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SubjectRe: 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@suse.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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