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SubjectRe: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks?
Date
On Monday, 2 of June 2008, Hugh Dickins wrote:
> On Sun, 1 Jun 2008, Andrew Morton wrote:
> > On Mon, 2 Jun 2008 01:00:40 +0200 Pavel Machek <pavel@suse.cz> wrote:
> > > > 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 ;-).
> >
> > Something like this:
>
> Though this fits very easily into the current kernel implementation,
> I don't think it's the right interface for userspace.
>
> If we do go this kind of a way, then I'd say SYNC_FILE_RANGE_NON_BLOCKING
> needs to tell the caller how far it got before giving up, rather than just
> success or failure. Why? um, um, because it feels right; and would help
> the caller help the kernel by not overloading it with needlessly repeated
> loop ranges - any stronger reasons? But sync_file_range() was defined
> to return int rather than ssize_t, so that becomes awkward.
>
> Never mind, I don't think it is the right way anyway. We don't need
> additions to the existing sync_file_range() interface, we just need it
> to behave as naive people like Pavel and I expected it to behave in the
> first place: SYNC_FILE_RANGE_WRITE should be nonblocking (with respect
> to queue congestion, and maybe page locking also).

Well, frankly, I'm not sure if we need anything better than we already have.
In fact my numbers show that SYNC_FILE_RANGE_WRITE works quite well -
please see
http://sourceforge.net/mailarchive/message.php?msg_name=200806020122.36193.rjw%40sisk.pl
"early writeout" means that SYNC_FILE_RANGE_WRITE is used and the file with
the results is attached for convenience.

My interpretation of the results is here:
http://sourceforge.net/mailarchive/forum.php?thread_name=200806021238.17100.rjw%40sisk.pl&forum_name=suspend-devel

Thanks,
Rafael
Labels:
W+ - early writeout enabled
W- - early writeout disabled
C+ - compression enabled
C- - compression disabled
E+ - encryption enabled
E- - encryption disabled
T+ - threads enabled
T- - threads disabled

The numbers are speed values in MB/s.


Box 1 (Single-core, Athlon 64 3000+ 1.8 GHz, 1 MB L2 cache, 1.5 GB RAM)

W-C-E-T- W+C-E-T- W-C+E-T- W+C+E-T-
Write: 26.2 27.4 46.5 78.6
Read: 27.5 27.5 83.5 83.8

W-C-E+T- W+C-E+T- W-C+E+T- W+C+E+T-
Write: 15.6 19.1 28.4 38.7
Read: 18.0 18.1 46.1 46.2

W-C-E+T+ W+C-E+T+ W-C+E+T+ W+C+E+T+
Write: N/A N/A 27.9 37.5
Read: N/A N/A 46.4 46.4

For compressed images the compression ratio was approx. 0.30


Box 2 (Dual-core, Turion X2 TL-60 2 GHz, 2 x 512 KB L2 cache, 2 GB RAM)

W-C-E-T- W+C-E-T- W-C+E-T- W+C+E-T-
Write: 31.9 34.1 42.7 74.5
Read: 33.0 32.9 79.8 80.1

W-C-E+T- W+C-E+T- W-C+E+T- W+C+E+T-
Write: 16.7 22.0 23.9 34.0
Read: 22.2 22.2 47.6 47.9

W-C-E+T+ W+C-E+T+ W-C+E+T+ W+C+E+T+
Write: 16.4 22.2 34.7 57.2
Read: 22.2 22.1 47.9 48.0

For compressed images the compression ratio was approx. 0.40

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