Helping people with computers... one answer at a time.

The system idle process's purpose is to get out of the way to let other processes run. So what does it mean if the SIP has 100% CPU on your slow system?

I understand, Leo, that in your article What is the System Idle Process and why is it using most of the CPU? you are saying the System Idle Process (or SIP) is actually doing nothing. But I, like several others, do find that, while we are using the computer (so it is not idle), all processing slows down and becomes sporadic. When that happens, Task Manager (in XP) shows only SIP using the CPU. If SIP is not causing the slowdown, what is?

I do get a lot of pushback on that article from people who are absolutely convinced the system idle process is somehow evil and must be eradicated simply because their computer is slow and "System Idle Process" is what's at the top of the CPU usage list.

They, of course, are wrong.

System idle is benign. The CPU has to do something 100% of the time, so when it has nothing to do for you or for the system, it's assigned the idle task to while away the time. It's the CPU equivalent of twiddling your thumbs, waiting for something to do.

So why's your system as slow as molasses?

Well, I'll give you one hint: the CPU is not the only thing in your computer that affects its speed.

The CPU or central processing unit is often referred to as the brain of your computer. It runs programs which are sequences of instructions that cause it to perform calculations and just generally tell the rest of the computer what to do.

"If the CPU is mostly idle - spending the majority of its time in the System Idle Process - then the CPU is not slowing your system down."

We often place a lot of importance on the speed of our CPU. A faster CPU can perform those instructions faster and get us faster results. Similarly, if we run too many programs at once that are all making demands on the CPU the system gets slow. The CPU simply can't execute those instructions fast enough to keep up with everything we're asking of it.

That's so common, in fact, that whenever the system slows down people automatically assume that it must be due to the CPU being overloaded - even if the task manager shows that the CPU is actually spending most of it's time doing absolutely nothing:

Task Manager showing System Idle Process at the top

I'll put it very clearly:

If the CPU is mostly idle - spending the majority of its time in the System Idle Process - then the CPU is not slowing your system down. The System Idle Process is not "hogging" your machine. It just isn't.

Fine, so it's not the System Idle Process... then what the heck is it?

It can be many, many things - almost all of them related to your hardware.

By far the most common is your disk. If your disk drive is continuously active while the CPU is idling, then whatever is using the disk is what is making your system slow.

And the most common cause of that?

Not enough RAM.

Here's how that scenario plays out:

You have some amount of RAM, and you end up running software that requires more RAM than you actually have. It could be running too many programs simultaneously, it could be a program that is requesting a lot of RAM for whatever it's doing, it could even be a program that has a bug that causes it to continually ask for more RAM without reason. Whatever the cause, the net result is that the software or combination of software that you're running needs more memory that you have installed on your system. (And to be clear, I'm talking about having enough RAM memory, which is completely unrelated to your available disk space.)

When a program requests more RAM than you have, the system uses what's called "virtual memory" to satisfy the request. Through a somewhat complex process it uses some space on your hard disk in the form of a "swap" or "paging" file to make it look like you machine has more RAM than it really does.

All that happens transparently - the programs themselves actually don't know that it's going on. To them, it simply appears that the machine has enough memory, but sometimes things take longer than you might expect.

This is where performance enters into the picture.

Virtual memory involves writing to ("swapping out") and reading from ("swapping in") the hard drive. And if the software being run is requesting a lot of additional memory over what the machine really has, it can be a lot of writing to and reading from the hard drive.

As fast as they are, hard disks are very slow compared to RAM. Even the newer solid state flash-based drives are still orders of magnitude slower than actual traditional RAM. That means these disk accesses that are happening to swap memory in and out take time - sometimes noticeable time.

In fact, if there's enough virtual memory activity, your system can slow down to a crawl as all the software attempting to run causes memory to be swapped in and out repeatedly.

While all this is happening, you wait. Your system is slow.

You know what else is waiting?

The computer's CPU.

You might even say that while all this is happening your CPU is ...


Like I said, hard disks are slow. When this type of virtual memory "thrashing" is going on it's not at all uncommon for the system to get into a state where it's spending the majority of its time waiting for the hard drive to read or write the memory being swapped in and out.

The CPU is mostly idle, but your system is slow as molasses because its spending all its time waiting for the hard disk.

The solution depends on your situation, of course.

  • Don't run so much software at the same time.

  • Don't run as much software that requires a lot of memory.

  • Don't perform operations that require a lot of memory - perhaps editing a huge picture or video, for example.

  • Run the latest versions of the system and your software so as to get fixes for any "memory leaks" that are discovered.

  • And of course, consider getting more RAM. It's cheap these days.

And while hard disk thrashing is perhaps the most common scenario that can lead to this symptom it's most certainly not the only one.

The bottom line, though, is that you can stop blaming the System Idle Process for hogging your machine - it's not. In fact, if it looks like it is - if the System Idle Process is using lots of CPU while you experience a performance problem - then that's a clear sign to look somewhere else for the source of your computer's troubles.

(I hate to say it, but because I know what'll happen: comments to this article that continue to insist that the SIP is the problem will be deleted. You didn't read the article.)

