StackOverflow2013

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copied!<blockquote> <p>What is the best way to determine how much memory to allocate?</p> </blockquote> <p>Due to how virtual memory is used, non-swappable kernel memory, it is nearly impossible to identify how much of installed memory can be accessed by an application.</p> <p>Best I can come up with is to allow user to configure how much memory to use for buffering.</p> <blockquote> <p>Am I limited to just allocating a slightly smaller block than the reported free memory, </p> </blockquote> <p>Reported free memory is not really "free physical memory." Unfortunately.</p> <blockquote> <p>or can I interface more directly with the linux virtual memory manager?</p> </blockquote> <p>That can be done by using a custom device driver, allocating memory directly in kernel space and providing access to it via <code>mmap()</code>. Generally not recommended, yet would works in specialized cases such as yours.</p> <blockquote> <p>However, I also need to make sure that there will be no disk-swapping</p> </blockquote> <p>At pace of the Linux kernel development, knowledge becomes obsolete quite fast, so take with grain of salt what I'm saying here. You can try to play with the following:</p> <ol> <li><p>SysV shared memory. It is generally not swapped. See <code>man shmget</code>.</p></li> <li><p>tmpfs - in-memory file system. The memory was pinned to RAM at least in early 2.6 kernels and thus was not swappable. To use it as memory, create a file on tmpfs, <code>write()</code> something into the file (to force the memory being actually allocated) and then mmap() the file.</p></li> </ol>

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