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SubjectRe: [PATCH] Performance Improvement in CRC16 Calculations.
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Date
Hi,

Don't be so sure that slice by 16 provides the best performance.

Some time ago, I did the comparison for CRC32 between slice by 4 and
slice by 8 (as included in the kernel), and the result is the following
on an mpc8xx (powerpc/32)

With CONFIG_CRC32_SLICEBY8:
[ 1.109204] crc32: CRC_LE_BITS = 64, CRC_BE BITS = 64
[ 1.114401] crc32: self tests passed, processed 225944 bytes in 15118910
nsec
[ 1.130655] crc32c: CRC_LE_BITS = 64
[ 1.134235] crc32c: self tests passed, processed 225944 bytes in 4479879
nsec

With CONFIG_CRC32_SLICEBY4:
[ 1.097129] crc32: CRC_LE_BITS = 32, CRC_BE BITS = 32
[ 1.101878] crc32: self tests passed, processed 225944 bytes in 8616242 nsec
[ 1.116298] crc32c: CRC_LE_BITS = 32
[ 1.119607] crc32c: self tests passed, processed 225944 bytes in 3289576
nsec

As you can see, slice by 4 is better than slice by 8 on that CPU.

So I'm sure it is worth doing the test for CRC16 as well.

Christophe

Le 16/08/2018 à 16:02, Jeffrey Lien a écrit :
> Eric,
> We did not test the slice by 4 or 8 tables. I'm not sure of the value of doing that since the slice by 16 will provide the best performance gain. If I'm missing anything here, please let me know.
>
> I'm working on a new version of the patch based on the feedback from others and will also change the pointer variables to start with p and fix the indenting you mentioned below in the new version of the patch.
>
> Thanks
>
> Jeff Lien
>
> -----Original Message-----
> From: Eric Biggers [mailto:ebiggers@kernel.org]
> Sent: Friday, August 10, 2018 3:16 PM
> To: Jeffrey Lien <Jeff.Lien@wdc.com>
> Cc: linux-kernel@vger.kernel.org; linux-crypto@vger.kernel.org; linux-block@vger.kernel.org; linux-scsi@vger.kernel.org; herbert@gondor.apana.org.au; tim.c.chen@linux.intel.com; martin.petersen@oracle.com; David Darrington <david.darrington@wdc.com>; Jeff Furlong <jeff.furlong@wdc.com>
> Subject: Re: [PATCH] Performance Improvement in CRC16 Calculations.
>
> On Fri, Aug 10, 2018 at 02:12:11PM -0500, Jeff Lien wrote:
>> This patch provides a performance improvement for the CRC16
>> calculations done in read/write workloads using the T10 Type 1/2/3
>> guard field. For example, today with sequential write workloads (one
>> thread/CPU of IO) we consume 100% of the CPU because of the CRC16
>> computation bottleneck. Today's block devices are considerably
>> faster, but the CRC16 calculation prevents folks from utilizing the
>> throughput of such devices. To speed up this calculation and expose
>> the block device throughput, we slice the old single byte for loop into a 16 byte for loop, with a larger CRC table to match. The result has shown 5x performance improvements on various big endian and little endian systems running the 4.18.0 kernel version.
>>
>> FIO Sequential Write, 64K Block Size, Queue Depth 64
>> BE Base Kernel: bw=201.5 MiB/s
>> BE Modified CRC Calc: bw=968.1 MiB/s
>> 4.80x performance improvement
>>
>> LE Base Kernel: bw=357 MiB/s
>> LE Modified CRC Calc: bw=1964 MiB/s
>> 5.51x performance improvement
>>
>> FIO Sequential Read, 64K Block Size, Queue Depth 64
>> BE Base Kernel: bw=611.2 MiB/s
>> BE Modified CRC calc: bw=684.9 MiB/s
>> 1.12x performance improvement
>>
>> LE Base Kernel: bw=797 MiB/s
>> LE Modified CRC Calc: bw=2730 MiB/s
>> 3.42x performance improvement
>
> Did you also test the slice-by-4 (requires 2048-byte table) and slice-by-8 (requires 4096-byte table) methods? Your proposal is slice-by-16 (requires 8192-byte table); the original was slice-by-1 (requires 512-byte table).
>
>> __u16 crc_t10dif_generic(__u16 crc, const unsigned char *buffer,
>> size_t len) {
>> - unsigned int i;
>> + const __u8 *i = (const __u8 *)buffer;
>> + const __u8 *i_end = i + len;
>> + const __u8 *i_last16 = i + (len / 16 * 16);
>
> 'i' is normally a loop counter, not a pointer.
> Use 'p', 'p_end', and 'p_last16'.
>
>>
>> - for (i = 0 ; i < len ; i++)
>> - crc = (crc << 8) ^ t10_dif_crc_table[((crc >> 8) ^ buffer[i]) & 0xff];
>> + for (; i < i_last16; i += 16) {
>> + crc = t10_dif_crc_table[15][i[0] ^ (__u8)(crc >> 8)] ^
>> + t10_dif_crc_table[14][i[1] ^ (__u8)(crc >> 0)] ^
>> + t10_dif_crc_table[13][i[2]] ^
>> + t10_dif_crc_table[12][i[3]] ^
>> + t10_dif_crc_table[11][i[4]] ^
>> + t10_dif_crc_table[10][i[5]] ^
>> + t10_dif_crc_table[9][i[6]] ^
>> + t10_dif_crc_table[8][i[7]] ^
>> + t10_dif_crc_table[7][i[8]] ^
>> + t10_dif_crc_table[6][i[9]] ^
>> + t10_dif_crc_table[5][i[10]] ^
>> + t10_dif_crc_table[4][i[11]] ^
>> + t10_dif_crc_table[3][i[12]] ^
>> + t10_dif_crc_table[2][i[13]] ^
>> + t10_dif_crc_table[1][i[14]] ^
>> + t10_dif_crc_table[0][i[15]];
>> + }
>
> Please indent this properly.
>
> crc = t10_dif_crc_table[15][i[0] ^ (__u8)(crc >> 8)] ^
> t10_dif_crc_table[14][i[1] ^ (__u8)(crc >> 0)] ^
> t10_dif_crc_table[13][i[2]] ^
> t10_dif_crc_table[12][i[3]] ^
> t10_dif_crc_table[11][i[4]] ^
> ...
>
> - Eric
>

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