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Operating Systems Linux Memory issue on My CentOS 5.8 x64 bit server Post 302770808 by DGPickett on Monday 18th of February 2013 12:10:03 PM
Old 02-18-2013
Under VM, not dirty pages stay mapped in case anyone wants to read them through a mmap(). If you need a page, they can be taken, but they are not free. You can run a job that mmap()'s a file, read the whole file into ram, exit, run it again, and when it mmap()s the file again, the old pages are mapped into the new process memory range and it runs much faster. This is what happens with libc.so: everyone runs printf() from the same ram page.
 

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MMAP2(2)						     Linux Programmer's Manual							  MMAP2(2)

NAME
mmap2 - map files or devices into memory SYNOPSIS
#include <sys/mman.h> void *mmap2(void *addr, size_t length, int prot, int flags, int fd, off_t pgoffset); DESCRIPTION
The mmap2() system call operates in exactly the same way as mmap(2), except that the final argument specifies the offset into the file in 4096-byte units (instead of bytes, as is done by mmap(2)). This enables applications that use a 32-bit off_t to map large files (up to 2^44 bytes). RETURN VALUE
On success, mmap2() returns a pointer to the mapped area. On error -1 is returned and errno is set appropriately. ERRORS
EFAULT Problem with getting the data from userspace. EINVAL (Various platforms where the page size is not 4096 bytes.) offset * 4096 is not a multiple of the system page size. mmap2() can return any of the same errors as mmap(2). VERSIONS
mmap2() is available since Linux 2.3.31. CONFORMING TO
This system call is Linux-specific. NOTES
Nowadays, the glibc mmap() wrapper function invokes this system call rather than the mmap(2) system call. On ia64, the unit for offset is actually the system page size, rather than 4096 bytes. SEE ALSO
getpagesize(2), mmap(2), mremap(2), msync(2), shm_open(3) COLOPHON
This page is part of release 3.27 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2008-04-22 MMAP2(2)
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