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Reply-To: dgilbert@interlog.com
Subject: Re: [PATCH] Performance Improvement in CRC16 Calculations.
To:     Joe Perches <joe@perches.com>,
        Nicolas Pitre <nicolas.pitre@linaro.org>
Cc:     Jeff Lien <jeff.lien@wdc.com>, 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@wdc.com, jeff.furlong@wdc.com
References: <1533928331-21303-1-git-send-email-jeff.lien@wdc.com>
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From:   Douglas Gilbert <dgilbert@interlog.com>
Message-ID: <b36d5eb6-3ee8-1bcd-0060-a4d4083a0fe6@interlog.com>
Date:   Sun, 12 Aug 2018 23:36:17 -0400
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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).

Doug Gilbert

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diff --git a/crypto/crct10dif_common.c b/crypto/crct10dif_common.c
index 4eb1c50c3688..bf5fab98aebb 100644
--- a/crypto/crct10dif_common.c
+++ b/crypto/crct10dif_common.c
@@ -591,23 +591,30 @@ __u16 crc_t10dif_generic(__u16 crc, const unsigned char *buffer, size_t len)
 {
 	const u8 *ptr = (const __u8 *)buffer;
 	const u8 *ptr_end = ptr + len;
+	const u16 *flat_tbl = (const u16 *)t10dif_crc_table;
 #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);
 
+	/*
+	 * t10dif_crc_table is two dimensional but access as vector via
+	 * flat_tbl for speed. t[k][j] is equivalent to tt[k*num_cols + j].
+	 * num_cols in this case is 256 allowing tt[k<<8 + j]. Perhaps
+	 * there should be a compile time assert that num_cols==256 .
+	 */
 	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];
+		t = flat_tbl[(--index << 8) + (*ptr++ ^ (u8)(crc >> 8))];
+		t ^= flat_tbl[(--index << 8) + (*ptr++ ^ (u8)crc)];
 		crc = t;
 		while (index > 0)
-			crc ^= t10dif_crc_table[--index][*ptr++];
+			crc ^= flat_tbl[(--index << 8) + *ptr++];
 	}
 #endif
 	while (ptr < ptr_end)
-		crc = t10dif_crc_table[0][*ptr++ ^ (u8)(crc >> 8)] ^ (crc << 8);
+		crc = flat_tbl[*ptr++ ^ (u8)(crc >> 8)] ^ (crc << 8);
 
 	return crc;
 }

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