linux-kernel - Re: prioritize PCI traffic ?

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Message-ID: <45AB956A.5070103@linux.vnet.ibm.com>
Date:	Mon, 15 Jan 2007 20:23:30 +0530
From:	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>
To:	Soeren Sonnenburg <kernel@....de>
CC:	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: prioritize PCI traffic ?

Soeren Sonnenburg wrote:
> On Mon, 2007-01-15 at 19:23 +0530, Vaidyanathan Srinivasan wrote:
>> Soeren Sonnenburg wrote:
>>> Dear all,
>>>
>>> is it possible to explicitly tell the kernel to prioritize PCI traffic
>>> for a number of cards in pci slots x,y,z ?
>>>
>>> I am asking as severe ide traffic causes lost frames when watching TV
>>> using 2 DVB cards + vdr... This is simply due to the fact that the PCI
>>> bus is saturated...
>> How do you know that the bus is saturated?
> 
> I simply dd if=/dev/sd? of=/dev/null from four brand new sata-harddisks.
> 
>> Are you streaming data to/from the ide hard disks/CDROM?
> 
> yes.
>
>> Do you have DMAs 'ON' for the hard disks?
> 
> yes.

Good.

>> Is everything just fine if there are no IDE traffic?
> 
> yes.

Good.

>> Are you running 2.6 kernel with preempt 'ON'?
> 
> no: CONFIG_PREEMPT_NONE=y
> 

Try CONFIG_PREEMPT=y it may help. It made the system more responsive
for me.

>> Are all hardware on the same IRQ line? (shared interrupts)
> 
> no: libata devices are on IRQ 16 and DVB devices on IRQ 20

Watch the interrupt count (/proc/interrupts) and interrupt rate,
that may give some clue on what is happening on the PCI bus.

> 
>>> So, is any prioritizing of the PCI bus possible ?
>> The drivers + application indirectly can control priority on the
>> bus.  Just reduce the priority of the application that uses IDE and
>> see if adjusting nice values of applications can change the scenario.
> 
> That unfortunately did not help... no change...

33Mhz 32-bit PCI bus on typical PC can do around 100MB/sec... watch
your individual disk transfer rates and data rate to each DVB.
dd from 4 new SATA drive will probably fillup the bus.  However,
depending on your motherboard, the integrated SATA controllers may
be on a different PCI bus than the one for the slots.

You can watch the lspci output to determine which bus the devices
are located.

--Vaidy

> 
> Soeren
> --
> Sometimes, there's a moment as you're waking, when you become aware of
> the real world around you, but you're still dreaming.
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