Messages in this thread Patch in this message | | | Date | Sun, 1 Jun 2008 13:40:09 +0200 | From | Pavel Machek <> | Subject | Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks? |
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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?
And andrew, something for you:
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SYNC_FILE_RANGE_WRITE may and will block. Document that.
Signed-off-by: Pavel Machek <pavel@suse.cz>
--- commit 5db78da3d8e6fa527bfe384ded2ff7c835592fe2 tree 4c405e07be12f0a2260492fb43d19802ff7ebab1 parent 0ea376de01be797f9563c2c2464149f8f0af6329 author Pavel <pavel@amd.ucw.cz> Sun, 01 Jun 2008 13:39:25 +0200 committer Pavel <pavel@amd.ucw.cz> Sun, 01 Jun 2008 13:39:25 +0200
fs/sync.c | 3 ++- 1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/fs/sync.c b/fs/sync.c index 228e17b..54e9f20 100644 --- a/fs/sync.c +++ b/fs/sync.c @@ -139,7 +139,8 @@ asmlinkage long sys_fdatasync(unsigned i * before performing the write. * * SYNC_FILE_RANGE_WRITE: initiate writeout of all those dirty pages in the - * range which are not presently under writeback. + * range which are not presently under writeback. Notice that even this this + * may and will block if you attempt to write more than request queue size. * * SYNC_FILE_RANGE_WAIT_AFTER: wait upon writeout of all pages in the range * after performing the write.
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