• Dan Williams's avatar
    libata/ahci: accommodate tag ordered controllers · cfd192c9
    Dan Williams authored
    commit 8a4aeec8d2d6a3edeffbdfae451cdf05cbf0fefd upstream.
    
    The AHCI spec allows implementations to issue commands in tag order
    rather than FIFO order:
    
    	5.3.2.12 P:SelectCmd
    	HBA sets pSlotLoc = (pSlotLoc + 1) mod (CAP.NCS + 1)
    	or HBA selects the command to issue that has had the
    	PxCI bit set to '1' longer than any other command
    	pending to be issued.
    
    The result is that commands posted sequentially (time-wise) may play out
    of sequence when issued by hardware.
    
    This behavior has likely been hidden by drives that arrange for commands
    to complete in issue order.  However, it appears recent drives (two from
    different vendors that we have found so far) inflict out-of-order
    completions as a matter of course.  So, we need to take care to maintain
    ordered submission, otherwise we risk triggering a drive to fall out of
    sequential-io automation and back to random-io processing, which incurs
    large latency and degrades throughput.
    
    This issue was found in simple benchmarks where QD=2 seq-write
    performance was 30-50% *greater* than QD=32 seq-write performance.
    
    Tagging for -stable and making the change globally since it has a low
    risk-to-reward ratio.  Also, word is that recent versions of an unnamed
    OS also does it this way now.  So, drives in the field are already
    experienced with this tag ordering scheme.
    
    Cc: Dave Jiang <dave.jiang@intel.com>
    Cc: Ed Ciechanowski <ed.ciechanowski@intel.com>
    Reviewed-by: 's avatarMatthew Wilcox <matthew.r.wilcox@intel.com>
    Signed-off-by: 's avatarDan Williams <dan.j.williams@intel.com>
    Signed-off-by: 's avatarTejun Heo <tj@kernel.org>
    Signed-off-by: 's avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
    cfd192c9
libata-core.c 178 KB