linux-kernel - Re: sync_file_range(SYNC_FILE_RANGE

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Message-ID: <20080601114008.GC16843@elf.ucw.cz>
Date:	Sun, 1 Jun 2008 13:40:09 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>, mtk.manpages@...il.com
Cc:	Hugh Dickins <hugh@...itas.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>
Subject: Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?

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:

--- 

SYNC_FILE_RANGE_WRITE may and will block. Document that.

Signed-off-by: Pavel Machek <pavel@...e.cz>

---
commit 5db78da3d8e6fa527bfe384ded2ff7c835592fe2
tree 4c405e07be12f0a2260492fb43d19802ff7ebab1
parent 0ea376de01be797f9563c2c2464149f8f0af6329
author Pavel <pavel@....ucw.cz> Sun, 01 Jun 2008 13:39:25 +0200
committer Pavel <pavel@....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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(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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