Messages in this thread | | | Date | Tue, 3 Jun 2008 10:05:15 +0200 | From | Pavel Machek <> | Subject | Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks? |
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On Tue 2008-06-03 10:01:22, Michael Kerrisk wrote: > On Sun, Jun 1, 2008 at 1:40 PM, Pavel Machek <pavel@suse.cz> wrote: > > Hi! > > > >> > > > All I can say so far is that I find the same as you do: > >> > > > SYNC_FILE_RANGE_WRITE (after writing) takes a significant amount of time, > >> > > > more than half as long as when you add in SYNC_FILE_RANGE_WAIT_AFTER too. > >> > > > > >> > > > Which make the sync_file_range call pretty pointless: your usage seems > >> > > > perfectly reasonable to me, but somehow we've broken its behaviour. > >> > > > I'll be investigating ... > >> > > > >> > > It will block on disk queue fullness - sysrq-W will tell. > >> > > >> > Ah, thank you. What a disappointment, though it's understandable. > >> > Doesn't that very severely limit the usefulness of the system call? > >> > >> A bit. The request queue size is runtime tunable though. > > > > Which /sys is that? What happens if I set the queue size to pretty > > much infinity, will memory management die horribly? > > > >> I expect major users of this system call will be applications which do > >> small-sized overwrites into large files, mainly databases. That is, > >> once the application developers discover its existence. I'm still > >> getting expressions of wonder from people who I tell about the > >> five-year-old fadvise(). > > > > Hey, you have one user now, its called s2disk. But for this call to be > > useful, we'd need asynchronous variant... is there such thing? > > > > Okay, I can fork and do the call from another process, but... > > > >> > I admit the flag isn't called SYNC_FILE_RANGE_WRITE_WITHOUT_WAITING, > >> > but I don't suppose Pavel and I are the only ones misled by it. > >> > >> Yup, this caveat/restriction should be in the manpage. > > > > Michael, this is something for you I guess? > > Pavel, > > Just to confirm: you are meaning that the sentence > > Notice that even this this may and will block if you attempt > to write more than request queue size. > > should be added to the man page under the description of > SYNC_FILE_RANGE_WRITE, right?
Yes. Pavel
-- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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