Why is used in linux

Why swap memory is used in linux?

Swap space in Linux is used when the amount of physical memory (RAM) is full. If the system needs more memory resources and the RAM is full, inactive pages in memory are moved to the swap space. While swap space can help machines with a small amount of RAM, it should not be considered a replacement for more RAM.

Why is swap memory used?

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 does Linux need swap?

Think of a swap partition as overflow space. When the main memory is full, the overflow space can be used for additional memory. . You can create a swap partition that is used by Linux to store idle processes when the physical RAM is low. The swap partition is disk space set aside on a hard drive.

What is swap and what is it used for 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. Swap space can take the form of either a dedicated swap partition or a swap file.

Is swap memory necessary for Linux?

Why is swap needed? . If your system has RAM less than 1 GB, you must use swap as most applications would exhaust the RAM soon. If your system uses resource heavy applications like video editors, it would be a good idea to use some swap space as your RAM may be exhausted here.

What is Linux swap?

18 related questions found

Can I run Linux without swap?

It’s a bit dangerous but you CAN run without a swap space. But if you finally exceed the amount of memory you have your system will nearly immediately crash without notice.

Is swap memory needed?

Swap space is used when your operating system decides that it needs physical memory for active processes and the amount of available (unused) physical memory is insufficient. When this happens, inactive pages from the physical memory are then moved into the swap space, freeing up that physical memory for other uses.

What does swap mean 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. Swap space can take the form of either a dedicated swap partition or a swap file.

How does swap work in Linux?

Linux divides its physical RAM (random access memory) into chucks of memory called pages. Swapping is the process whereby a page of memory is copied to the preconfigured space on the hard disk, called swap space, to free up that page of memory.

What does a swap file do?

A swap file allows an operating system to use hard disk space to simulate extra memory. When the system runs low on memory, it swaps a section of RAM that an idle program is using onto the hard disk to free up memory for other programs. . This combination of RAM and swap files is known as virtual memory.

Why is swapping needed?

The purpose of the swapping in operating system is to access the data present in the hard disk and bring it to RAM so that the application programs can use it. The thing to remember is that swapping is used only when data is not present in RAM.

Why is swap used when there is free memory?

It’s also a last resort should your applications actually request more memory than you actually have. In this case, swapping is necessary to prevent an out-of-memory situation which will often result in an application crashing or having to be forcibly killed.

Does 16GB RAM need swap space?

16GB of ram, or even 8GB of ram is more than enough. . You should however have the same size of swap equal to your ram size or if you are planning to hibernate, since the process of hibernation grabs everything in ram and puts it on swap, which is why you need a minimum size equal to your ram size for swap.

What is using swap memory?

A Swap Memory is a space in the Hard Disk of your computer that Operating Systems will use to put the info that is actually on the RAM to free it for another application.

Is using swap memory bad?

Swap memory is not detrimental. It may mean a bit slower performance with Safari. As long as the memory graph stays in the green there’s nothing to worry about. You want to strive for zero swap if possible for optimal system performance but it’s not detrimental to your M1.

Why is my swap usage so high?

A higher percentage of swap use is normal when provisioned modules make heavy use of the disk. High swap usage may be a sign that the system is experiencing memory pressure. However, the BIG-IP system may experience high swap usage under normal operating conditions, especially in later versions.

What is swap in and swap out in Linux?

SWAP-IN: Moving data from Swap space to main memory of the machine. SWAP-OUT: Moving main memory contents to Swap disk when main memory space fills up. You can monitor this using the performance charts. The Memory (MBps) chart displays the swap in and swap out rates for a host.

What is the difference between RAM and swap?

Simply put, virtual memory is a combination of RAM and disk space that running processes can use. Swap space is the portion of virtual memory that is on the hard disk, used when RAM is full.

What is swap in free command?

The free command provides information about unused and used memory and swap space on any computer running Linux or another Unix-like operating system. . Swap space is is a portion of a hard disk drive (HDD) that is used to simulate additional main memory (i.e., which is used for virtual memory).

Does 32gb RAM need swap space?

If you want hibernation to work, it has to save everything in RAM to swap space so that it can be restored when the computer is turned on again, so you’d need at least 32 GB of swap space.

