Right. That makes sense.
On Monday, July 16, 2018, 7:27:18 PM PDT, valdis.kletnieks@xxxxxx <valdis.kletnieks@xxxxxx> wrote:
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).
> 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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