/dev/shm is nothing but implementation of traditional shared memory concept. It is an efficient means of passing data between programs. One program will create a memory portion, which other processes (if permitted) can access. This will result into speeding up things on Linux.
shm / shmfs is also known as tmpfs, which is a common name for a temporary file storage facility on many Unix-like operating systems. It is intended to appear as a mounted file system, but one which uses virtual memory instead of a persistent storage device.
If you type the mount command you will see /dev/shm as a tempfs file system. Therefore, it is a file system, which keeps all files in virtual memory. Everything in tmpfs is temporary in the sense that no files will be created on your hard drive. If you unmount a tmpfs instance, everything stored therein is lost. By default almost all Linux distros configured to use /dev/shm:
$ df -h
Sample outputs:
Filesystem Size Used Avail Use% Mounted on
/dev/mapper/wks01-root
444G 70G 351G 17% /
tmpfs 3.9G 0 3.9G 0% /lib/init/rw
udev 3.9G 332K 3.9G 1% /dev
tmpfs 3.9G 168K 3.9G 1% /dev/shm
/dev/sda1 228M 32M 184M 15% /boot
Nevertheless, where can I use /dev/shm?
You can use /dev/shm to improve the performance of application software such as Oracle or overall Linux system performance. On heavily loaded system, it can make tons of difference. For example VMware workstation/server can be optimized to improve your Linux host's performance (i.e. improve the performance of your virtual machines).
In this example, remount /dev/shm with 8G size as follows:
# mount -o remount,size=8G /dev/shm
To be frank, if you have more than 2GB RAM + multiple Virtual machines, this hack always improves performance. In this example, you will give you tmpfs instance on /disk2/tmpfs which can allocate 5GB RAM/SWAP in 5K inodes and it is only accessible by root:
# mount -t tmpfs -o size=5G,nr_inodes=5k,mode=700 tmpfs /disk2/tmpfs
Where,
- -o opt1,opt2 : Pass various options with a -o flag followed by a comma separated string of options. In this examples, I used the following options:
- remount : Attempt to remount an already-mounted filesystem. In this example, remount the system and increase its size.
- size=8G or size=5G : Override default maximum size of the /dev/shm filesystem. he size is given in bytes, and rounded up to entire pages. The default is half of the memory. The size parameter also accepts a suffix % to limit this tmpfs instance to that percentage of your pysical RAM: the default, when neither size nor nr_blocks is specified, is size=50%. In this example it is set to 8GiB or 5GiB. The tmpfs mount options for sizing ( size, nr_blocks, and nr_inodes) accept a suffix k, m or g for Ki, Mi, Gi (binary kilo, mega and giga) and can be changed on remount.
- nr_inodes=5k : The maximum number of inodes for this instance. The default is half of the number of your physical RAM pages, or (on a machine with highmem) the number of lowmem RAM pages, whichever is the lower.
- mode=700 : Set initial permissions of the root directory.
- tmpfs : Tmpfs is a file system which keeps all files in virtual memory.
How do I restrict or modify size of /dev/shm permanently?
You need to add or modify entry in /etc/fstab file so that system can read it after the reboot. Edit, /etc/fstab as a root user, enter:
# vi /etc/fstab
Append or modify /dev/shm entry as follows to set size to 8G
none /dev/shm tmpfs defaults,size=8G 0 0
Save and close the file. For the changes to take effect immediately remount /dev/shm:
# mount -o remount /dev/shm
Verify the same:
# df -h
Recommend readings:
- See man pages of mount regarding tmpfs options.
- Details regarding tmpfs is available in /usr/share/doc/kernel-doc-
/Documentation/filesystems/tmpfs.txt file.
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{ 38 comments… read them below or add one }
Very good Description.
I want to know the concept of /dev/shm in FreeBSD.
I cannot see where allocating an 8G ram disk on a machine with 8G of physical ram is a good thing. The OS will start swapping out the ram disk pages long before you fill it up, slowing the whole system to a crawl.
I you have 32G add 8G for virtual machine. I’m not asking to allocate all ram.
HTH
I don’t quite understand why u need the tmpfs thing – if u have that much of memory, just write direct to memory, why set up swapspace and then write to swapspace via tmpfs?
