Messages in this thread | | | Date | Thu, 9 Feb 2006 20:13:33 -0800 | From | Andrew Morton <> | Subject | Re: msync() behaviour broken for MS_ASYNC, revert patch? |
| |
Nick Piggin <nickpiggin@yahoo.com.au> 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |