linux-kernel - Re: sync_file_range(SYNC_FILE_RANGE

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Message-ID: <20080601230040.GC2255@elf.ucw.cz>
Date:	Mon, 2 Jun 2008 01:00:40 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	mtk.manpages@...il.com, 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?

On Sun 2008-06-01 15:47:36, Andrew Morton wrote:
> On Mon, 2 Jun 2008 00:22:02 +0200 Pavel Machek <pavel@...e.cz> wrote:

Hi!

> > > I sense a strangeness.  What are you actually trying to do with all of this?
> > 
> > Okay, so I have around 400MB of data, I want it compressed, optionally
> > encrypted and written to partition.
> > 
> > Now, if I do it "naturally", I do writes, followed by fsync.
> > 
> > That's bad, because kernel does not start write out immediately, and
> > we waste time with idle disk. (If data compress really well, or
> > encryption is off, this is significant).
> > 
> > So we improve on this, by doing sync_file_range(SYNC_FILE_RANGE_WRITE)
> > periodically. That keeps the disk busy, but occassionaly blocks the
> > cpu... wasting time (which mostly hurts in compression+encryption
> > case).
> 
> yep.  That's another use of sync_file_range(): to allow smart userspace
> to optimise the kernel's IO scheduling decisions.
> 
> > So... how can I keep _both_ cpu and disk busy?
> 
> pthread_create() ;)

Actually it is easy enough to do with fork(), but...

> How about this:
> 
> - Add a new SYNC_FILE_RANGE_NON_BLOCKING
> 
> - If userspace set that flag, turn on writeback_control.nonblocking
>   in __filemap_fdatawrite_range().
> 
> - test it a lot.

Works for me. Is the expectation that I code this? I can certainly
provide testing ;-).

> It will be userspace's responsibility to avoid burning huge amounts of
> CPU repeatedly calling sync_file_range() and having it not actually write
> anything.

Ok... I guess doing 10x sync_file_range() when writing 400MB of data
is not excessive?
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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