Re: [PATCH] lib/libcrc32c

[Clay Haapala <chaapala@cisco.com>]

On Tue, 30 Mar 2004, Bartlomiej Zolnierkiewicz told this:

> On Tuesday 30 of March 2004 21:11, Clay Haapala wrote:
>> On Tue, 30 Mar 2004, Bartlomiej Zolnierkiewicz outgrape:
>> > +
>> > +static u32 crc32c_table[256] = {
>> >
>> > Tables are build time generated in case of CRC32
>> > (lib/gen_crc32table.c) so you can trade some performance for
>> > smaller size of the table.
>> >
>> > [ However I don't know how useful is this. ]
>>
>> As the table was statically in code in sctp and in the iSCSI driver
>> where it originally came from, I left it that way.  Why would the
>> table be of a different size?
> 
> See lib/crc32defs.h and lib/gen_crc32table.c.
> 
> lib/crc32defs.h:
> 
> /* How many bits at a time to use.  Requires a table of
> 4<<CRC_xx_BITS bytes. */ /* For less performance-sensitive, use 4 */
> #ifndef CRC_LE_BITS # define CRC_LE_BITS 8 #endif #ifndef
> CRC_BE_BITS # define CRC_BE_BITS 8 #endif
> 
> lib/gen_crc32table.c:
> 
> #define LE_TABLE_SIZE (1 << CRC_LE_BITS)
> #define BE_TABLE_SIZE (1 << CRC_BE_BITS)
> 
> static u_int32_t crc32table_le[LE_TABLE_SIZE];
> static u_int32_t crc32table_be[BE_TABLE_SIZE];
> 
> 
This is a legitimate question.

The immediate user of this crc32c is the iscsi-sfnet driver, and my
mind-set for that is always towards gigabit-ethernet wire-rate, so
that bias may prevent me from making the right choice here.

Regarding performance vs. size, we have the BITS == 8 table for "high"
performance and bigger size, and BITS == 1 bit-wise computation, for the
least size, albeit slower performance.  Is that good enough?

The last time I wrote CRC lookup code for nibble-sized tables, it was
for an 8051 microcontroller. :-)
-- 
Clay Haapala (chaapala@cisco.com) Cisco Systems SRBU +1 763-398-1056
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