Similarly question for /dev/shm?
Is it because of 4GB limit for memory – in 32bit OS scenario?
Peter you can’t always write direct to memory, for example I have a cgi program I want to load very fast placing it in tmpfs will permit that. If I simply place the file on the disk and ‘hope’ the disk cache caches it then it may be swapped out during times of high disk activity. by placing it in shared memory it will stay put. until of course the next reboot
DC
I am a new to linux
After reading the discussion of this article i am
really confused
firstly, what is tmpfs and swap space? where they are stored?
second, if there are any difference how this will affect the performance of a system?
third, is there any method to allocate tmpfs or swap space for a particular program?
if my english is bad please forgive
/dev/shm is a good place for separate programs to communicate with each other. For example, one can implement a simple resource lock as a file in shared memory. It is fast because there is no disk access. It also frees you up from having to implement shared memory blocks yourself. You get (almost) all of the advantages of a shared memory block using stdio functions. It also means that simple bash scripts can use shared memory to communicate with each other.
As an example, I have an external piece of hardware which only one program at a time can communicate with. I want to log data from this device as often as possible, but also run an “assay” – ie. tell it to do a sequence of tasks. I have implemented this in a client/server mode where the “server” does all of the communication and the clients request actions. The data logger requests readings as often as it can and the assay controller requests the device to change states at the appropriate times. In order to avoid conflicts each client program must check a “lock file” in /dev/shm to see if the server is available. If it is, then the client writes it own process ID ($$ in bash) to the lock runs it request, and then releases the lock (don’t forget that step!) by writing 0 back to the lock so other clients know that the server is available again.
All of this _could_ be done with a “normal” file, but would be a bit slower.
Dear Susan,
Good explanation one thing more i wana add that when we install oracle in linux os if don’t have enough size in /dev/shm then we can not increase the size of sga at first stage we have to modify the size of this shm then we can alter memory_max_target parameter. thanks hope it was useful for every one.
I’m curious to know why when I executed “mount -o remount,size=8G /dev/shm” to increase the swap, the disk size of other partitions remain the same? Where is the additional 6 GB coming from if I already have 2G of /dev/shm ?
Something I see done that’s not commented on here is using this setup for single programs that use a lot of I/O during normal operation – like qmail (as long as your willing to loose mail in the queue if the server crashes or needs to reboot).
Not sure why you think making a ramdisk for swapping/paging is a good idea. Any page fault is still overhead. If you’re having heavy swapping or page faulting but have immense amounts of memory, the issue may be in the kernel parameters, not in reducing available memory to make a ramdisk.
Oracle uses large /dev/shm to improve communication between processes.
If your /dev/shm is not large enough Oracle will not install, worse will most likely
crash.
So is /dev/shm effectivly a ramdisk? but using virtual memory?
Yes and No
Yes /dev/shm is Ramdisk
No it doesn’t use virtual memory unless you run out of ram then the OS may swap to disk (virtual RAM). I don’t know enough to say for sure if this happens actually happens but it would be most likely.
If you run out of space on /dev/shm it would report an error like any other type of disk
It’s not quite the same as a ramdisk. A ramdisk is guaranteed to be in RAM, tmpfs may be swapped out. Depending what else is using RAM at the time it may or may not be faster that writing to a disk; it generally will be of course. Personally I run with very small /dev/shm as nothing I use uses it, as far as I can see.
The equivalent for the BSDs is mfs, but there’s no standard of having it in any particular location – as far as I know anyway.
Dear friend,
It’s really helpful and i can solve a giant oracle configuration problem easily with the above description..
thank you..
I think It’s worth to mention to /dev/shm with nosuid,noexec options
Hello
It is corect this permision of ramdisk ?
drwxrwxrwt 2 root root 40 2010-05-24 15:53 ramdisk
This is a corect mount ?
# mount -t tmpfs /dev/shm /media/ramdisk
# mount -o remount,size=1756M /dev/shm
Why you use this ?
nr_inodes=5k,mode=700
Thank you !
I’m not finding this very helpful.
Like almost every manpage I’ve ever seen, this documentation assumes that you already know the stuff, and need a reminder of how to use it.
