Re: [PATCH] Performance Improvement in CRC16 Calculations.

[Joe Perches]

On Sat, 2018-08-11 at 11:36 -0400, Martin K. Petersen wrote:
> Jeff,
> 
> > 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.
> 
> The reason I went with a simple slice-by-one approach was that the
> larger tables had a negative impact on the CPU caches. So while
> slice-by-N numbers looked better in synthetic benchmarks, actual
> application performance started getting affected as the tables grew
> larger.
> 
> These days we obviously use the hardware-accelerated CRC calculation so
> the software table approach mostly serves as a reference
> implementation. But given your big vs. little endian performance
> metrics, I'm assuming you guys are focused on embedded processors
> without support for CRC acceleration?
> 
> I have no problem providing a choice for bigger tables. My only concern
> is that the selection heuristics need to be more than one-dimensional.
> Latency and cache side effects are often more important than throughput.
> At least on the initiator side.
> 
> Also, I'd like to keep the original slice-by-one implementation for
> reference purposes.

Did you see the suggested patch that allows
either 1, 2, 4, 8 or 16 block table sizes?

Perhaps you have a comment on that?