How much swap memory is too much?

Your memory is 2000mb/s or higher, and your swap file is 60-100mb/s (unless you have a ssd). Anytime you depend on your swap file because you ran out of real memory slows down whatever programs are using it. If you have 8gb or more windows will swap small amounts out over time and it won’t affect performance.

Should I disable swap?

However, swap space should be set to 2 or 4 GB on systems with more than 4 GB of RAM. If your server has sufficient RAM memory or does not need the use of swap space or your device output is significantly reduced by swapping, you can consider disabling the swap field.

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Why is «./» used when executing a program? [duplicate]

If I have a program named foo, why do I need to say ./foo and not simply foo? Doesn’t ./ just mean within the current directory?

1 Answer 1

When you type in just a command – eg. foo – the shell (usually bash ) will only look for it in certain directories – like /bin, /usr/bin, /usr/X11R6/bin and so on. It will typically not look in your «current directory» (ie. «.» ) because of security concerns (especially if you’re root). If «.» is added, it should be added to the end of the search-path.

If you want to run a executable that’s not in this search-path, you need to specify the whole absolute or relative path to the command. For example:

/home/bok/foo # Absolute in my homedir ~/src/bar # Shorthand for my homedir and a subdir in it ../Download/foobar # Relative path – up one step, then down in Download 

Or – if the command is in my current directory – simply add a ./ :

The search-path for bash is stored in the PATH environmental-variable, so you can see it by typing:

To add «.» – which you really shouldn’t do – type:

The PATH is searched in order, so if there are several executables with the same name in different directories, it’s the one in the directory first in the PATH that the shell will execute.

First after looking in all the listed directories, will you get a «command not found» error.

The problem with adding the current directory – . – to the PATH, is that you don’t really know if an executable in a «non standard» directory you may just happen to be in should be executed or not.

Lets say I’m evil an make a destructive program that delete as many files as possible and leave it in my home directory – or worse, in /tmp. I call this program sl – a common miss-type of ls – and wait.

Some random user is in /tmp, and miss-spells ls with sl . With a normal PATH – without . – nothing happens, except he gets an error for «command not found». If he got ‘.’ at the end of his PATH, then he won’t get an error for typing the command. Instead my sl command will run, and delete all the files he owns. If he happens to be root, the command will be able to delete much of the system. (This is why root really should make it a rule to always use the full path of commands, rather than trusting PATH.)

If he had placed ‘.’ first in his PATH and I made program called ls (correct spelling) and put it in a directory, then it would be my program – not the normal ls – that would be run if he typed ls in that directory. Because bash would look in the current directory (.) first – before looking in the system-directories with the genuine ls .

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Why is select used in Linux

I was going through a serial program and I observed that they use select() before using read() . Why exactly is this required. Why cant we just directly call read() and check if it fails or not ? Also why do we have to increment the file descriptor by 1 and pass it while I am passing the file descriptor set already to select() ? Example: r=select(fd+1, &fds, NULL, NULL, &timeout); where fds already has the value of fd

A call to read(2) may block. Also, read about poll(2) syscall (which is better than select(2) for multiplexing purposes; read more about the C10K problem).

Both poll(2) and the old select(2) are waiting and mulltiplexing on several file descriptors. Their role is conceptually similar (but poll is more C10K friendly, since able to multiplex on more than 1024 file descriptors).

3 Answers 3

The select() system call tells you whether there is any data to read on the file descriptors that you’re interested in. Strictly, it is a question of whether a read operation on the file descriptor will block or not.

If you execute read() on a file descriptor — such as that connected to a serial port — and there is no data to read, then the call will hang until there is some data to read. Programs using select() do not wish to be blocked like that.

Why do we have to increment the file descriptor by 1 and pass it while I am passing the file descriptor set already to select ?

That’s probably specifying the size of the FD_SET. The first argument to select() is known as nfds and POSIX says:

The nfds argument specifies the range of descriptors to be tested. The first nfds descriptors shall be checked in each set; that is, the descriptors from zero through nfds-1 in the descriptor sets shall be examined.

So, to test a file descriptor n , the value in nfds must be at least n+1 .

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