Messages in this thread | | | Date | Thu, 10 Sep 2009 15:38:11 -0700 | Subject | mmap vs. real memory | From | Brian McGrew <> |
| |
Good afternoon all,
Right now, I'm in what appears [to me] to be a very tricky situation. We have several programs that all need access to the same memory. Currently, we use mmap and map to a file on disk like this:
const u_long kCommsegSize = 1024L * 1024L * 1024L; const u_long kCommsegMappedAddress = 0xb0000000;
And then the code is:
char *cseg_addr = reinterpret_cast<char *>(mmap( reinterpret_cast<char *>(kCommsegMappedAddress), size, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_FIXED, fd, 0)); gShmemOffset = kCommsegMappedAddress - kCommseg; return(cseg_addr);
And, all of this has worked great ever since we went from a physical shared memory board on a vme bus to emulating shared memory on the computer. In the beginning, kCommsegSize was only 4MB (4L * 1024L * 1024L).
Now that we are needing more and more shared memory, we're seeing random performance issues. With 16MB, 64MB and 256MB (in recent past) all was good. But now, trying to allocate 1GB of shared memory, we see it taking anywhere from .6 to 9 SEONDS to access the file. No good!
What I'm wondering and needed to do is map a chunk of mymoery (1GB today, maybe 2GB later on) so that all my apps can access it. Short of creating a ramdisk and moving the mmap'ed file to ramdisk, what is the best way to do this???
Thanks,
-brian
| |