What is paging and swapping in linux

Linux: The difference between “paging on major page fault” and “swapping enabled manually” [closed]

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sudo fallocate -l 500M /data/swapfile sudo chmod 600 /data/swapfile sudo mkswap /data/swapfile sudo swapon /data/swapfile 

But even when this is not enabled, the kernel still does paging when a page is not in memory. We can verify this by running the sar -B 1 30 command on a machine without setting any swap file.

03:08:40 AM pgpgin/s pgpgout/s fault/s majflt/s pgfree/s pgscank/s pgscand/s pgsteal/s %vmeff 03:08:41 AM 0.00 0.00 3.00 0.00 44.00 0.00 0.00 0.00 0.00 03:08:42 AM 0.00 0.00 19.00 0.00 30.00 0.00 0.00 0.00 0.00 03:08:43 AM 0.00 0.00 0.00 0.00 3.00 0.00 0.00 0.00 0.00 03:08:44 AM 24.00 0.00 2.00 1.00 7.00 0.00 0.00 0.00 0.00 03:08:45 AM 364.00 60.00 18.00 3.00 4.00 0.00 0.00 0.00 0.00 03:08:46 AM 140.00 0.00 392.00 2.00 243.00 0.00 0.00 0.00 0.00 

• Can we say there are two types of swapping on the OS?
• How do the two mechanisms work differently?
• If there is always a paging mechanism working, why is there still a need to enable swap manually?

Swapping refers to copying the entire process address space, or at any rate, the non-shareable-text data segment, out to the swap device, or back, in one go (typically disk).

Whereas paging refers to copying in/out one or more pages of the address space. In particular, this is at a much finer grain. For example, there are ~250,000 4 KB pages in a 1 GB RAM address space.

However, in the book Understanding the Linux Virtual Memory Manager,it doesn’t seem to be this way in Linux.

Strictly speaking, Linux does not swap as “swapping” refers to coping an entire process address space to disk and “paging” to copying out individual pages. Linux actually implements paging as modern hardware supports it, but traditionally has called it swapping in discussions and documentation. To be consistent with the Linux usage of the word, we too will refer to it as swapping.

Could someone shed some light on this? Thanks!

You seem to have done your homework, so kudos on that. Are you sure you’re reading the output from sar correctly? Since programs are loaded into memory as «page faults» mapped to the executable on disk, you’ll always have MAJFLTs.

By the way, I’ve never found sar -B to be helpful. I suggest vmstat 1 -SM for a live view of your swapfile activity. Just look at the si/so (swap in / swap out) columns.

Oh, and to answer your question about why the need for a swapfile if we have paging: Paging occurs when we access a memory mapped file or (as I mentioned) running an executable. A swapfile is what is needed when you run out of RAM as it provides a place to page out parts of a running process.

Hi! @hackerb9 Yes, I know the si/so in vmstat shows the activity in swap file. So based on what you said, can I say there are 2 swapping mechanisms? And could you give me more detail on how the two mechanisms works? Or some good references are welcomed 🙂

@JohntheTraveler please don’t copy and paste the same question everywhere serverfault.com/questions/1008629/…

2 Answers 2

There is still majflt which will trigger paging out data to the disk.

It’s the other way round: major fault are page faults which can only be addressed by paging in data from a disk.

Can we say there are two types of swapping on the OS?

Not quite; the distinction here is that paging out (copying the content of memory “somewhere else” so pages can be discarded) can use different backing stores. Pages in memory have different targets for paging out: mapped files are typically their own backing store, most other pages need a swap file or partition as backing store. Swapping refers to the latter. When the kernel needs to free a page of memory, if it wants to free an unmodified page from a file it can simply discard it, knowing that the page can be restored from the file whenever it’s needed. When it needs to discard a modified page from a file, if that file is mapped read/write with no sharing, the page will be written to the file before being discarded; again, the kernel then knows it can restore the page from the file. Anything else needs some other form of storage, or it can’t be discarded.

How do the two mechanisms work differently?

If there is always a paging mechanism working, why is there still a need to enable swap manually?

See above, mostly. The point of swap is to provide a backing store for pages which don’t have their own backing store already.

Hi @Stephen Could I ask one more follow-up question? Do these swapping operations backed with different stores have names for us to refer to?

Not really. My answer isn’t quite accurate, I’ll rephrase it; the general term used when copying the content of memory to “somewhere else” is paging out, and swapping is used specifically when paging out to a swap file or partition.

Your two citations have the historical/original meaning of swap. wikipedia Paging:

In the 1960s, swapping was an early virtual memory technique. An entire program would be «swapped out» (or «rolled out») from RAM to disk, and another one would be swapped in (or rolled in).

The Intel Manual is explicit:

When paging is implemented properly in the operating-system or executive, the swapping of pages between physical memory and the disk is transparent to the correct execution of a program.

