Are you sure you want to use MMAP in your database management system? [pdf]

Post by jstimpfle » Sat, Jan 15, 2022, 8:09 PM UTC

CyberDildonics wrote: post #98

jstimpfle wrote: post #95

Earlier quoted context omitted.

First, I didn't assume it a requirement to have two processes read() and write() _directly_ to the same memory (I suppose you meant "file region" here). And idk, it might not be a good idea to require that. Also, you can use normal (non-file-backed) memory to do the necessary synchronization (lock-free or not). I'm still not seeing why the memory should be backed by a file, that's why I was genuinely asking. One reas…

that's why I was genuinely asking. You weren't asking, you were saying it wasn't necessary, which you did in the sentence right before this one: Also, you can use normal (non-file-backed) memory to do the necessary synchronization (lock-free or not). Again, this is just a repeated claim, it isn't an explanation. How do you have two processes writing to the same place in memory without memory mapping a file? have two…

    #include 
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 

    int main(int argc, const char **argv)
    {
            int fd = shm_open("/TESTOBJECT", O_CREAT | O_RDWR, 0664);
            if (fd == -1)
            {
                    perror("shm_open()");
                    exit(1);
            }

            if (ftruncate(fd, 4) != 0)
            {
                    perror("ftruncate()");
                    exit(1);
            }

            void *mapping = mmap(NULL, 4, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);

            if (mapping == MAP_FAILED)
            {
                    perror("mmap()");
                    exit(1);
            }

            volatile int32_t *ptr = mapping;

            for (;;)
            {
                    printf("%d\n", (int) *ptr);
                    if (argc > 1)
                        *ptr = rand();
                    sleep(1);
            }

            return 0;
    }

Post by CyberDildonics » Sat, Jan 15, 2022, 8:43 PM UTC

jstimpfle wrote: post #102

CyberDildonics wrote: post #98

Earlier quoted context omitted.

that's why I was genuinely asking. You weren't asking, you were saying it wasn't necessary, which you did in the sentence right before this one: Also, you can use normal (non-file-backed) memory to do the necessary synchronization (lock-free or not). Again, this is just a repeated claim, it isn't an explanation. How do you have two processes writing to the same place in memory without memory mapping a file? have two…

> You weren't asking, you were saying it wasn't necessary, which you did in the sentence right before this one: quoting my OP: " Why do you need lockless atomic updates to a file-backed memory area? Genuinely curious. " . Dude. > it seems now you were making claims without much behind them, which is disappointing. Well thank you very much. I get the feeling we might just be talking about the same thing. Or we might b…

Post by CyberDildonics » Sat, Jan 15, 2022, 9:09 PM UTC

pritambaral wrote: post #103

CyberDildonics wrote: post #98

Earlier quoted context omitted.

that's why I was genuinely asking. You weren't asking, you were saying it wasn't necessary, which you did in the sentence right before this one: Also, you can use normal (non-file-backed) memory to do the necessary synchronization (lock-free or not). Again, this is just a repeated claim, it isn't an explanation. How do you have two processes writing to the same place in memory without memory mapping a file? have two…

> How do you have two processes writing to the same place in memory without memory mapping a file? In increasing order of modernity: System V SHM, POSIX SHM, and `mmap(... MAP_SHARED | MAP_ANONYMOUS ...)` (e.g., on Linux).

Post by CyberDildonics » Sat, Jan 15, 2022, 10:05 PM UTC

jstimpfle wrote: post #105

CyberDildonics wrote: post #104

Earlier quoted context omitted.

If this example approach is "file-backed" to you, then so be it but I think you have willfully misread my comments up to here. You kept making the same claim and I asked you to explain it, then you just made the same claim again. shm_open("/TESTOBJECT" That's a file path. Maybe you just wanted to bait an argument by not explaining yourself.

Homework: go back through my comments and identify all the places where I was VERY CLEARLY pointing out that my statement is that no disk-backed file is needed, or where you could reasonably infer this from my use of the term "file-backed", as well as from the general context of the discussion. > shm_open("/TESTOBJECT" >>That's a file path Pedantically, no. It's a name ( https://man7.org/linux/man-pages/man3/shm_open…

Post by CyberDildonics » Sat, Jan 15, 2022, 11:58 PM UTC

jstimpfle wrote: post #109

CyberDildonics wrote: post #108

Earlier quoted context omitted.

You conflated files with disks on your own. No one did that for you. rather argue than try to get my point. I still don't know what your point is. You have to have something that coordinates between two processes for shared memory interprocess communication and that ends up being file paths for the OS. You asked questions, they were answered and you could have learned something. The whole point was actually that you…

> If you didn't want to waste time, you would have explained what you meant or asked questions. You clearly haven't done your homework, because I did. > You conflated files with disks on your own. No one did that for you. I did not really conflate this. It is just conventional but imprecise terminology, and everyone who gets into such a discussion (especially when starting personal attacks) is expected to know to be…