On Fri, Oct 28, 2016 at 08:42:44AM +1100, Dave Chinner wrote:

On Fri, Oct 28, 2016 at 03:17:47AM +1100, Nicholas Piggin wrote:

quoted

Hi guys,

We're seeing crc32c_le show up in xfs log checksumming on a MySQL benchmark
on powerpc. I could reproduce similar overheads with dbench as well.

1.11%  mysqld           [kernel.vmlinux]            [k] __crc32c_le
        |
        ---__crc32c_le
           |          
            --1.11%--chksum_update
                      |          
                       --1.11%--crypto_shash_update
                                 crc32c
                                 xlog_cksum
                                 xlog_sync
                                 _xfs_log_force_lsn
                                 xfs_file_fsync
                                 vfs_fsync_range
                                 do_fsync
                                 sys_fsync
                                 system_call
                                 0x17738
                                 0x17704
                                 os_file_flush_func
                                 fil_flush

2-3% is the typical CRC CPU overhead I see on metadata/log intensive
workloads on x86-64, so this doesn't seem unreasonable.

FWIW, I just noticed that qemu is now passing through hardware CRC
feature bits to the guest, so it's now using the hardware
accelerated intel module for CRCs. That "2-3%" I mentioned I used
to see from __crc32c_le is now:

   0.35%  [kernel]  [k] crc32c_pcl_intel_update

And there's about 250MB/s of log+metadata IO being CRC'd in this
workload, so I don't think that the way XFS splits the CRCs into
multiple segments is really all that significant...

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com