Brad Roberts wrote:
bearophile wrote:
>> What the heck is going on? When does memory mapping actually help?<
You are scanning the file linearly, and the memory window you use is
probably very small. In such situation a memory mapping is probably
not the best thing. A memory mapping is useful when you for example
operate with random access on a wider sliding window on the file.
You can drop the 'sliding' part. mmap tends to help when doing random
access (or sequential but non-contiguous maybe) over a file. Pure
streaming is handled pretty well by both patterns. One nicity with mmap
is that you can hint to the os how you'll be using it via madvise. You
can't do that with [f]read.
This all would make perfect sense if the performance was about the same
in the two cases. But in fact memory mapping introduced a large
*pessimization*. Why? I am supposedly copying less data and doing less
work. This is very odd.
Andrei
