I heard (read it on the internet somewhere) that mmap() is faster than sequential IO. Is this correct? If yes then why it is faster? It can be – there are pros and cons, listed below. When you really have reason to care, always benchmark both. Quite apart from the actual IO efficiency, there are … Read more
From IEEE 1003.1: The mmap() function shall establish a mapping between a process’ address space and a file, shared memory object, or [TYM] typed memory object. It needs all the virtual address space because that’s exactly what mmap() does. The fact that it isn’t really running out of memory doesn’t matter – you can’t map … Read more
If you are using MAMP be sure to add the unix_socket key with a value of the path that the mysql.sock resides in MAMP. ‘mysql’ => array( ‘driver’ => ‘mysql’, ‘host’ => ‘localhost’, ‘unix_socket’ => ‘/Applications/MAMP/tmp/mysql/mysql.sock’, ‘database’ => ‘database’, ‘username’ => ‘root’, ‘password’ => ‘root’, ‘charset’ => ‘utf8’, ‘collation’ => ‘utf8_unicode_ci’, ‘prefix’ => ”, ),
Look folks, contrary to common believe, mmap is indeed a memory allocation function similar to malloc.. the mmaped file is one use of it.. you can use it as memory allocation function passing -1 as file descriptor.. so.. the common use is to use malloc for tiny objects and mmap for large ones.. this is … Read more
The following analysis applies only to glibc (based on the ptmalloc2 algorithm). There are certain options that seem helpful to return the freed memory back to the system: mallopt() (defined in malloc.h) does provide an option to set the trim threshold value using one of the parameter option M_TRIM_THRESHOLD, this indicates the minimum amount of … Read more
MAP_PRIVATE mappings require a memory reservation, as writing to these pages may result in copy-on-write allocations. This means that you can’t map something too much larger than your physical ram + swap. Try using a MAP_SHARED mapping instead. This means that writes to the mapping will be reflected on disk – as such, the kernel … Read more
Both methods are viable. mmap method is a little bit more restrictive then shmget, but easier to use. shmget is the old System V shared memory model and has the widest support. mmap/shm_open is the new POSIX way to do shared memory and is easier to use. If your OS permits the use of POSIX … Read more
No, at least not on Linux it’s fine to close the file. The manual page clearly states: On the other hand, closing the file descriptor does not unmap the region. For portability, I also checked the POSIX manual, it says the same thing (although even more clearly): The mmap() function adds an extra reference to … Read more
Linux has an execution domain called READ_IMPLIES_EXEC, which causes all pages allocated with PROT_READ to also be given PROT_EXEC. Older Linux kernels used to use this for executables that used the equivalent of gcc -z execstack. This program will show you whether that’s enabled for itself: #include <stdio.h> #include <sys/personality.h> int main(void) { printf(“Read-implies-exec is … Read more
I was trying to find the final word on mmap / read performance on Linux and I came across a nice post (link) on the Linux kernel mailing list. It’s from 2000, so there have been many improvements to IO and virtual memory in the kernel since then, but it nicely explains the reason why … Read more