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Diffstat (limited to 'module/zcommon/zfs_fletcher.c')
| -rw-r--r-- | module/zcommon/zfs_fletcher.c | 246 |
1 files changed, 246 insertions, 0 deletions
diff --git a/module/zcommon/zfs_fletcher.c b/module/zcommon/zfs_fletcher.c new file mode 100644 index 000000000..fa43ce6bd --- /dev/null +++ b/module/zcommon/zfs_fletcher.c @@ -0,0 +1,246 @@ +/* + * CDDL HEADER START + * + * The contents of this file are subject to the terms of the + * Common Development and Distribution License (the "License"). + * You may not use this file except in compliance with the License. + * + * You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE + * or http://www.opensolaris.org/os/licensing. + * See the License for the specific language governing permissions + * and limitations under the License. + * + * When distributing Covered Code, include this CDDL HEADER in each + * file and include the License file at usr/src/OPENSOLARIS.LICENSE. + * If applicable, add the following below this CDDL HEADER, with the + * fields enclosed by brackets "[]" replaced with your own identifying + * information: Portions Copyright [yyyy] [name of copyright owner] + * + * CDDL HEADER END + */ +/* + * Copyright 2009 Sun Microsystems, Inc. All rights reserved. + * Use is subject to license terms. + */ + +/* + * Fletcher Checksums + * ------------------ + * + * ZFS's 2nd and 4th order Fletcher checksums are defined by the following + * recurrence relations: + * + * a = a + f + * i i-1 i-1 + * + * b = b + a + * i i-1 i + * + * c = c + b (fletcher-4 only) + * i i-1 i + * + * d = d + c (fletcher-4 only) + * i i-1 i + * + * Where + * a_0 = b_0 = c_0 = d_0 = 0 + * and + * f_0 .. f_(n-1) are the input data. + * + * Using standard techniques, these translate into the following series: + * + * __n_ __n_ + * \ | \ | + * a = > f b = > i * f + * n /___| n - i n /___| n - i + * i = 1 i = 1 + * + * + * __n_ __n_ + * \ | i*(i+1) \ | i*(i+1)*(i+2) + * c = > ------- f d = > ------------- f + * n /___| 2 n - i n /___| 6 n - i + * i = 1 i = 1 + * + * For fletcher-2, the f_is are 64-bit, and [ab]_i are 64-bit accumulators. + * Since the additions are done mod (2^64), errors in the high bits may not + * be noticed. For this reason, fletcher-2 is deprecated. + * + * For fletcher-4, the f_is are 32-bit, and [abcd]_i are 64-bit accumulators. + * A conservative estimate of how big the buffer can get before we overflow + * can be estimated using f_i = 0xffffffff for all i: + * + * % bc + * f=2^32-1;d=0; for (i = 1; d<2^64; i++) { d += f*i*(i+1)*(i+2)/6 }; (i-1)*4 + * 2264 + * quit + * % + * + * So blocks of up to 2k will not overflow. Our largest block size is + * 128k, which has 32k 4-byte words, so we can compute the largest possible + * accumulators, then divide by 2^64 to figure the max amount of overflow: + * + * % bc + * a=b=c=d=0; f=2^32-1; for (i=1; i<=32*1024; i++) { a+=f; b+=a; c+=b; d+=c } + * a/2^64;b/2^64;c/2^64;d/2^64 + * 0 + * 0 + * 1365 + * 11186858 + * quit + * % + * + * So a and b cannot overflow. To make sure each bit of input has some + * effect on the contents of c and d, we can look at what the factors of + * the coefficients in the equations for c_n and d_n are. The number of 2s + * in the factors determines the lowest set bit in the multiplier. Running + * through the cases for n*(n+1)/2 reveals that the highest power of 2 is + * 2^14, and for n*(n+1)*(n+2)/6 it is 2^15. So while some data may overflow + * the 64-bit accumulators, every bit of every f_i effects every accumulator, + * even