Re: [PATCH] Performance Improvement in CRC16 Calculations.

[Joe Perches <joe@perches.com>]

On Sun, 2018-08-12 at 23:36 -0400, Douglas Gilbert wrote:
> On 2018-08-10 08:11 PM, Joe Perches wrote:
> > On Fri, 2018-08-10 at 16:02 -0400, Nicolas Pitre wrote:
> > > On Fri, 10 Aug 2018, Joe Perches wrote:
> > > 
> > > > On Fri, 2018-08-10 at 14:12 -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.
> > > > 
> > > > Thanks.
> > > > 
> > > > This seems a sensible tradeoff for the 4k text size increase.
> > > 
> > > More like 7.5KB.  Would be best if this was configurable so the small
> > > version remained available.
> > 
> > Maybe something like: (compiled, untested)
> > ---
> >   crypto/Kconfig            |  10 +
> >   crypto/crct10dif_common.c | 543 +++++++++++++++++++++++++++++++++++++++++++++-
> >   2 files changed, 549 insertions(+), 4 deletions(-)
> > 
> > diff --git a/crypto/Kconfig b/crypto/Kconfig
> > index f3e40ac56d93..88d9d17bb18a 100644
> > --- a/crypto/Kconfig
> > +++ b/crypto/Kconfig
> > @@ -618,6 +618,16 @@ config CRYPTO_CRCT10DIF
> >   	  a crypto transform.  This allows for faster crc t10 diff
> >   	  transforms to be used if they are available.
> >   
> > +config CRYPTO_CRCT10DIF_TABLE_SIZE
> > +	int "Size of CRCT10DIF crc tables (as a power of 2)"
> > +	depends on CRYPTO_CRCT10DIF
> > +	range 1 5
> > +	default 1 if EMBEDDED
> > +	default 5
> > +	help
> > +	  Set the table size used by the CRYPTO_CRCT10DIF crc calculation
> > +	  Larger values use more memory and are faster.
> > +
> >   config CRYPTO_CRCT10DIF_PCLMUL
> >   	tristate "CRCT10DIF PCLMULQDQ hardware acceleration"
> >   	depends on X86 && 64BIT && CRC_T10DIF
> > diff --git a/crypto/crct10dif_common.c b/crypto/crct10dif_common.c
> > index b2fab366f518..4eb1c50c3688 100644
> > --- a/crypto/crct10dif_common.c
> > +++ b/crypto/crct10dif_common.c
> > @@ -32,7 +32,8 @@
> >    * x^16 + x^15 + x^11 + x^9 + x^8 + x^7 + x^5 + x^4 + x^2 + x + 1
> >    * gt: 0x8bb7
> >    */
> > -static const __u16 t10_dif_crc_table[256] = {
> > +static const __u16 t10dif_crc_table[][256] = {
> 
> <snip table>
> 
> >   };
> >   
> >   __u16 crc_t10dif_generic(__u16 crc, const unsigned char *buffer, size_t len)
> >   {
> > -	unsigned int i;
> > +	const u8 *ptr = (const __u8 *)buffer;
> > +	const u8 *ptr_end = ptr + len;
> > +#if CONFIG_CRYPTO_CRCT10DIF_TABLE_SIZE > 1
> > +	size_t tablesize = 1 << (CONFIG_CRYPTO_CRCT10DIF_TABLE_SIZE - 1);
> > +	const u8 *ptr_last = ptr + (len / tablesize * tablesize);
> >   
> > -	for (i = 0 ; i < len ; i++)
> > -		crc = (crc << 8) ^ t10_dif_crc_table[((crc >> 8) ^ buffer[i]) & 0xff];
> > +	while (ptr < ptr_last) {
> > +		size_t index = tablesize;
> > +		__u16 t;
> > +
> > +		t = t10dif_crc_table[--index][*ptr++ ^ (u8)(crc >> 8)];
> > +		t ^= t10dif_crc_table[--index][*ptr++ ^ (u8)crc];
> > +		crc = t;
> > +		while (index > 0)
> > +			crc ^= t10dif_crc_table[--index][*ptr++];
> > +	}
> > +#endif
> > +	while (ptr < ptr_end)
> > +		crc = t10dif_crc_table[0][*ptr++ ^ (u8)(crc >> 8)] ^ (crc << 8);
> >   
> >   	return crc;
> >   }
> > 
> > 
> 
> The attached patch is on top of the one above. I tested it in the user space
> where it is around 20% faster (with a full size table). Also tried swab16 but
> there was no gain from that (perhaps around a 2% loss).

I don't get a significant difference in performance.
gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)