Article C4338 - June 10, 2010 « »

Share this article with your friends:

Share this article on Facebook Tweet this article Email a link to this article
Leo Leo A. Notenboom has been playing with computers since he was required to take a programming class in 1976. An 18 year career as a programmer at Microsoft soon followed. After "retiring" in 2001, Leo started Ask Leo! in 2003 as a place for answers to common computer and technical questions. More about Leo.

Not what you needed?

June 11, 2010 12:56 AM

Yet again, Leo has the answers.

I use a free program that monitors my memory useage, and it gives me a nice readout in the taskbar of unused physical memory in Mb.
I have noticed that this figure drops very low on some occations (I have seen it as low as 6) and if below around 150 my laptop slows down and even sometimes freezes for anything up to a minute while my HD light is on continuously. And yes, CPU is twiddling it's digital thumbs (I minimize Task manager to moniter exactly that).

Ken B
June 11, 2010 5:57 AM

Then there are people who say that there's something wrong if the CPU usage ever gets to 100%. (ie: some program is "broken" and "hogging the CPU".) They can't grasp the concept of "CPU-bound computation", and the report generator's sorting routine is not "stealing" the CPU away from anything else. It's simply using up the otherwise-idle time.

On a separate-but-related question, when the CPU is not at 100%, but the system is dragging its feet and the HD light is on nearly continuously, is there a way to find out what process is causing the disk thrashing? (It's not caused by over-swapping in my case, as there is still physical memory available.) I have Process Explorer, but I can't seem to find a "sort by who's using the HD most" type of option.


I actually have a couple of techniques that I use. With Process Explorer I add the "I/O Delta Reads" and "I/O Delta Writes" columns, and just sort by those to see what process is doing the most. That's a good 90% solution to the question "who's thrashing?".

To answer the inevitable "why?" I fire up Process Monitor which will show all disk activity right down the level of what objects are being accessed. More here: I have constant disk activity, and I don't know why. How can I tell what program is doing it? - the only problem is that to do this Process Monitor can itself eat up resources and affect the problem, particularly if that problem is swap/memory and not application activity related.

June 11, 2010 8:19 AM

Ken: You might be able to use the I/O graph to see what process is reading/writing most bytes to disk. If it's the system it's probably paging, or it might be a driver that's used when you're writing to disk (I'm thinking about the TrueCrypt driver, when you copy something the system uses that to encrypt things on the fly).
You can also add columns to monitor the reads and writes for each program: view>select columns...>process performance>i/o delta reads/writes. I'm not sure what the I/O other would be for, maybe Leo can explain.

June 11, 2010 9:28 AM

I use Anvir's task manager to keep track of the CPU, disk read and writes, RAM and connectivity. By mousing over its icons in the taskbar I can see what programs are eating up which resources.

What I've found is that when my machine slows down it is most often due to one of two culprits:my security software updating or wuauclt.exe. I've recently changed security software from McAfee to Norton its gotten a little better, but not by a lot.

Dan Geiser
June 12, 2010 10:37 AM

Great article, Leo! I find it funny that people think that the computer is somehow monitoring this process called "System Idle Process" rather than just displaying a value there that equals 100 minus The Total CPU Usage of All The Processes.

Ken B
June 14, 2010 7:30 AM


Thanks for the follow-up. I've got procmon downloaded and ready to go the next time it happens.

I think it's time to buy you another latte...

June 15, 2010 8:47 AM

Not only is the article good -- even the comments are good. Worth a Latte.

Nicholas Gimbrone
June 15, 2010 10:03 AM

Not only the disks, but also the network can serve as a source of slow response... and then there is the dreaded "lock contention" (all sorts of reasons) that can serve as another source (ok, we went geeky internals there... bygones...). The nice(?) thing about this is that all of these can (and do! ;-) occur in all operating systems. ;-)

June 15, 2010 10:03 AM

I don't have a disk in my laptop...
What am I missing?

June 15, 2010 2:49 PM

Absolutely great article for people not using Windows 7 :). In windows 7, in the taskmanager when clicking processes from all users, the SIP is described as percentage of the time the CPU is idle. That imho underlines once again how well Win7 is put together.

June 15, 2010 3:58 PM

When you hear hard drive thrashing , check the processes, [ R click bottom task bar, L click on task bar menu ]
The noisy culprit is usually indicated my high CPU usage.
Check that your "virtual memory" settings are correct{do a Google search for directions} and do normal sruff like disk clean and defrag all helps.

Actually if you re-read the article carefully you'll see that's exactly not what we're talking about here. The CPU is primarily idle.

June 15, 2010 4:42 PM

....Google search if you are not sure what the other processes running are.


June 15, 2010 6:39 PM

It might be useful to point out to people worried about System Idle CPU usage that, by MS logic, the figure seems to mean % of idle not usage. If you look at the next tab in Task Manager, "Performance", a "System Idle Process" of say 98 will show 2% CPU usage.

