newly add MAP_SYNC flag to mmap

[valdis.kletnieks at vt.edu]

On Tue, 17 Jul 2018 02:15:17 -0000, David Frank said:

> inking. I'm checking out if the flag does what is is said to do-- I don't have 
> to call msync function, which would boost performance.

Note that this can actually *kill* performance, because this means that the
kernel has to flush to backing store every single time it notes a change,
whereas if you use msync only at those points your software needs a sync point,
it can do it at only those points....

Thought experiment:  Imagine a workflow that needs to checkpoint every 1000
changes to the shared segment (for instance, if you've mapped an array with
1000 rows, do a for() loop across it incrementing one item, and checkpoint when
they're all incremented).  msync after the loop completes is one sync, while a worst-case
using MAP_SYNC could result in a flush after every single increment (if the system
is rescheduling the process over and over - for instance, if there's also another
syscall inside the loop).

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