Re: msync() behaviour broken for MS_ASYNC, revert patch?

[linux@horizon.com]

> 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.