Re: [PATCH] Performance Improvement in CRC16 Calculations.

[Eric Biggers]

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