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CPU use is too low while load is high?!

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April 26, 2009 8:37:17 PM

Hi all :) 


I have a very weird problem if I can call it that. I have an intel Quad 9300 with an Asus P5Q-Deluxe motherboard (P45 chipset) and 4GB of DDR2 1066 RAM. No overclocking or anything like that. OS is Vista ultimate x64.

I always have very low CPU use, even when my system is doing something CPU heavy e.g. processing graphics filters on pictures in GIMP, encoding audio or video, or doing heavy compression operations (I have many many large archives where I store data).

When I perform one of these tasks I often have to wait and wait but my CPU utilization (an pretty much all cores) never goes above 50%. Only with a CPU benchmark was I able to see the CPU go to 90% and that was only for a few seconds.

So what gives? Do I have a faulty processor? Could my motherboard or RAM be the problem? (I doubt the latter, as I have occasionaly chnage the chips and don't recall any difference).

Any suggestions please please? I'm going crazy here!

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April 26, 2009 8:47:18 PM

At POST, does the motherboard report the correct CPU mosel? If you open Task Managerdoes it show 4 graphs? Run CPU-Z and check correct model and speed.

Mike.
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April 27, 2009 12:25:32 AM

Yes thanks, that stuff is all fine. POST reports correct CPU/speed as well as Ram timings etc. CPU-z also reports all the details correctly and I can see all 4 cores active on task mgr.

I'm even thinking of installing another OS to see if this could a vista x64 issues (which I admit seem unlikely).
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April 27, 2009 12:28:35 AM

Likely everything is working fine, but the CPU isn't the bottleneck (or the programs cannot use more than 2 threads).
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April 27, 2009 6:11:23 AM

That was my thought as well i.e. that something else is a bottleneck.

I don't think the problem is the lack of multiu-threading in applications because *none* of the cores utilization goes up significantly. I do typically notice that one is higher than others but still rarely above 50%.

To contrast with my work laptop which runs XP and a host of crappy coporate apps and has a mobile dual core intel CPU, the utilization rarely goes below 60% (really ugly) :-)
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April 27, 2009 8:02:33 AM

What hard drives are you using? Run HDTune to measure transfer rates and access times. You DID install 64bit motherboard drivers?

Mike.
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April 27, 2009 8:38:49 AM

perhaps thermal throttling? download prime 95 (newer multi-threaded version) and see if full load shows in task manager as 100%

also make sure your bios revision supports that paticular cpu

mike99 - you cant install 32bit drivers on a 64 bit OS

low end/budget/old apps also dont really use modern cpus too well and may use two threads max (50% load in task manager) as someone previously mentioned etc

also if you have insufficent system resources or another bottlekneck elsewhere the system wont be able to "feed" the cpu sufficent data to process efficently (100%) etc - low ram can do that, aswell as disk thrashing.

Post back with the Prime results for us
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April 27, 2009 9:37:26 AM

check you havent got "six-engine" set to power saver, it always keeps the cpu throttled to its lowest speeds if you do...
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April 27, 2009 9:40:02 AM

Thanks guys, I'll get prime95 later.

Regarding all the other possible problems you listed, know none of these apply - CPU is supported, bios is OK (or as OK as possible as this particular mobo had issues with buggy bios for a long time - I use a revision generally considered 'safe' as the latest version breaks a few things).

System drive is a velociraptor 300GB with other drives used for data etc.

Software is optimized - i.e. no unnecessary apps running (not that it made a real difference) no old or buggy software and I run native x64 apps whenever possible. I do not run a lot of things in the background, and regardless my issue is not high CPU use, it is too low! :lol

Honestly I'm most suspicious of the motherboard but maybe the prime95 test will help. I'll post results this evening!
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April 27, 2009 12:12:33 PM

If you have a quadcore and use a single-threaded application, you still would see maximum 25% cpu usage even if your CPU is the bottleneck because one core is at full capacity. With single-threaded apps its MHz that counts and not the number of cores since only 1 will be used.
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April 27, 2009 1:24:45 PM

I'm pretty sure many/most of these apps are multithreaded: (a) I know it for a fact for some of them (b) I can see the number of threads spawned in task manager.

Anyway if all cores appear to be at 25% despite one of them being at 100% it means that task manager reports garbage. Who cares about average CPU use and what can you with it if you cannot even see which core is overloaded.

However I do not believe that is the case because I can often see one core being more heavily loaded than others. If task manager reported averages, they would all always be the same, so this is clearly not the case.

The question is, what is stopping these apps from using all the available CPU cycles from even ONE of the cores? Because none of the cores ever goes above 50-60% and that's only for seconds.
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April 27, 2009 1:38:10 PM

'if you cannot even see which core is overloaded' - operating systems will switch threads from core to core so no core will ever get overloaded. If you run prime95 without explicitly binding it to a core (using the CPU affinity option) you should see 100% usage on one core, but it could switch to other cores over time.

