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

Post by jstimpfle » Tue, Jan 18, 2022, 3:15 PM UTC

CyberDildonics wrote: post #127

jstimpfle wrote: post #123

Earlier quoted context omitted.

Why are you constantly ignoring what I'm writing? > It doesn't matter what the OS does or doesn't do in the background, atomically reading and writing to memory is unaffected. That's not true. If this thing is file backed there is usually no guarantee that the page of virtual memory (i.e. a page of the file data) you're accessing is present in physical memory. You'll cause page faults and data transfers to/from disk.…

This can delay the execution of an atomic read or write You can play "what if" all you want if you don't know what else running, but this was always about lock free interprocess communication, which is not broken by a page fault or process suspension. An atomic instruction by design will do everything it needs to when the instruction runs. Saying the OS can ultimately control the execution of a process is a nonsense…

Post by CyberDildonics » Tue, Jan 18, 2022, 3:32 PM UTC

pritambaral wrote: post #124

CyberDildonics wrote: post #120

Earlier quoted context omitted.

This just goes back to the same question - what do two processes use to map the same memory into their memory space if it isn't a path to a file? I'm not saying there isn't anything, I'm just seeing an extreme avoidance to an actual answer. The other guy went down a rabbit hole of syncing that memory to storage, which has nothing to do with anything.

Please just RTFM on `mmap(... MAP_SHARED | MAP_ANONYMOUS, -1, 0);`. There is no file path involved. That's all I can say to you now, apart from "this is not StackOverflow".

Post by [deleted] » Tue, Jan 18, 2022, 3:44 PM UTC

CyberDildonics wrote: post #134

jstimpfle wrote: post #131

Earlier quoted context omitted.

Dude, the example I gave you with shm_open() is creating anonymous memory. That's just what non-file-backed mappings are called, no matter how long you want to keep obsessing about any "file paths".

This doesn't even seem like a reply to what I said. If you map memory anonymously you aren't doing interprocess communication. If you don't, you have a file path that the other program can use to map the same memory. That's it, there is nothing wrong with this. I don't know why this is so upsetting. Mapping memory anonymously is local to the process tree and doesn't work for two different programs communicating.

Post by pritambaral » Tue, Jan 18, 2022, 4:33 PM UTC

CyberDildonics wrote: post #133

pritambaral wrote: post #124

Earlier quoted context omitted.

Please just RTFM on `mmap(... MAP_SHARED | MAP_ANONYMOUS, -1, 0);`. There is no file path involved. That's all I can say to you now, apart from "this is not StackOverflow".

But then that isn't interproccess communication. If your boss said "we need these two programs to have lock free IPC through memory" and you said "use MAP_ANONYMOUS" they would say "that is local to the process tree and won't work". You can try to ignore the context of this thread, but if someone wants IPC, this doesn't work.