Re: Serial Console and Slow SCSI Disk Access?

From: David Lang
Date: Wed Jun 21 2006 - 20:32:00 EST

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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