Questions tagged [shared-memory]

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Can a process share unused memory with others?

I want to run about 200 processes with my experiments. Every of them really needs less than a GB: PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 24688 rinkman 20 0 ...
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Shared memory using shmget()

According to my understanding about shared memory is that it is a memory segment that is shared between two or more processes. Shared memory can be implemented using shmget() . However , since any ...
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Values from sysctl -A don't match /etc/sysctl.conf even after restart

I'm on Mac Monterey 12.1 and increased my shared memory limits in /etc/sysctl.conf file: kern.sysv.shmmax: 16777216 kern.sysv.shmmin: 1 kern.sysv.shmmni: 128 kern.sysv.shmseg: 512 kern.sysv.shmall: ...
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ipcrm won't delete some shared memory blocks

I have shared memory I am trying to delete using ipcrm on a Mac. I managed to delete all segments except the below: T ID KEY MODE OWNER GROUP Shared Memory: m 2359296 ...
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How does a process and its children use memory in case of mmap()?

Let's say, we are creating a shared memory using mmap(). Let's say the total memory size is 4096. If we use a fork() system call to create children, would the children use the same memory, or will ...
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When should I alter overcommit_memory and what should I take into consideration when doing so?

I'm having a PC freeze issue that I can't seem to figure out. I have three identical PCs. They are each custom builds with i7 and 64GB of RAM. The OS drives are 512GB nvme drives. They each run ...
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Can't find cached and buffer from 'cat /proc/meminfo'

I want to find cached memory and buffer memory here. But unlike linux, bash does not give me all the information as expected. How can I find cached memory, shared memory and buffer memory? I used the ...
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shmat returns error (void *) -1 in selinux enabled rhel environment

as per the documentaion:- If SHM_RDONLY is specified in shmflg, the segment is attached for reading and the process must have read permission for the segment. Otherwise the segment is attached for ...
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Shared memory as reported by free not equal df reported one. Cache takes huge amount of memory despite recent drop_caches

I'm running Linux Mint 20. I often encounter low memory situations. Today I do not understand why available is so low. As per What is the meaning of `shared` memory in the `free` command? shared is ...
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Selinux avc denied read write to shm key

I am new to selinux concept and I want httpd to access shared memory, but some how httpd is not able to access the shared memory, as per audit log the message tells read write is denied for shm class ...
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Am I making invalid assumptions with regard to my kernel module's shared memory?

I have written a "device driver" (see source code here: https://bitbucket.org/wothke/websid/src/master/raspi/websid_module/ ) that runs fine for most of the time (see https://www.youtube.com/...
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Shared memory usage is piling up

I'm using Ubuntu 20.04.2 LTS with kernel 5.4.0-70-generic. Shared memory keeps piling up until system hangs as it doesn't have any memory left. I'm using Prometheus + Grafana to monitor my system ...
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FFmpeg cannot write file to /dev/shm: Permission Denied

Issue: I have an FFmpeg command that I've been running for months now to stream video into the /dev/shm directory. It had been working fine until relatively recently (e.g. within a week), now it ...
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Delete POSIX shared memory owned by different user

I have a program that creates four shared memory objects. The memory creation routine calls shm_unlink() before attempting to create them, and the program calls another routine to delete them with ...
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Do anonymous memory mapping and shared memory allocate space from physical memory only?

In Linux, when using mmap() for anonymous memory mapping, or using malloc(), do they allocate "space" from only physical memory, or either physical memory or swap or their combination? (I ...
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How to disable X11 extension (MIT-SHM) for systemd-nspawn container?

X Error of failed request: BadValue (integer parameter out of range for operation) Major opcode of failed request: 130 (MIT-SHM) Minor opcode of failed request: 3 (X_ShmPutImage) Value in ...
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SIGBUS immediately after successful mremap for shared memory

I am using POSIX shared memory. Initially I map four shared memory buffers. One of them exceeds its allocated size so I call mremap: void * remap_shm(void *old_address, size_t old_size, size_t ...
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How to revoke write permissions on a shared memory object s.t. subsequent writes to aleady mapped pages by other processes will fail?

Is there any way to revoke write permissions to shared memory by the process who created that shared object, s.t. any other process who has mapped the shared memory to its virtual space with write ...
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2 votes
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AMDGPU driver vramlimit

I have a console installation of debian and i found that from the available 16GB of ram i can see only 14GB, this is because my APU (AMD) is reserving 2GB which is way too much for my needs. The bad ...
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How to understand PBS output "mem" and "vmem" keep the same when the task is x-fold increased with mpirun -np x task

Here is a simple experiment I tried: Given a task called "sim.exe" which doing a model simulation, I then use MPI to launch x "sim.exe" simultaneously on one node (shared memory system). I have tried ...
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Will swap file engage automatically when I write too much to /dev/shm

Text analytics programming today. My program is going to write about 45 million small temporary files to /dev/shm and their size estimate is 177.0 GB. I'm going to delete them soon after the next ...
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Are sharing a memory-mapped file and sharing a memory region implemented based on each other?

Are sharing a memory-mapped file and sharing a memory region implemented based on each other? The following two quotes seem to say so, and seem a chicken-egg problem to me. Operating System Concepts ...
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calling fsync() on in-memory files

From fsync(2): fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor fd to the disk device (or other ...
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Processor not seeing changes to POSIX shared memory?

Context: I am using POSIX shared memory to provide a set of processes with a shared memory space. I have used this scheme for some time now in order to share data, and it's working okay. However, I ...
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Is the shared library object loaded as shared memory for the program?

