Re: Serial Console and Slow SCSI Disk Access?

[David Lang <dlang@digitalinsight.com>]

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.
>
> Linux can share interrupts without big performance problems - IF both of
> the hardware drivers are written with interrupt sharing in mind.  Many
> linux drivers are, but some are not.  So check if interrupt sharing is 
> happening,
> and if so, contact the maintainers of your scsi driver and your serial
> port driver.  Ask them if such sharing is ok or not.  If you don't
> understand the /proc/interrupt listing, just send it to the
> maintainers so they can have a look.
>
> Shared interrupts are to some extent set up by the bios, and to some
> extent by linux.  So different kernel versions (or booting with a different
> set of drivers loaded, or a different distribution) may make a difference.
>
> You can often get a pci device to use a different interrupt by moving it
> to another slot.  That tends to solve interrupt sharing problems. I assume
> your scsi adapter is pci.

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.

David Lang

# cat /proc/interrupts
           CPU0       CPU1
  0:  458865186      29148    IO-APIC-edge  timer
  1:        216          7    IO-APIC-edge  i8042
  3:         78          1    IO-APIC-edge  serial
  4:         76          1    IO-APIC-edge  serial
  8:          8          1    IO-APIC-edge  rtc
  9:          0          0   IO-APIC-level  acpi
 12:         84         10    IO-APIC-edge  i8042
 14:        332         10    IO-APIC-edge  ide0
 24:    4426414          1   IO-APIC-level  eth2
 26:  426930991          1   IO-APIC-level  eth0
 27:      17795  365692411   IO-APIC-level  eth1
 29:     553120         87   IO-APIC-level  ioc0
NMI:          0          0
LOC:  458903065  458904855
ERR:          0
MIS:          0

# lspci
0000:00:06.0 PCI bridge: Advanced Micro Devices [AMD] AMD-8111 PCI (rev 07)
0000:00:07.0 ISA bridge: Advanced Micro Devices [AMD] AMD-8111 LPC (rev 05)
0000:00:07.1 IDE interface: Advanced Micro Devices [AMD] AMD-8111 IDE (rev 03)
0000:00:07.2 SMBus: Advanced Micro Devices [AMD] AMD-8111 SMBus 2.0 (rev 02)
0000:00:07.3 Bridge: Advanced Micro Devices [AMD] AMD-8111 ACPI (rev 05)
0000:00:0a.0 PCI bridge: Advanced Micro Devices [AMD] AMD-8131 PCI-X Bridge (rev 
12)
0000:00:0a.1 PIC: Advanced Micro Devices [AMD] AMD-8131 PCI-X APIC (rev 01)
0000:00:0b.0 PCI bridge: Advanced Micro Devices [AMD] AMD-8131 PCI-X Bridge (rev 
12)
0000:00:0b.1 PIC: Advanced Micro Devices [AMD] AMD-8131 PCI-X APIC (rev 01)
0000:00:18.0 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:18.1 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:18.2 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:18.3 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:19.0 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:19.1 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:19.2 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:00:19.3 Host bridge: Advanced Micro Devices [AMD] K8 NorthBridge
0000:01:00.0 USB Controller: Advanced Micro Devices [AMD] AMD-8111 USB (rev 0b)
0000:01:00.1 USB Controller: Advanced Micro Devices [AMD] AMD-8111 USB (rev 0b)
0000:01:06.0 VGA compatible controller: ATI Technologies Inc Rage XL (rev 27)
0000:02:02.0 PCI bridge: IBM PCI-X to PCI-X Bridge (rev 02)
0000:02:03.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5702X 
Gigabit Ethernet (rev 02)
0000:02:04.0 Ethernet controller: Broadcom Corporation NetXtreme BCM5702X 
Gigabit Ethernet (rev 02)
0000:03:04.0 Ethernet controller: Intel Corp. 82546EB Gigabit Ethernet 
Controller (rev 01)
0000:03:04.1 Ethernet controller: Intel Corp. 82546EB Gigabit Ethernet 
Controller (rev 01)
0000:03:06.0 Ethernet controller: Intel Corp. 82546EB Gigabit Ethernet 
Controller (rev 01)
0000:03:06.1 Ethernet controller: Intel Corp. 82546EB Gigabit Ethernet 
Controller (rev 01)
0000:04:01.0 SCSI storage controller: LSI Logic / Symbios Logic 53c1030 PCI-X 
Fusion-MPT Dual Ultra320 SCSI (rev 07)



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