Date:	Mon, 12 Dec 2011 22:32:28 -0800
From:	"Darrick J. Wong" <djwong@...ibm.com>
To:	Bob Pearson <rpearson@...temfabricworks.com>
Cc:	"'Herbert Xu'" <herbert@...dor.apana.org.au>,
	"'Andrew Morton'" <akpm@...ux-foundation.org>,
	"'Theodore Tso'" <tytso@....edu>,
	"'Joakim Tjernlund'" <joakim.tjernlund@...nsmode.se>,
	"'linux-kernel'" <linux-kernel@...r.kernel.org>,
	"'Andreas Dilger'" <adilger.kernel@...ger.ca>,
	"'linux-crypto'" <linux-crypto@...r.kernel.org>,
	"'linux-fsdevel'" <linux-fsdevel@...r.kernel.org>,
	"'Mingming Cao'" <cmm@...ibm.com>, linux-ext4@...r.kernel.org
Subject: Re: [PATCH 14/14] crc32: Select an algorithm via kconfig

On Mon, Dec 12, 2011 at 05:10:45PM -0600, Bob Pearson wrote:
> That choice was for Joakim who measured better performance on his 32 bit PPC
> platform with "by 4".

Ok.  On my 1.33GHz PowerBook I get ~255MB/s with slice by 4 and ~270MB/s with
slice by 8.  I think it's a PPC 7447, and definitely 32-bit.  In any case, it
reports having 32K of L1D cache.