I was coding some C when I started asking myself about free() inside a shared library and SIGKILL, and so on. Then, after some reading and re-thinking, I came up to this simple question. CONTEXT : ...
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Two processes sharing memory under different privileged users

I have two processes sharing memory (via Boost Interprocess). Each process is run under a different user. The first process is run from one of our privileged users. The second process is ran via a ...
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Is it wrong to think of "memfd"s as accounted "to the process that owns the file"?

https://dvdhrm.wordpress.com/2014/06/10/memfd_create2/ Theoretically, you could achieve [memfd_create()] behavior without introducing new syscalls, like this: int fd = open("/tmp", O_RDWR | ...
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Change ownership of shared memory

When I run ipcs -m, I can see a list of the shared memory segments on the system, like ------ Shared Memory Segments -------- key shmid owner perms bytes nattch status ...
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Programs started to fail suddenly with messages pertaining to shared memory

No idea why this is happening it's the first time I see this error. There seems to be enough space left in ram and drive. First vnc fails now if I don't run it with -noshm option. Second one that ...
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Sharing file descriptors

If file descriptors are specific to each process (i.e. two processes may use the same file descriptor id to refer to different open files) then how is it possible to share transfer file descriptors (e....
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Why does `/dev/shm` on an EC2 instance have I/O throughput comparable to an EBS drive?

I recently learned about the convenient "shared memory" file system at /dev/shm. I wanted to see if I could use this to speed up a sometimes-disk-bound program that writes to and reads from a ...
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Issues due to independent concurrent writes to shared memory

Actually I would like to know the issues caused by independent concurrent writes to shared memory. To be more elobarote, consider we have three processes (proc1, proc2, proc3). These 3 processes are ...
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1 vote
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Interpetation of /proc/sys/kernel/shmall

Unless I am wrong. the value of /proc/sys/kernel/shmall indicates the the total amount of shared memory, in bytes, that can be allocated to the system. I am on a t2.micro ec2 debian instance, and I ...
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2GB of shared memory used as shown in free [closed]

OS: Ubuntu 16.04 free output indicated about 2GB of shared memory. The shared memory was still allocated after a logout, and no particular process from top list was occupying it. ipcs -m did not ...
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14 votes
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Understanding mmap

I was going through documentation regarding mmap here and tried to implement it using this video. I have a few questions regarding its implementation. Does mmap provide a mapping of a file and return ...
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2 answers
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Where does Linux set the default values for SHMMAX?

I'm just wondering where these values are being set and what they default to? Mine is currently 18446744073692774399. I didn't set it anywhere that I can see. $ cat /proc/sys/kernel/shmmax ...
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How is memory corruption handled by Linux when the process terminates?

There are many questions on Stack Overflow asking about how a system handles memory leaks and what happens on abnormal termination. Examples: https://stackoverflow.com/questions/6727383/dynamically-...
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How to fetch semaphores and sharedmem objects for a specific apache instance when more than one apache instances run with the same app id on the box

Not sure if the question is pertinent for this forum. We have a server with 3 apache instances running with the "same" app/functional ID These instances communicate with their corresponding ...
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Across systems, How a process communicate with other process?

I have learnt below IPC mechanisms, BSD style half-duplex pipes SYSV style message queues SYSV style semaphore sets SYSV style shared memory segments BSD style sockets Half-duplex pipe can be used ...
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How to know shared memory between two processes?

I need to know the amount of memory shared between two processes, that is, the intersection of their shared memories. Any ideas?
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The same program which uses 'mmap' behaves differently on the different machines

I am running an application which uses a huge memory mapped file on two different machines (CentOS 7). I found that the same program's performance is significantly different when it's running on two ...
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How can i calculate the size of shared memory available to the system

According to the rhel document, the total amount of shared memory available on the system equals to shmall*PAGE_SIZE. After I completed the installation of RHEL6, the value of the shmall kernel ...
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1 vote
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Discrepancy between shared memory from /proc/meminfo and ipcs

Running free -hm outputs: total used free shared buffers cached Mem: 15G 15G 389M 321M 169M 11G -/+ buffers/cache: ...
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How do you install Oracle on Linux when you get the error "ORA-27101: shared memory realm does not exist"?

I am installing Oracle 11 on CentOS 7.2. I have installed it before on servers with less RAM. I get this error: ORA-01034: ORACLE not available ORA-27101: shared memory realm does not exist ...
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Limit memory usage of a user

I have a computing cluster of 44 cores and 256GB memory running Ubuntu and I'd like to limit the number of CPUs and memory used by certain users. Limiting memory usage would be more important. So for ...
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3 votes
1 answer
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Kernel parameter SHMALL and SHMAX [closed]

I am using Red Hat Enterprise Linux Server release 5.11 (Tikanga) as OS and rdbms is 11gR2 software. I have created a database with memory_target=3 GB and physical RAM=7 GB. My DB service is getting ...
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Unix Domain Socket with VM

Is it a true statement that, shared memory does not work between a host OS and guest OS, but a Unix Domain Socket (specifically udp) can communicate between the two? An in depth explanation would be ...
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2 votes
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non-reentrant libraries in shared memory?

I found this Q&A saying shared libraries can be shared between processes using shared memory. It seems like it would be impossible, though, to share code between processes without some pretty ...
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2 votes
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Extremely high shmem usage

I'm trying to debug the issue that's periodically taking down my work machine. It's an OOM-type problem, and based on this /proc/meminfo output: Total: 32902608 kB MemFree: 4054100 kB ...
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1 vote
1 answer
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determine the actual memory usage of several processes that share a large memory segment

I just took a look at the output of top and it showed me (amongst other processes) the following: As once can see I have ten processes consuming approx 10GB each so 100GB in total. The computer ...
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