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@xxxxxxxxxx] > > 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@xxxxxxxxxxxxxxx > > 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@xxxxxxxxxx> > > > > > > > > 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@xxxxxxxxxx> > > --- > > > > 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. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
