a lot of paging in/out - What is it?

[Private_Member_19084]

Hi experts,

I am jumping via google from one page to another one and I am still not sure what paging is and what I am looking for

What I got is, that paging is working differently to swapping and just storing parts of a process on the disc.

Hopefully this is correct.

But what is not, page-in, page-out and faults?
Does it mean the following:

- page-in, bringing the pages from memory to disc

- page-out, bringing the page out of the disc

- faults, bringing the page back to memory as it was requested?

Next item is, in "sar -b" i find values like 13318 pgpgin/s.

We have pagesize 4096, so this means weave a paging-in from around 53272 kb/s.

But what means /s? Is it an average or total value? I can't imagine that the system would page 53 MB per second.

Next questions, why is it paging, when still a lot of memory is available?

Kind regards

[manoj2910]

There will be other applications along with Buffer Cache. Buffer Cache is created in the area of kernel Memory. So the memory would be full since other applications co-exists along with buffer catche.

[manoj2910 · Accepted solution]

Hi Christian,

To summarize what it means, your computer has a certain amount of RAM, which is its "memory".  It simulates more RAM by allowing extra data to be saved to the hard disk, which is known as Virtual Memory. To do this, it breaks your memory space up into "pages". Applications that need access to data that is in memory call the data by page. If an application calls a page and it is in the RAM, then it is a "page in" occurs. If an app calls for a page from memory, and that page is currently stored on the hard disk and has to be read back into the RAM, then a "Page Out" occurs. A "Page-out" slows the operation of the system down because it has to read the data from a hard disk into RAM first, rather than reading straight from the RAM. If page-outs exceed page-ins, you definitely don't have enough RAM.

Writing memory pages to the page file when there is still plenty of RAM is a good thing. As soon as a program requests more memory than there is free, the OS can start clearing out space as soon as possible. Better to prepare now than later. If the OS were to wait around, then you run into a performance bottleneck. If a program asks for more memory than is available, now you have to wait until the OS writes out changed memory block, and then free them up.

[Private_Member_19084 · Accepted solution]

So page in, are pages which are available in memory (RAM) and page out are memory which is not available in memory and has to be read from disc to memory?

I thought, one of the page in/out is the values which shows what is sent to disc and the other which is coming from disc?

And what is the fault value?

[manoj2910 · Accepted solution]

In a virtual memory system, memory is described as paged in if it is available in physical memory.  memory is described as paged out if it is not available in physical memory.

“Page fault” usually means an access to a page that has been paged out and hence requires fetching from disk, but it is sometimes also used to mean invalid page fault or protection fault.

[Private_Member_19084 · Accepted solution]

So, page in is the physical memory the system did really use?

And page out the virtual memory the sysetm did use?

So page out should be very low in comparison to page in.

Kind regards

[manoj2910 · Accepted solution]

Yes, Page-out slows the operation of the system because it has to read the data from a hard disk into RAM first, rather than reading straight from the RAM. If page-outs exceed page-ins, you definitely don't have enough RAM. Ideally, page-outs should be much less than the number of page-ins.

[Private_Member_19084 · Accepted solution]

Thx a lot Manoj,

but I don't really get what the information page-in is good for, if it is exactly the same information than used-RAM.

Or is there any difference?

Kind regards

[manoj2910 · Accepted solution]

OS uses a page file to store data that can’t be held by your computer’s RAM when it fills up. The Windows page file is somewhat misunderstood. People see it as the cause of slowdowns because it’s slower to use the page file than your computer’s RAM, but having a page file is better than not having one.Your computer stores files, programs, and other data you’re using in your RAM because it’s much faster to read from RAM than it is to read from a hard drive. For example, when you open Firefox, Firefox’s program files are read from your hard drive and placed into your RAM. The computer uses the copies in RAM rather than repeatedly reading the same files from your hard drive. Windows will try to move data you aren’t using to the page file. For example, if you’ve had a program minimized for a long time and it isn’t doing anything, its data may be moved to RAM. If you maximize the program later and notice that it takes a while to come back instead of instantly snapping to life, it’s being swapped back in from your page file. You’ll see your computer’s hard disk light blinking as this happens.

[Private_Member_19084 · Accepted solution]

Got it, but you wrote "If an application calls a page and it is in the RAM, then it is a "page in" occurs."

Or is the RAM not completly splitted into page-files?

Because if so, this would mean, that this would exactly show which RAM was used or am I completly wrong?

[manoj2910 · Accepted solution]

No the RAM does not have page files.  The virtual memory has page files which has the same size as that of blocks in RAM. Its maintained in Page table which is maintained in the operating system.

[juergen_weber · Accepted solution]

explained very well THX