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SubjectRe: [PATCH] Performance Improvement in CRC16 Calculations.
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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)

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