

Thank you for the idea - however, it's a bit tricky since I can't access Task Manager when the monitors are off. It seems to resolve itself when the main monitor is turned back on, and AFAIK there is no way in Task Manager to log status of CPU usage.

[QUOTE=LaurV;309072]...If you are using HD4000 integrated graphic to drive that monitor, you may want to look for this cause too. I may talking rubbish here (this mean take it with a big dose of skepticism) but I saw something similar long ago with a 2600S compared with a 2600K, that time I didn't care about, but now I remember it.[/QUOTE]

Thank you for your input. I was using discrete graphics (HD 7750) when I first noticed this problem. Now I have switched to integrated graphics to see what would happen. Now only one core (core 2) is affected, but the effect is far worse on that core (upwards of .5 sec per iteration time as opposed to "normal" of .027. With discrete graphics I was seeing something like .037 on core 2 and .045 on core 4. So I will switch back to HD 7750 now.

Do you have a screensaver? Switch to "Blank Screen".

[QUOTE=garo;309114]Do you have a screensaver? Switch to "Blank Screen".[/QUOTE]

No, haven't seen the need for that for a long time now. Just to be clear, I only see this when I switch the last monitor off, not when the monitors go into standby mode. I guess I should test that, too, just to see what happens.

I have switched back to the HD 7750 discrete graphics now, and now it is worker 3 (only) that is exhibiting this behavior. I tried moving the Task Manager window and the Prime 95 window to the secondary monitor and then switching off the primary monitor while leaving the second monitor on. However, the slowdown only occurs when both monitors are switched off, so it looks like Task Manager will not help in diagnosing this.

Could you try opening a console window, typing 'tasklist /v' to get the list of running tasks, then turning the monitors off and pressing up-arrow return to get the list of running tasks with the monitors off? See if any new task starts up when the last monitor goes off.

2 of 4 cores slow down significantly when I turn main monitor off - SOLVED
 

[QUOTE=fivemack;309142]Could you try opening a console window, typing 'tasklist /v' to get the list of running tasks, then turning the monitors off and pressing up-arrow return to get the list of running tasks with the monitors off? See if any new task starts up when the last monitor goes off.[/QUOTE]

Thank you for this suggestion - it worked perfectly. The culprit was Windows Indexing (SearchIndexer.exe in the task list). So once the initial index is built, it should return to normal.

drive indexing
 

fyi: since you said you have a 3xxx intel processor you may have a Solid State Drive.

If you do have a SSD you should turn off drive indexing all together to increase drive life with no performance decrease.