Does anyone allow unlimited or extremely large quotas?
pascal.gienger at uni-konstanz.de
Mon Nov 22 08:19:35 EST 2010
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.
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