Messages in this thread | | | Date | 10 Feb 2006 02:15:04 -0500 | From | linux@horizon ... | Subject | Re: msync() behaviour broken for MS_ASYNC, revert patch? |
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> 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. - 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/
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