Nick Piggin <[email protected]> wrote:
>
> I don't think anyone would use MS_ASYNC for anything other than
> performance improvement, so it is not like we need super well
> defined behaviour... the earlier it will start IO AFAIKS the better.
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.
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.
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