How does shared memory work behind the scene in Linux?

How does shared memory work behind the scene in Linux?

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How does shared memory work behind the scene in Linux?
Tag : development , By : Al Dimond
Date : November 25 2020, 03:01 PM

Hope this helps Every process has its own virtual memory space. To simplify things a bit, you can imagine that a process has all possible memory addresses 0x00000000..0xffffffff available for itself. One consequence of this is that a process can not use memory allocated to any other process – this is absolutely essential for both stability and security.
Behind the scenes, kernel manages allocations of all processes and maps them to physical memory, making sure they don't overlap. Of course, not all addresses are in fact mapped, only those that are being used. This is done by means of pages, and with the help of memory-mapping unit in the CPU hardware.

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Using shared memory on a shared hosting Linux account

Tag : php , By : Morbo
Date : March 29 2020, 07:55 AM
it fixes the issue There could be multiple if they are doing load balancing, you would have to ask them. No, shared memory is shared across the current host, not multiple Depends on system ram and limits in place by the system administrator.
A better solution would probarbly be to create a MySQL table with the engine type of HASH, which makes it a memory table. This would work around the issue of load balanced setups and also keep your data away from other users on the same server.

Shared memory with mmap returns when writing to mapped memory on linux

Tag : c , By : Yohan Lee
Date : March 29 2020, 07:55 AM
With these it helps You have the mode argument to shm_open wrong. This is supposed to be a mode specification as for open. Probably your version here by conincidence forbids writing to the address, so the process then crashes when you try to write to it.
BTW: you should always check the return of library calls such as shm_open and mmap.

How do I get copy-on-write to work on shared memory on linux

Tag : cpp , By : Viv
Date : March 29 2020, 07:55 AM
it fixes the issue I tried to write a small application to get familiar with the concept of copy-on-write in user space. I've read through the answer by MSalters and figured that it would only work if I started with a mmap'ed file to store my data in. As I don't need file based persistency, I tried to do the same thing with shared memory. First I mmap'ed and initialized a shm fd, then I mapped a second copy with MAP_PRIVATE and read from it again. However, just reading from it causes the kernel to copy the whole thing, taking considerably more time and eating up twice the memory. Why does it not do COW? , There was no copying. The smaps file has a hint:
Size:            4194304 kB
Rss:             4194304 kB
Pss:             2097152 kB

How does limits on the shared memory work on Linux

Tag : linux , By : pttr
Date : March 29 2020, 07:55 AM
I hope this helps . The limit only applies to physical memory, that is the real shared memory allocated for all segments, because shmat() just maps that allocated segment into process address space.
You can trace it in the kernel, there is only one place where this limit is checked — in the newseg() function that allocates new segments (ns->shm_ctlall comparison). shmat() implementation is busy with a lot of stuff, but doesn't care at all about shmall limit, so you can map one segment as many times as you want to (well, address space is also limited, but in practice you rarely care about this limit).
#define _GNU_SOURCE
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/ipc.h>
#include <sys/shm.h>
#include <unistd.h>

unsigned long int get_shmall() {
        FILE *f = NULL;
        char buf[512];
        unsigned long int value = 0;

        if ((f = fopen("/proc/sys/kernel/shmall", "r")) != NULL) {
                if (fgets(buf, sizeof(buf), f) != NULL)
                        value = strtoul(buf, NULL, 10); // no proper checks                                                                                                                                                             
                fclose(f); // no return value check                                                                                                                                                                                     
        return value;

int set_shmall(unsigned long int value) {
        FILE *f = NULL;
        char buf[512];
        int retval = 0;

        if ((f = fopen("/proc/sys/kernel/shmall", "w")) != NULL) {
                if (snprintf(buf, sizeof(buf), "%lu\n", value) >= sizeof(buf) ||
                    fwrite(buf, 1, strlen(buf), f) != strlen(buf))
                        retval = -1;
                fclose(f); // fingers crossed                                                                                                                                                                                           
        } else
                retval = -1;
        return retval;

int main()
        int shm_id1 = -1, shm_id2 = -1;
        unsigned long int shmall = 0, shmused, newshmall;
        void *ptr1, *ptr2;
        struct shm_info shminf;

        if ((shmall = get_shmall()) == 0) {
                printf("can't get shmall\n");
                goto out;
        printf("original shmall: %lu pages\n", shmall);
        if (shmctl(0, SHM_INFO, (struct shmid_ds *)&shminf) < 0) {
                printf("can't get SHM_INFO\n");
                goto out;
        shmused = shminf.shm_tot * getpagesize();
        printf("shmused: %lu pages (%lu bytes)\n", shminf.shm_tot, shmused);
        newshmall = shminf.shm_tot + 1;
        if (set_shmall(newshmall) != 0) {
                printf("can't set shmall\n");
                goto out;
        if (get_shmall() != newshmall) {
                printf("something went wrong with shmall setting\n");
                goto out;
        printf("new shmall: %lu pages (%lu bytes)\n", newshmall, newshmall * getpagesize());
        printf("shmget() for %u bytes: ", (unsigned int) getpagesize());
        shm_id1 = shmget(IPC_PRIVATE, (size_t)getpagesize(), IPC_CREAT | 0666);
        if (shm_id1 < 0) {
                printf("failed: %s\n", strerror(errno));
                goto out;
        printf("ok\nshmat 1: ");
        ptr1 = shmat(shm_id1, NULL, 0);
        if (ptr1 == 0) {
                goto out;
        printf("ok\nshmat 2: ");
        ptr2 = shmat(shm_id1, NULL, 0);
        if (ptr2 == 0) {
                goto out;
        if (ptr1 == ptr2) {
                printf("ptr1 and ptr2 are the same with shm_id1\n");
                goto out;
        printf("shmget() for %u bytes: ", (unsigned int) getpagesize());
        shm_id2 = shmget(IPC_PRIVATE, (size_t)getpagesize(), IPC_CREAT | 0666);
        if (shm_id2 < 0)
                printf("failed: %s\n", strerror(errno));
                printf("ok, although it's wrong\n");
        if (shmall != 0 && set_shmall(shmall) != 0)
                printf("failed to restrore shmall\n");

        if (shm_id1 >= 0 && shmctl(shm_id1, IPC_RMID, NULL) < 0)
                printf("failed to remove shm_id1\n");

        if (shm_id2 >= 0 && shmctl(shm_id2, IPC_RMID, NULL) < 0)
                printf("failed to remove shm_id2\n");

        return 0;
$ sudo ./a.out 
original shmall: 18446744073708503040 pages
shmused: 21053 pages (86233088 bytes)
new shmall: 21054 pages (86237184 bytes)
shmget() for 4096 bytes: ok
shmat 1: ok
shmat 2: ok
shmget() for 4096 bytes: failed: No space left on device

Is shared memory for fork shared if the program is run from two terminals in linux

Tag : cpp , By : Blaise Roth
Date : March 29 2020, 07:55 AM
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