Le 19 nov. 2010 à 17:48, Michel Sébastien a écrit : > Is mmap still efficient ? map a gigabit file should cost a lot of I/O and a > relatively long reponse time to just access the records of the most recent > emails.
mmap does nothing besides mapping the file as virtual memory to your process. Read requests on memory addresses within the mmap range yield in page-ins of "pages" of the mmap'd file - it behaves like swap space. if you write in this mmap'd virtual memory range then you'll trigger a pageout when the system's vm system thinks it's time to write the page out (or you unmap the file). This is a very efficient way to access a file. So mapping a Gigabyte file does not need much i/o - and 10 read accesses result in a maximum of 10 pages read. The file (or the offset given with mmap() ) must fit in the process vm, so mmap'd files can be much bigger in size on 64 bit platforms. Pascal ---- Cyrus Home Page: http://www.cyrusimap.org/ List Archives/Info: http://lists.andrew.cmu.edu/pipermail/info-cyrus/
