@@ -124,7 +124,7 @@ LDIRT = gen_crc32table crc32table.h crc32selftest
default: crc32selftest ltdepend $(LTLIBRARY)
-crc32table.h: gen_crc32table.c
+crc32table.h: gen_crc32table.c crc32defs.h
@echo " [CC] gen_crc32table"
$(Q) $(BUILD_CC) $(BUILD_CFLAGS) -o gen_crc32table $<
@echo " [GENERATE] $@"
@@ -135,7 +135,7 @@ crc32table.h: gen_crc32table.c
# systems/architectures. Hence we make sure that xfsprogs will never use a
# busted CRC calculation at build time and hence avoid putting bad CRCs down on
# disk.
-crc32selftest: gen_crc32table.c crc32table.h crc32.c
+crc32selftest: gen_crc32table.c crc32table.h crc32.c crc32defs.h
@echo " [TEST] CRC32"
$(Q) $(BUILD_CC) $(BUILD_CFLAGS) -D CRC32_SELFTEST=1 crc32.c -o $@
$(Q) ./$@
@@ -1,4 +1,38 @@
/*
+ * Use slice-by-8, which is the fastest variant.
+ *
+ * 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.
+ *
+ * The Linux kernel uses this as the default implementation "unless you
+ * have a good reason not to". The reason why Kconfig urges you to pick
+ * SLICEBY8 is because people challenged the assertion that we should
+ * always use slice by 8, so Darrick wrote a crc microbenchmark utility
+ * and ran it on as many machines as he could get his hands on to show
+ * that sb8 was the fastest.
+ *
+ * Every 64-bit machine (and most of the 32-bit ones too) saw the best
+ * results with sb8. Any machine with more than 4K of cache saw better
+ * results. The spreadsheet still exists today[1]; note that
+ * 'crc32-kern-le' corresponds to the slice by 4 algorithm which is the
+ * default unless CRC_LE_BITS is defined explicitly.
+ *
+ * FWIW, there are a handful of board defconfigs in the kernel that
+ * don't pick sliceby8. These are all embedded 32-bit mips/ppc systems
+ * with very small cache sizes which experience cache thrashing with the
+ * slice by 8 algorithm, and therefore chose to pick defaults that are
+ * saner for their particular board configuration. For nearly all of
+ * XFS' perceived userbase (which we assume are 32 and 64-bit machines
+ * with sufficiently large CPU cache and largeish storage devices) slice
+ * by 8 is the right choice.
+ *
+ * [1] https://goo.gl/0LSzsG ("crc32c_bench")
+ */
+#define CRC_LE_BITS 64
+
+/*
* There are multiple 16-bit CRC polynomials in common use, but this is
* *the* standard CRC-32 polynomial, first popularized by Ethernet.
* x^32+x^26+x^23+x^22+x^16+x^12+x^11+x^10+x^8+x^7+x^5+x^4+x^2+x^1+x^0
The crc32c code used in xfsprogs was copied directly from the Linux kernel. However, that code selects slice-by-4 by default, which isn't the fastest -- that's slice-by-8, which trades table size for speed. Fix some makefile dependency problems and explicitly select the algorithm we want. With this patch applied, I see about a 10% drop in CPU time running xfs_repair. Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com> --- libxfs/Makefile | 4 ++-- libxfs/crc32defs.h | 34 ++++++++++++++++++++++++++++++++++ 2 files changed, 36 insertions(+), 2 deletions(-) -- To unsubscribe from this list: send the line "unsubscribe linux-xfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html