for 128k blocks. + * + * If we wanted to make a stronger version of fletcher4 (fletcher4c?), + * we could do our calculations mod (2^32 - 1) by adding in the carries + * periodically, and store the number of carries in the top 32-bits. + * + * -------------------- + * Checksum Performance + * -------------------- + * + * There are two interesting components to checksum performance: cached and + * uncached performance. With cached data, fletcher-2 is about four times + * faster than fletcher-4. With uncached data, the performance difference is + * negligible, since the cost of a cache fill dominates the processing time. + * Even though fletcher-4 is slower than fletcher-2, it is still a pretty + * efficient pass over the data. + * + * In normal operation, the data which is being checksummed is in a buffer + * which has been filled either by: + * + * 1. a compression step, which will be mostly cached, or + * 2. a bcopy() or copyin(), which will be uncached (because the + * copy is cache-bypassing). + * + * For both cached and uncached data, both fletcher checksums are much faster + * than sha-256, and slower than 'off', which doesn't touch the data at all. + */ + +#include <sys/types.h> +#include <sys/sysmacros.h> +#include <sys/byteorder.h> +#include <sys/zio.h> +#include <sys/spa.h> + +void +fletcher_2_native(const void *buf, uint64_t size, zio_cksum_t *zcp) +{ + const uint64_t *ip = buf; + const uint64_t *ipend = ip + (size / sizeof (uint64_t)); + uint64_t a0, b0, a1, b1; + + for (a0 = b0 = a1 = b1 = 0; ip < ipend; ip += 2) { + a0 += ip[0]; + a1 += ip[1]; + b0 += a0; + b1 += a1; + } + + ZIO_SET_CHECKSUM(zcp, a0, a1, b0, b1); +} + +void +fletcher_2_byteswap(const void *buf, uint64_t size, zio_cksum_t *zcp) +{ + const uint64_t *ip = buf; + const uint64_t *ipend = ip + (size / sizeof (uint64_t)); + uint64_t a0, b0, a1, b1; + + for (a0 = b0 = a1 = b1 = 0; ip < ipend; ip += 2) { + a0 += BSWAP_64(ip[0]); + a1 += BSWAP_64(ip[1]); + b0 += a0; + b1 += a1; + } + + ZIO_SET_CHECKSUM(zcp, a0, a1, b0, b1); +} + +void +fletcher_4_native(const void *buf, uint64_t size, zio_cksum_t *zcp) +{ + const uint32_t *ip = buf; + const uint32_t *ipend = ip + (size / sizeof (uint32_t)); + uint64_t a, b, c, d; + + for (a = b = c = d = 0; ip < ipend; ip++) { + a += ip[0]; + b += a; + c += b; + d += c; + } + + ZIO_SET_CHECKSUM(zcp, a, b, c, d); +} + +void +fletcher_4_byteswap(const void *buf, uint64_t size, zio_cksum_t *zcp) +{ + const uint32_t *ip = buf; + const uint32_t *ipend = ip + (size / sizeof (uint32_t)); + uint64_t a, b, c, d; + + for (a = b = c = d = 0; ip < ipend; ip++) { + a += BSWAP_32(ip[0]); + b += a; + c += b; + d += c; + } + + ZIO_SET_CHECKSUM(zcp, a, b, c, d); +} + +void +fletcher_4_incremental_native(const void *buf, uint64_t size, + zio_cksum_t *zcp) +{ + const uint32_t *ip = buf; + const uint32_t *ipend = ip + (size / sizeof (uint32_t)); + uint64_t a, b, c, d; + + a = zcp->zc_word[0]; + b = zcp->zc_word[1]; + c = zcp->zc_word[2]; + d = zcp->zc_word[3]; + + for (; ip < ipend; ip++) { + a += ip[0]; + b += a; + c += b; + d += c; + } + + ZIO_SET_CHECKSUM(zcp, a, b, c, d); +} + +void +fletcher_4_incremental_byteswap(const void *buf, uint64_t size, + zio_cksum_t *zcp) +{ + const uint32_t *ip = buf; + const uint32_t *ipend = ip + (size / sizeof (uint32_t)); + uint64_t a, b, c, d; + + a = zcp->zc_word[0]; + b = zcp->zc_word[1]; + c = zcp->zc_word[2]; + d = zcp->zc_word[3]; + + for (; ip < ipend; ip++) { + a += BSWAP_32(ip[0]); + b += a; + c += b; + d += c; + } + + ZIO_SET_CHECKSUM(zcp, a, b, c, d); +} |