Just Me
June 15, 2010 8:42 PM

"The CPU has to do something 100% of the time"

as a project, I proved the same thing with DOS,

that the processor never idle doing absolutely nothing
it's always doing something

I loaded the DOS 6.20 into a debugger and it showed a tiny 4 or 5 instruction loop that just looped waiting for input at the C:\

Joseph Bernard
June 16, 2010 3:48 AM

Am using a Window 7 Home Premium and my system is very slow. When I am playing Window Media Player it become unreasonable slow

June 16, 2010 6:33 AM

Dear Leo,

in this article you show how to use the Windows Task Manager to see what is happening. The tab "Performance" shows some interesting graphs and below that a table with twelve parameters. Could you please spend an article explaining what these parameters mean? I tried to find information on Microsoft's website but was not successful.

Karl Medisch
June 16, 2010 7:22 AM

Leo, I inherited a computer with XP Home. Occasionally after start up it would not respond. I would try to open a folder or a file and the hourglass would appear and then nothing. Waits of up to five minutes and nothing. A reboot would usually fix that except that many times the computer had to be crashed since not even Task Manager would respond. The malevolent spirit that had the computer enthralled simply kept the computer hostage. I did all the checking of running processes to determine their origin and killed everything that was unnecessary to run the basic machine. No change. I did a reinstall of the OS plus service packs but did not format the drive so as to retain as much of the data as possible. No change. Finally I formatted the hard drive reinstalled OS and service packs and problem gone. The only conclusion I came to was that an OCR program listed in the ADD/REMOVE programs could not be uninstalled and I could not find folders, start up entries or whatever for the program. There was also no evidence of that program in Task Manager. My guess is that it had been damaged / removed incorrectly and left some piece of dross that gummed up the OS. As a standard practice I now use Revo Uninstaller in the most brutal mode and also CCleaner to check for left over entries.

June 18, 2010 7:05 AM

Yes this issue can occure if you are running too much software simultaneously.

so only run the once you need at that particular time and close the rest.

then check your startup programs and disable the ones you don't need at startup and this should speed up your system


July 18, 2012 5:47 PM

This is a complete lie, my Systems Idle Process is the only reason my computer is so slow, it is the only thing running besides Firefox and I am trying to watch a 2min video and it keeps messing up because System Idle is taking up too much of my CPU, its making me go to 100% CPU when I should be at 10% total. There is no reason System Idle should be on if I am doing something on my computer it should only be on when my computer is not being used but still on.

August 28, 2012 5:55 AM

Seriously Jacob? You apparently did not read this article at all. So by your reasoning, your SIP (system idle process) should be at 0% while you are doing something on your PC. If this were the truth, this would mean that your cpu would have no time to do anything but that single process /program you are working with therefore not allowing it to "think" about anything else and become extremely slow. Also, even though you may not be working on your PC, your cpu is constantly working regardless. If it weren't your computer wouldn't function at all.

Here, let me put this into easy terms for you:

Take your brain for example. If you are just sitting in one spot motionless, doing nothing, does your brain stop? No it is still working. If you are sleeping does it stop? No, otherwise you would cease to live. However it allows you to multitask and function on a daily basis (no offense to people that have a hard time walking and chewing bubble gum).

Now take your brain and throw 10 simultaneous tasks at it at once. Your brain is now very far from idle but you still exist and breath even though you don't have the time to even think about it.

Now how is this idle at 100% bad? It's not. It basically means it has some down time to sit and do almost nothing at all. Do some more reading and educate yourself please.

August 28, 2012 5:56 AM

Oh, by the way Leo....I though you did explain that very well.

Maya Computer-Isslow
October 11, 2012 8:57 AM

Hi Leo, thanks for your numerous explanations of the SIP non-process. I think you've hit many good points, however, I did a lot of staring at my Processes in Task Manager this afternoon, and I noticed that some of my intensive (Excel) process never got > 25% of my CPU's attention. I remember a long time ago, years, that some of my larger Excel files used to get 100% of my CPU time in the crunch. Kind of like a governor plate in NASCAR, something appears to limit my CPU's dedication to processes. Is there some other invisible hand that makes my CPU "lazy and undedicated" to the important tasks at hand, and how do I give it motivation to "change". ;-)

That would be this article: Why won't my program use more than 25% of the CPU?

November 6, 2012 9:32 AM

The biggest problem really are the inadequate tools provided in every edition of Windows to quickly find what the wait event is.

Anyone that regularly uses a unix/linux machine will know what I mean.


will tell you instantly where your wait event is.

If on a server somewhere without a bunch of additional tools... resorting to vmstat, sar, iostat, and the whole array of vendor dependent commands you see in AIX and Solaris... just to name 2... is also not a bad solution (tho you do need some knowledge to use them).

But like Leo says... it's almost always memory and swapping. Windows is terrible at managing it.

February 16, 2013 1:05 PM

Just a week ago I noticed an much intensive CPU usage for SIP. I also compacted old files, so, maybe, it takes much longer to encode back those files and use them for daily usage... It seriously slows computer even to the state that even typing gets glitchy or mouse moves in chunks... I was even considering a virus of some "disturbing" manner, because this is just

Comments on this entry are closed.

If you have a question, start by using the search box up at the top of the page - there's a very good chance that your question has already been answered on Ask Leo!.

If you don't find your answer, head out to to ask your question.