--D
> 
> > -----Original Message-----
> > From: Darrick J. Wong [mailto:djwong@...ibm.com]
> > Sent: Monday, December 12, 2011 4:59 PM
> > To: Herbert Xu; Bob Pearson
> > Cc: Andrew Morton; Theodore Tso; Joakim Tjernlund; linux-kernel; Andreas
> > Dilger; linux-crypto; linux-fsdevel; Mingming Cao;
> linux-ext4@...r.kernel.org
> > Subject: Re: [PATCH 14/14] crc32: Select an algorithm via kconfig
> > 
> > On Fri, Dec 02, 2011 at 06:36:46PM -0800, Darrick J. Wong wrote:
> > > On Fri, Dec 02, 2011 at 08:25:05AM +0800, Herbert Xu wrote:
> > > > On Thu, Dec 01, 2011 at 12:15:17PM -0800, Darrick J. Wong wrote:
> > > > > Allow the kernel builder to choose a crc32* algorithm for the
> kernel.
> > > > >
> > > > > Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> > > >
> > > > I don't like this at all.  How do you expect distros or indeed
> > > > anyone to make this choice? For generic C implementations like
> > > > this we should only have one, and not many.
> > >
> > > Slice-by-8 should be picked automatically if the builder doesn't
> explicitly
> > > pick another one.  The other choices are provided for people who want a
> > slimmer
> > > cache footprint.  I guess I could make the Kconfig file a bit more
> explicit
> > > about slice-by-8 being default, or I guess we could just ignore this one
> > patch
> > > and thereby keeping us with the old method where anyone who wants the
> > slimmer
> > > implementations patches the #defines.
> > 
> > Ok, here's a patch that makes it more explicit that sliceby8 is the
> default.
> > I expect distros and anyone else to simply hit <Enter>.  The only people
> who
> > should do otherwise are people who know they are building for machines
> > that
> > have small cache sizes such that the crc table fights for cache lines with
> the
> > data being checksummed.
> > 
> > I made a quick survey of CPU L1 cache quantities:
> > 
> > All Intel CPUs since the Pentium MMX have > 8KiB of L1.
> > All AMD CPUs since the K5 have had > 8KiB of L1.
> > Most SPARC64 CPUs except the UltraSparc T1 and T2 CPUs have > 8KiB of L1.
> > Most PowerPC CPUs since the 601 seem to have > 8KiB of L1.
> > All IBM POWER CPUs since at least the POWER2 have had > 8KiB of L1.
> > There are too many different ARM cores for me to track.   My smartphones
> > and
> > embedded ARM controllers all have > 8KIB of L1, but that's not enough to
> > generalize.
> > 
> > While I might've been tempted to agree with Herbert and hardwire the code
> > to
> > use slice by 8, there are enough CPUs out there that *could* have
> too-small
> > L1
> > caches that I'm not comfortable with _removing_ the Kconfig option to use
> a
> > slimmer algorithm.  I can't gate the decision on 64-bitness either, since
> I've
> > seen plenty of i386 CPUs that benefit from slice by 8, and the UltraSparc
> T2 is
> > a 64-bit processor that seems likely to suffer cache thrashing.
> > 
> > I think having a configurable menu that steers people towards slice by 8
> is
> > fine.  Bob, was there a reason for picking slice by 4 for 32-bit machines?
> > 
> > D
> > ---
> > Allow the kernel builder to choose a crc32* algorithm for the kernel.
> > 
> > Signed-off-by: Darrick J. Wong <djwong@...ibm.com>
> > ---
> > 
> >  lib/Kconfig     |   43 +++++++++++++++++++++++++++++++++++++++++++
> >  lib/crc32defs.h |   18 ++++++++++++++++++
> >  2 files changed, 61 insertions(+), 0 deletions(-)
> > 
> > diff --git a/lib/Kconfig b/lib/Kconfig
> > index cfddafc..029c0e3 100644
> > --- a/lib/Kconfig
> > +++ b/lib/Kconfig
> > @@ -70,6 +70,49 @@ config CRC32_SELFTEST
> >  	  and crc32_be over byte strings with random alignment and length
> >  	  and computes the total elapsed time and number of bytes
> > processed.
> > 
> > +choice
> > +	prompt "CRC32 implementation"
> > +	depends on CRC32
> > +	default CRC32_SLICEBY8
> > +
> > +config CRC32_SLICEBY8
> > +	bool "Slice by 8 bytes"
> > +	help
> > +	  Calculate checksum 8 bytes at a time with a clever slicing
> algorithm.
> > +	  This is the fastest algorithm, but comes with a 8KiB lookup table.
> > +	  Most modern processors have enough cache to hold this table
> > without
> > +	  thrashing the cache.
> > +
> > +	  This is the default implementation choice.  Choose this one unless
> > +	  you have a good reason not to.
> > +
> > +config CRC32_SLICEBY4
> > +	bool "Slice by 4 bytes"
> > +	help
> > +	  Calculate checksum 4 bytes at a time with a clever slicing
> algorithm.
> > +	  This is a bit slower than slice by 8, but has a smaller 4KiB
> lookup
> > +	  table.
> > +
> > +	  Only choose this option if you know what you are doing.
> > +
> > +config CRC32_SARWATE
> > +	bool "Sarwate's Algorithm (one byte at a time)"
> > +	help
> > +	  Calculate checksum a byte at a time using Sarwate's algorithm.
> This
> > +	  is not particularly fast, but has a small 256 byte lookup table.
> > +
> > +	  Only choose this option if you know what you are doing.
> > +
> > +config CRC32_BIT
> > +	bool "Classic Algorithm (one bit at a time)"
> > +	help
> > +	  Calculate checksum one bit at a time.  This is VERY slow, but has
> > +	  no lookup table.  This is provided as a debugging option.
> > +
> > +	  Only choose this option if you are debugging crc32.
> > +
> > +endchoice
> > +
> >  config CRC7
> >  	tristate "CRC7 functions"
> >  	help
> > diff --git a/lib/crc32defs.h b/lib/crc32defs.h
> > index 6fd1917..64cba2c 100644
> > --- a/lib/crc32defs.h
> > +++ b/lib/crc32defs.h
> > @@ -13,6 +13,24 @@
> >   */
> >  #define CRC32C_POLY_LE 0x82F63B78
> > 
> > +/* Try to choose an implementation variant via Kconfig */
> > +#ifdef CONFIG_CRC32_SLICEBY8
> > +# define CRC_LE_BITS 64
> > +# define CRC_BE_BITS 64
> > +#endif
> > +#ifdef CONFIG_CRC32_SLICEBY4
> > +# define CRC_LE_BITS 32
> > +# define CRC_BE_BITS 32
> > +#endif
> > +#ifdef CONFIG_CRC32_SARWATE
> > +# define CRC_LE_BITS 8
> > +# define CRC_BE_BITS 8
> > +#endif
> > +#ifdef CONFIG_CRC32_BIT
> > +# define CRC_LE_BITS 1
> > +# define CRC_BE_BITS 1
> > +#endif
> > +
> >  /*
> >   * How many bits at a time to use.  Valid values are 1, 2, 4, 8, 32 and
> 64.
> >   * For less performance-sensitive, use 4 or 8 to save table size.
> 
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