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SubjectRe: [PATCH] Performance Improvement in CRC16 Calculations.
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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
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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