I’d like to use this–my system has 3 cpu’s and 4G memory, but runs erratically–freezing for anything from a fraction of a second to over a minute–particularly bothersome when viewing video from the hard drive, but it happens even when all I am doing is browsing.
While I understand the concept, (it is, after all an old idea,) this doesn’t help me apply it to my problem–if indeed it would help.
I’m still new to doing system admin on a linux box, though I have done many years of doing so on IBM mini’s and mainframes.
I’m old enough that my memory isn’t reliable, and my hands have tremors–making the use of the command line a pain in the (&O.
I know that I have a lot of study yet to do, but like all new systems, the amount of documentation is daunting, and much of it doesn’t help very much because it assumes a great many things–like all too much documentation written by programmers, it ignores the problems of those learning how to use things, and tends to be minimalistic and cryptic until you reach a threshold of knowledge (which I have yet to acquire.)
While reminder documentation is extremely useful, more details descriptions of what programs do and how they do it seem to me to be needed.
In general I’ve found that things which are used daily are over-documented, and things used rarely are massively under-documented. And programmers writing their own documentation are extremely likely to write only the most obscure parts of the tool instructions…because they are already intimately familiar with the main functions.
This is the kind of thing that leads to the extremely useless error messages generated by Windoze (Or an old database program I used once that gave only 4-digit error codes, all of which seemed to map to “call tech support,” bad enough, but the company no longer existed….
Am I alone in this perception?
Charles wrote:
“Am I alone in this perception?”
Yes you are!
Hardly. Linux is a very unprofessional, badly written, poorly documented “OS”.
It’s only allure is the fact it’s free. If it weren’t free – nobody would use it. Unix was an interesting concept, but not good in practice. IBM’s DOS was able to destroy the Unix market almost immediately.
Linux is just a re-do of Unix – but worse. The original Linux source code was written by a novice: and as usual, everything flows “from the top”. That’s why we now have such horrible documentation, poorly-styled code without comments and hare-brained concepts such as /dev/shm. Everything is a Band-Aid fix for a gaping wound. There’s no consistency, no style, no rules. While some people may think these are features, the fact is these are what keeps Linux from winning over the bigger market.
The fact some people act like Linux is a product of nature shows how ignorant they are. It’s free. That’s why people use it. If Linus started charging for downloads it would lead to the end of Linux.
This is the most uneducated post I have read on the internet in all my life! My god, are you working for M$ or is this mr gates himself.
ROFL!
XSeSiV you are right, if this guy is not gates himself s a complete ignorant.
Fact, Linux rules the world of IT. The person responsible for the evolution of Linux basically won the “Nobel Prize” for technology. The poster Bob is probably a point and click monkey who needs a Vendor to tell him how to do his job.
Charles — I don’t think you’re alone in that perception. However, with enough searching it seems I eventually find *somebody’s* site that gives wonderful insight for exactly what I need to know. The main thing is I don’t expect every blogger to write a follow-up to fill me in with what is assumed I know…I go and find out. Yes it takes time, but I think that’s the place everybody learning something new has to start :(… Unless you personally know an experienced Linux guru who volunteers time to mentor you.
I think as far as /dev/shm goes, the beauty of this being mounted like a normal file system is that you can write programs (and/or scripts) to use it like you would any other file system. If you have enough RAM available, it will be much more efficient than reading/writing a hard disk. If you’re not writing programs, then /dev/shm is of little consequence to you and does not do anything for you (unless you use a program which uses this). It really should be as if it does not even exist to the average user…but for a sysadmin, it’s good to be aware of it in the event you have a program (like Oracle) that is having strange problems. Then your awareness will lead you in the right direction when researching a solution.
As far as comments about using up RAM and slowing down the whole system…well you simply need to make good decisions about using it when it makes sense to use it, and not making your entire program use it. For example, you have to make the same decision about how much data you load into RAM at a time whether using /dev/shm or just by using a big buffer and passing a pointer between applications…lots of data uses lots of RAM and doesn’t matter whether it’s shm or somewhere else.
