Re: Serial Console and Slow SCSI Disk Access?

Andrey Melnikoff
2006-06-22T16:31:57Z

David Lang <dlang@digitalinsight.com> wrote:
> On Tue, 20 Jun 2006, Helge Hafting wrote:

> > Jeff Gold wrote:
> >> Helge Hafting wrote:
> >>> With nothing attached, any write to the serial device might go
> >>> through a lengthy timeout because of flow control. [...] But I can't
> >>> see why it'd make scsi disks slower. The scsi host adapter initialization
> >>> writes some messages of course, but there should be no
> >>> more console accesses during a hdparm test run.
> >>
> >> This makes sense to me. When I attach a serial cable and use that to login
> >> (I've got agetty running), hdparm produces no console messages that I can
> >> see using minicom. Still, the disk throughput is around 1.5 MB/sec for
> >> some reason. When I disable the serial console in grub.conf and reboot I
> >> get over 70 MB/sec again.
> >>
> >> A combination of out-of-tree patches (mainly network related but also one
> >> to disable PM_TIMERS) seem to eliminate the issue even with the serial
> >> console enabled, at least for the moment. That means I no longer have a
> >> problem, but the whole thing is mysterious to me.
> > I can see one possibility, that I didn't think of yesterday.
> > Do the scsi host adapter share its interrupt with the serial line?
> > (Boot a kernel that has the problem, and when scsi is slow, do a
> > cat /proc/interrupts
> > If the scsi and the serial driver share an interrupt, then that is the source
> > of the problem.

[skipp]

> I ran into a similar problem (extremely slow scsi performance), but hadn't
> reported it as I made a lot of changes at the same time (moved from 2.6.7 to
> 2.6.12.3 as well as enabling the serial console). but with this report I wanted
> to chime in with a weak 'me-too'. I haven't rebooted without the serial console
> yet to confirm this, but I will do so shortly, and try 2.6.17 to see if it has a
> similar problem or not.

2.6.17.1 here, MB Intel SCB20, aic7899 onboard. aic7xxx compilled in, serial
core compilled in, serial console enabled.
With/without serial sonsole - speed 66.0 Mb/s.

Maybe problem related to MB chipset?