I'm not sure if all versions of windows behave the same in this regard, but you may check if its true in your case. Also, are you sure the harddrive isn't being accessed while you're doing processing tasks. If its used, it may be a bottleneck of its own, especially mechanical disks since they have a very high latency (i.e. the cpu has to wait on the HDD and could do like 40.000 other operations in the meantime. In Linux you can see this quite easily by looking at "IOwait" percentage. If the cpu is used 13% but has 87% IOwait, that means the CPU can't work faster because it has to wait on the disk alot, basically spoiling the cpu power that could have been used if the disk was faster.
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April 27, 2009 2:00:38 PM

Thanks sub mesa - believe me I get it but I'm still not sure this is it.

I have four other computers in the house alone (server, HTPC, wife's PC) running a variety of OS Vista, XP and ubuntu). None of them behave this way. E.g. the Core2Duo HTPC routinely hits 70% utilization when upscaling video from 720p to 1080p with ffdshow post-processing, and it also runs vista (albeit the x86 version). But I've never seen a system where you sit and wait for a video to be transcoded while the CPU is at 22%. This has gotta be a mobo issue or an OS issue or something. :o 

I don't think its the disks - while slow they are as fast as it gets before you move to SAS or SSD. I cannot believe they are such a bottleneck in this PC while others are not.

This is really weird. I will run prime95 tonight and report back.
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April 27, 2009 2:48:12 PM

If a duo is at 70%, a quad would be at around 35%, all other things being equal.
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April 27, 2009 4:16:35 PM

Have you tried setting the affinity to two cores rather than all four?
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April 28, 2009 6:31:41 AM

cjl said:
If a duo is at 70%, a quad would be at around 35%, all other things being equal.


True but that wasn't my point - I meant only that I have seen much higher CPU use in systems other than the quad. When the c2d is busy CPU will go all the way up to 90% until the load lessens.

What the quad is doing is taking 5 minutes to complete a task at 10-25% load when it could be completing it faster (theoretically :kaola:  ) if it was going up to say 50% utilization for a few seconds.

Mousemonkey said:
Have you tried setting the affinity to two cores rather than all four?

No, but I will test if it makes any difference. Kind of doubt it at this stage but its worth a shot.
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April 28, 2009 6:37:29 AM

Just want to add a Thank You to everyone for your replies, I appreciate it very much!
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April 28, 2009 7:53:20 AM

dimitrik said:
True but that wasn't my point - I meant only that I have seen much higher CPU use in systems other than the quad. When the c2d is busy CPU will go all the way up to 90% until the load lessens.

What the quad is doing is taking 5 minutes to complete a task at 10-25% load when it could be completing it faster (theoretically :kaola:  ) if it was going up to say 50% utilization for a few seconds.


No, but I will test if it makes any difference. Kind of doubt it at this stage but its worth a shot.

My point was more that if your duo isn't going to 100% though, something else is the bottleneck. Since something else is the bottleneck, upgrading to a quad will not inherently speed up your applications, unless you also eliminated the bottleneck.
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April 28, 2009 11:55:40 PM

OK, prime 95 and process explorer have solved the problem (such as it was). IT turns out that windows does try to load balance threads accross all CPUs, as much as possible. It takes a large number of cyclle hungry threads to overload the quad.

Running prim95 torture test with 4 threads took all 4 cores to 100%. Running 3, thne 2, and 1 threads progressively decreased the overall load to 80%, 60% and 40% respectively, when all threads were evenly distributed.

Changing the affinity allowed me to load one core to 100% leaving the others idle.

So obviously I was worrying about the wrong thing - the bottleneck in my system has to be the number of i/o operations it is capable off. I checked the slow processes using perfmon and they all suffered from longer i/o queues & wait time.

Since i don't think this is a hardware issue (the spec is reasonably high end for workstation) I will test ubuntu on this machine and see if that makes a difference. Otherwise I better hope windows 7 is a lot better than vista x64 ;) 

The good thing is this has stopped from upgrading to an i7 which I will now not do, until I know I'm reaching the limits of the quad. Unless of course the i7 platforms have more efficient i/o, maybe I should investigate... :kaola: 

Thanks to all of you for your help with this - your advice has been invaluable and saved me a bundle of cash!
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April 29, 2009 2:09:36 AM

Yea, thats a 'feature' of Vista.
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April 29, 2009 6:15:36 PM

If your bottleneck is data access you could try a second drive in RAID 0 stripe. Pendrives are fast access times, but the transfer rate can be slow. Try copying your data to the hard drive and see if it makes a difference.

Mike,
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April 29, 2009 6:40:24 PM

Hey, turns out i was right. ;) 

Seems you might be better off using an SSD to 'fix' the storage bottleneck in your situation. Mechanical HDDs will not be fast when the I/O is not sequential (enough). An SSD is king in IOps performance, both for reads and writes if you pick a good SSD. I believe it's worth investing in them already, since HDDs can really slowdown PC's with otherwise high-performance hardware.

Mechanical disks are not in league to match the performance of the other components, alot is done to hide this limitation inherit to mechanical storage, but ultimately you will pay in slowdowns and your CPU just waiting for the data to arrive. In the meantime, it could have done alot of work, many tens or hundreds of thousands of operations can pass before the harddrive return the data of one I/O request.
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