(It goes on explaining it transparently works also for 16-bit virtual 8086 mode. )

AMD has also very readable parts:

Finally, system software can use the paging mechanism to map multiple, large virtual-address spaces into a much smaller amount of physical memory. Each application can use the entire 32-bit or 64-bit virtual-address space. System software actively maps the most-frequently-used virtual-memory pages into the available pool of physical-memory pages. The least-frequently-used virtual-memory pages are swapped out to the hard drive. This process is known as demand-paged virtual memory.

It does not consider a setup without swapping out to hard drive. In that case the LFUs would have to be written back to the file (if any) or dropped (if clean).

A little bit related is this new 64 instruction:

The SWAPGS instruction provides a fast method for system software to load a pointer to system data structures. SWAPGS can be used upon entering system-software routines as a result of a SYSCALL instruction or as a result of an interrupt or exception.

.

This exchange allows the system-kernel software to quickly access kernel data structures by using the GS segment-override prefix during memory references.

It always goes back to this segmentation:

Although segmentation provides a great deal of flexibility in relocating and protecting software and data, it is often more efficient to handle memory isolation and relocation with a combination of software and hardware paging support. For this reason, most modern system software bypasses the segmentation features. However, segmentation cannot be completely disabled, and an understanding of the segmentation mechanism is important to implementing long-mode system software.

So you can say there is this page-wise swapping out of RAM, and the context-switch side of swapping, reflected by the 60s implementation, but also the modern SWAPGS instruction.

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Difference Between Paging and Swapping

Swapping occurs when whole process is transferred to disk. Paging occurs when some part of process is transferres to disk. In this process is swapped temporarily from main memory to secondary memory. In this the contiguous block of memory is made non-contiguous but of fixed size called frame or pages.

1. What is the difference between page file and swap file?
2. What is paging and swapping in Linux?
3. What is swapping explain?
4. What is the purpose of paging or swapping in an operating system?
5. Does Windows 10 use a swap file?
6. Do you need pagefile with 16GB RAM?
7. What are the advantages of swapping?
8. What is difference between segmentation and paging?
9. What is swapping in Linux?
10. What are two advantages of swapping?
11. Why is swapping needed?
12. Why is paging used?

What is the difference between page file and swap file?

The swap file deals with modern Windows apps (the kind you download from the Windows Store), moving them to the hard drive in a sort of hibernation state when not in use, while the page file takes individual pages (4KB in size) of a process and moves them back and forth as needed.

What is paging and swapping in Linux?

Paging refers to writing portions, termed pages, of a process’ memory to disk. Swapping, strictly speaking, refers to writing the entire process, not just part, to disk. In Linux, true swapping is exceedingly rare, but the terms paging and swapping often are used interchangeably.

What is swapping explain?

Swapping. Swapping is a mechanism in which a process can be swapped temporarily out of main memory (or move) to secondary storage (disk) and make that memory available to other processes. At some later time, the system swaps back the process from the secondary storage to main memory.

What is the purpose of paging or swapping in an operating system?

Paging allows the memory address space of a process to be noncontiguous. Swapping allows multiple programs to run parallelly in the operating system. Paging is more flexible as only pages of a process are moved.

Does Windows 10 use a swap file?

Windows 10 (and 8) include a new virtual memory file named swapfile. . Windows swaps out some types of data that aren’t being used to the swap file. Currently, this file is used for those new “universal” apps — formerly known as Metro apps. Windows may do more with it in the future.

Do you need pagefile with 16GB RAM?

You don’t need a 16GB pagefile. I have mine set at 1GB with 12GB of RAM. You don’t even want windows to try to page that much.

What are the advantages of swapping?

• The process helps the CPU to manage multiple processes within the same main memory.
• The method helps to create and use Virtual Memory.
• The method is economical.
• Swapping makes a CPU perform several tasks simultaneously. Hence, processes do not have to wait for too long before they are executed.

What is difference between segmentation and paging?

In Paging, a process address space is broken into fixed sized blocks called pages. In Segmentation, a process address space is broken in varying sized blocks called sections. Operating System divides the memory into pages. . During segmentation, a logical address is divided into section number and section offset.

What is swapping in Linux?

Swap is a space on a disk that is used when the amount of physical RAM memory is full. When a Linux system runs out of RAM, inactive pages are moved from the RAM to the swap space. . In most cases, when running Linux on a virtual machine, a swap partition is not present, so the only option is to create a swap file.

What are two advantages of swapping?

• Borrowing at Lower Cost:
• Access to New Financial Markets:
• Hedging of Risk:
• Tool to correct Asset-Liability Mismatch:
• Swap can be profitably used to manage asset-liability mismatch. .
• Additional Income:

Why is swapping needed?

Swap is used to give processes room, even when the physical RAM of the system is already used up. In a normal system configuration, when a system faces memory pressure, swap is used, and later when the memory pressure disappears and the system returns to normal operation, swap is no longer used.

Why is paging used?

Paging is used for faster access to data. . When a program needs a page, it is available in the main memory as the OS copies a certain number of pages from your storage device to main memory. Paging allows the physical address space of a process to be noncontiguous.

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