http://bugzilla.kernel.org/show_bug.cgi?id=15565 --- Comment #1 from Anonymous Emailer <anonymous@xxxxxxxxxxxxxxxxxxxx> 2010-03-18 02:20:02 --- Reply-To: dgilbert@xxxxxxxxxxxx bugzilla-daemon@xxxxxxxxxxxxxxxxxxx wrote: > http://bugzilla.kernel.org/show_bug.cgi?id=15565 > > Summary: SCSI Generic queueing completes commands in reverse > order > Product: IO/Storage > Version: 2.5 > Kernel Version: 2.6.18-2.6.32 > Platform: All > OS/Version: Linux > Tree: Mainline > Status: NEW > Severity: normal > Priority: P1 > Component: SCSI > AssignedTo: linux-scsi@xxxxxxxxxxxxxxx > ReportedBy: mh-linux-kernel@xxxxxxxx > Regression: No > > > I've noticed after queueing the first command, subsequent commands > appear to be executed and complete in reverse order. The SCSI Generic > HOWTO says "By default, read() will return the oldest completed > request that is queued up." > > This could also be a performance defect if it's what's really > happening since it isn't desirable behavior if, for example, ios are > typically ordered by lba and issued one at a time by kernel to a non > queueing block device. > > This reverse order behavior is trivial to reproduce; just queue 16 > concurrent INQUIRY commands. The following are typical results I get > from initially queueing 16 READ_10 commands. > > Completion Command # hdr.driver_duration (us) > Order > 1 22 14979 > 2 20 14981 > 3 19 14982 > 4 17 14984 > 5 16 14985 > 6 15 14986 > 7 14 14988 > 8 12 14990 > 9 11 14991 > 10 10 14992 > I have been told that is a feature :-) The SCSI mid level processes commands from pass-throughs (e.g. sg and bsg) in LIFO order. For certain types of error processing it makes sense. For READs and WRITEs it makes no sense. Doug Gilbert -- Configure bugmail: http://bugzilla.kernel.org/userprefs.cgi?tab=email ------- You are receiving this mail because: ------- You are the assignee for the bug. -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
