> Well, no. Consider a continuously-running application which modifies its
> data store via MAP_SHARED+msync(MS_ASYNC). If the msync() immediately
> started I/O, the disk would be seeking all over the place all the time. The
> queue merging and timer-based unplugging would help here, but it won't be
> as good as a big, infrequent ascending-file-offset pdflush pass.
>
> Secondly, consider the behaviour of the above application if it is modifying
> the same page relatively frequently (quite likely). If MS_ASYNC starts I/O
> immediately, that page will get written 10, 100 or 1000 times per second.
> If MS_ASYNC leaves it to pdflush, that page gets written once per 30
> seconds, so we do far much less I/O.
You're assuming a brain-dead application. Which can already thrash the
disk very nicely with O_SYNC. Yes, if you ask for control and then do
something stupid, you can send performance into the toilet.
That's not a reason to not do what the application asks unless it's
a serious DoS attack.
(For example, In my appliction, I'm using a raw device as a circular
buffer, so I'm already delivering perfectly sequential block numbers.
And it's a flash memory disk anyway.)
> We just don't know. It's better to leave it up to the application designer
> rather than lumping too many operations into the one syscall.
I know the operating system doesn't know. If it did, there wouldn't
be any need for the application to tell it by making a system call.
So do what the application asks for, which is what the SuS says
msync(MS_ASYNC) means, which is start the write immediately.
(I'd call it "I/O", but it's only "O".)
As I said, I'm actively looking for a way, on Linux 2.6.x, x <= 15,
to start disk writes on part of an mmapped file without either blocking
(yet) or writing other dirty pages that aren't complete yet.
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