I’m speaking out my arse here, but I think mounting 8G shm on 8G RAM is perfectly fine as it’s referencing paged memory like any other program. There is a danger that if your program uses a significant amount, you can starve other programs in RAM by forcing them to swap…but if you’re in that situation, then the active program using SHM is going to use swap if you limit SHM to say, 2G or something…so either way you lose. In the end, don’t write programs that want to load 8G into RAM during runtime. Only load things into RAM if you need to use them right away. When you’re done using it, then free it.
Hello there,
no Gentoo user here? So just my addon, shm worked like charm if you do -j3 ore more compiling with gcc, it takes under Gentoo full advantage and speed up thinks greatly, while in daily use it seems to not be every used.
Put i really would not suggest to use more than half of the memory for shm, even its used by the kernel dynamically by the need, if you have something fucked up in any service running and your shm is filling up the main memory, everything goes into swap and kills the system via I/O flood ,….
Greeting from Beijing
xuedi
/dev/shm memory is not allocated until it is needed, as it would be silly to do so otherwise.
It is fairly typical to make the size == the RAM, and just pray you (almost) never actually reach that and start swapping like crazy.
/dev/shm is essentially a ramdisk, only pretending to be a file, and it might get swapped to disk when the going gets tough. You can survive a page fault or two now and then. You can’t survive constant thrashing of your swap.
Because it pretends to be a file and lives in RAM (mostly) it makes an execellent vehicle for program communication with locks etc as a “shared memory” substitute. It’s easy to use; It lives in RAM (mostly); It is fault-tolerant (hitting swap if it has to); It’s fast
My RAM died last night. be careful if you do this stuff.
Is there a standard or something for using /dev/shm? In other words is any program use /dev/shm by default? In other other words ;) can I harmless unmount and delete /dev/shm on working computer?
I found malware stored in /dev/shm that looked like an irc bot. I too am considering unmounting and removing this.
@Rob: If you have a rootkit on your system it does not matter if it is in SHM or in the file system, your system is lost, better make backups ^^ SHM is just a file system as any other..
It was very simple, concise and useful. Thanks a lot
Charles / Bob :- Why troll? If you do not have anything to contribute, move along.
And yes, this was really useful to me. I could not find any good source to get rid of the following errors on my production Oracle which runs on CentOS 64bit system:
I increase /dev/shm from default 1G to 8G and now I do not get any errors.
Thank you for this post. Very concise and very useful!
Thank you for your comments! These also helped me to better understand this concept!
Charles, I understand your frustration. There are many situations when I have to search a lot to find something. But like RJB said, finally you will find what you need. But you have to want this! Really want!!
“tmpfs : Tmpfs is a file system which keeps all files in virtual memory.”
This should read:
tmpfs : Tmpfs is a file system which keeps all files in physical memory.
Swap is an example of virtual memory.
‘virtual memory’ is correct. In this case, it is referring to the virtual addressing structure used within the kernel which, through memory management, accounts for all memory, both physical and otherwise. tmpfs can be swapped out as well so it would be unfair/unwise to state that it keeps it in ‘physical’ memory when, in fact, the data could exist within a swapfile or swap partition.
Can I create shm partition on LVM?
Ramdisk/tmpfs is kind of last resort. It’s almost never needed. Linux kernel uses all the free RAM for the file cache by default, so all the frequently accessed files remain cached in RAM. Short-lived temporary files may stay in cache and not hit the disk at all. Linux kernel is smart. :)
There’re just a few cases when ramdisk/tmpfs is much faster than real filesystem (and most of them can be reported as a bug). Usually there’re better alternatives.
For example to reduce number of disk write attempts you can increase vm.dirty_* sysctls (http://www.kernel.org/doc/Documentation/sysctl/vm.txt). If you’re trying to reduce number of HDD spinups for a laptop, then vm.laptop_mode and laptop-mode-tools package would give you the best solution. If your software uses a lot of slow fsync’s that you need to avoid, then `eatmydata` wrapper is just for you.
Tmpfs takes your RAM from other apps even when not used, often it’s not easy to keep it in sync with your local data, and it’s MUCH slower than real filesystem when it’s swapped out. The only way to make sure that you need it is to benchmark it, not with some general benchmarking tool, but with your particular test case.