[email protected] wrote:
>
> > 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.
We've covered this. Handing pte-dirty pages over to pdflush for prompt
writeback is a perfectly valid, sensible and fast thing to do.
It efficiently solves the single biggest problem with using MAP_SHARED
instead of write().
> 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)
I cannot think of a way, sorry.
> or writing other dirty pages that aren't complete yet.
msync() will write all of the file's dirty pages and it has always has done
that.
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