[tim2718281]

But if IO wait is at 100% all the time then it will slow down my system?

No, it means your system is going as fast as the IO system can manage.

[freshwire]

It will mean that other processes wanting to use IO will be slow. Those just using the CPU for computation will not be slowed.

[antdgar]

I assume you're referring to "48% IO"

That means 48% of the time, the CPU had no work to do while IO was in progress.

There's no reason to try to reduce it.

But if IO wait is at 100% all the time then it will slow down my system?

[tim2718281]

What is IO and how can I reduce it?

I assume you're referring to "48% IO"

That means 48% of the time, the CPU had no work to do while IO was in progress.

There's no reason to try to reduce it.

But you might want to check overall transaction times, to see if it's worth spending effort trying to reduce them.

[Myatu]

In htop, the IO wait time is included with the idle time. This is actually is more accurate description, given that the kernel (and in turn, the user process) are waiting for a response (and thus, "sitting idle").

However, if you wish to see the details in htop to filter the good vs. bad idle time (a-la top), press F2 (for Setup - see the bottom row for what each key does) and use the arrow keys to navigate that menu.

First select "Meters" (default), then use the right-key to go to the "Available Meters" column. Now use the down-key to select "CPU Average". Press the F5 button to add it to the left-hand side column (you could add it to the right hand side, but we'll change it to show a lot of details). It should now appear in the left-hand column, under the bar graphs.

Use the left-arrow key to select the "Left Column" column, and the down-key to "CPU [bar]". Since a bar graph isn't going to provide you with much details in this case, change it to text by pressing the Enter or F4 key.

Finally, return back to the first column ("Setup") and select "Display Options" (under "Meters"). Press the right-key, then move to the last entry "Detailed CPU Time" and press the Enter key to tick the box. This is what you'll end up with:



The last entry is your wait time. As you can tell, you can do a lot more stuff with htop this way, such as re-arranging things the way you like them, specific details, etc., etc. When you're done, hit F10 and your settings will be saved for next time (so you don't have to do this each time you use htop).

[antdgar]

htop doesn't show %wa (I/O)?

[Myatu]

Why does htop not seem to be showing as many processes using CPU as top?

You did use PGUP / PGDN, right (just sort by the process ID to have a better side-to-side comparison).

[NickW]

I love htop - a bit more intuitive / cleaner.

Why does htop not seem to be showing as many processes using CPU as top? It's like this on all updates between the two not just that instance. I'm sure the one on top is how it should look.

[Neil]

I love htop - a bit more intuitive / cleaner.

Nice, like all the fancy colours.

[Myatu]

I love htop - a bit more intuitive / cleaner.

[HandsomeChap]

just press the 'q' key

[antdgar]

If you type TOP in ssh it should give you an output of high usage apps. Check the memory (and others) to see if anything has decided to behave bad.

Thanks, that was useful. How can I exit gracefully?

[Neil]

How can I find out exactly which process is causing this?
none of the processes are using much CPU... but hurting IO

If you type TOP in ssh it should give you an output of high usage apps. Check the memory (and others) to see if anything has decided to behave bad.

[antdgar]

How can I find out exactly which process is causing this?
none of the processes are using much CPU... but hurting IO

[NickW]

Ahh, I see. I believe my database software has an option to increase ram cache. Will increasing this reduce IO usage?

Yes, do that.

You probably have a high server load average with that amount of I/O clogging the CPU?

[antdgar]

A RAM cache could perhaps reduce the I/O time.

Ahh, I see. I believe my database software has an option to increase ram cache. Will increasing this reduce IO usage?

[Myatu]

From "Highest" to "Lowest" levels:

1. User mode: software applications (ie.: Mozilla browser)
2. Kernel mode: privileged software (ie.: Linux, Windows)
3. I/O: device communication time (ie.: "Write two bytes to the hard disk at position X" [more technically: the kernel waits for an answer from the SATA controller and the HDD, so it can report back to the software application "Yep, those two bytes were written without any problems, go do your next thing!"]).

A high I/O usage indicates a "congestion" communicating with devices. This could for example be when you are trying to write bytes to the HDD faster than it can cope. For example, a dozen different applications are writing to a MySQL database located on the same HDD. A lower I/O time is better, but it cannot always be lowered due to server demands (ie: database server) and hardware capabilities. A RAM cache could perhaps reduce the I/O time.

[yonatan]

CPU I/O - as in Input Output ,
its the actual processes the goes through your CPU.
in order to reduce it, close applications that are not in use ( if any ) , and optimize your applications ( website / sql databases etc ... ).

you can read more about I/O here:
http://en.wikipedia.org/wiki/Input/output

44% Idle is pretty good, you are not using it "too much".

[antdgar]

What is IO and how can I reduce it?