linux-kernel - Re: mmap vs. real memory

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Date:	Fri, 11 Sep 2009 10:25:17 +0100
From:	Luciano Rocha <luciano@...otux.com>
To:	Brian McGrew <brian@...ionpro.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: mmap vs. real memory

On Thu, Sep 10, 2009 at 03:38:11PM -0700, Brian McGrew wrote:
> 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???

Were are you storing the file? For memory-only, you should use /dev/shm.
Another more portable solution is to use shm_open(3), that will open the
file were the system wants it.

-- 
Luciano Rocha <luciano@...otux.com>
Eurotux Informática, S.A. <http://www.eurotux.com/>

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