Re: [GIT PULL] Update LZO compression

[Jeff Garzik]

On 08/16/2012 02:27 AM, Markus F.X.J. Oberhumer wrote:
> On 2012-08-15 16:45, Johannes Stezenbach wrote:
> > On Wed, Aug 15, 2012 at 02:02:43PM +0200, Markus F.X.J. Oberhumer wrote:
> > > On 2012-08-14 14:39, Johannes Stezenbach wrote:
> > > > On Tue, Aug 14, 2012 at 01:44:02AM +0200, Markus F.X.J. Oberhumer wrote:
> > > > > On 2012-07-16 20:30, Markus F.X.J. Oberhumer wrote:
> > > > > > As stated in the README this version is significantly faster (typically more
> > > > > > than 2 times faster!) than the current version, has been thoroughly tested on
> > > > > > x86_64/i386/powerpc platforms and is intended to get included into the
> > > > > > official Linux 3.6 or 3.7 release.
> > > > > >
> > > > > > I encourage all compression users to test and benchmark this new version,
> > > > > > and I also would ask some official LZO maintainer to convert the updated
> > > > > > source files into a GIT commit and possibly push it to Linus or linux-next.
> > > >
> > > > Sorry for not reporting earlier, but I didn't have time to do real
> > > > benchmarks, just a quick test on ARM926EJ-S using barebox,
> > > > and found in the new version decompression is slower:
> > > > http://lists.infradead.org/pipermail/barebox/2012-July/008268.html
> > >
> > > I can only guess, but maybe your ARM cpu does not have an efficient
> > > implementation of {get,put}_unaligned().
> >
> > Yes, ARMv5 cannot do unaligned access.  ARMv6+ could, but
> > I think the Linux kernel normally traps it for debug,
> > all ARM seem to use generic {get,put}_unaligned() implementation
> > which use byte access and shift.
>
> Hmm - I could imagine that we're wasting a lot of possible speed gain
> by not exploiting that feature on ARMv6+.

Or you could just realize that unaligned accesses are slow in the best 
case, and are simply not supported on some processors.

If you think a little bit, I bet you could come up with a solution that 
operates at cacheline-aligned granularity, something that would be _even 
faster_ than simply fixing the code to do aligned accesses.

	Jeff




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