hi
i have a SMP with 2x duo-core Opteron processors, and i have an ethernet card
(eth0). i want to balance the interrupts generated by the card to all 4
processors. i can do this by writing to smp_affinity, but on another machine it
get's done by "default", without doing anything.
is this irq balancing a feature in the chipset, or driver, or kernel ? i got
the idea of the smp_affinity stuff, and i've set it to "f", but nothing
happens, i mean i see only one CPU getting the interrupts.
i'm using kernel 2.6.18, but tried with 2.6.19, too.
thanks.
Imre Gergely wrote:
> hi
>
> i have a SMP with 2x duo-core Opteron processors, and i have an ethernet card
> (eth0). i want to balance the interrupts generated by the card to all 4
> processors. i can do this by writing to smp_affinity, but on another machine it
> get's done by "default", without doing anything.
>
> is this irq balancing a feature in the chipset, or driver, or kernel ? i got
> the idea of the smp_affinity stuff, and i've set it to "f", but nothing
> happens, i mean i see only one CPU getting the interrupts.
>
> i'm using kernel 2.6.18, but tried with 2.6.19, too.
You need to run the irqbalance daemon to do this, unless the interrupts
are distributed in hardware (which only some machines do).
--
Robert Hancock Saskatoon, SK, Canada
To email, remove "nospam" from [email protected]
Home Page: http://www.roberthancock.com/
On Fri, 2007-02-23 at 13:32 +0200, Imre Gergely wrote:
> hi
>
> i have a SMP with 2x duo-core Opteron processors, and i have an ethernet card
> (eth0). i want to balance the interrupts generated by the card to all 4
> processors.
you have only one card? then balancing like you say will only really
hurt you; neither linux nor the chipset will do this.
> is this irq balancing a feature in the chipset, or driver, or kernel ? i got
> the idea of the smp_affinity stuff, and i've set it to "f", but nothing
> happens, i mean i see only one CPU getting the interrupts.
if you have multiple real interrupt sources you can balance between them
using irqbalance, see http://www.irqbalance.org
But note that this program will NOT move one irq across 4 cpus all the
time, since that gives *really* bad performance.
--
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org
Arjan van de Ven wrote:
> On Fri, 2007-02-23 at 13:32 +0200, Imre Gergely wrote:
>> hi
>>
>> i have a SMP with 2x duo-core Opteron processors, and i have an ethernet card
>> (eth0). i want to balance the interrupts generated by the card to all 4
>> processors.
>
> you have only one card? then balancing like you say will only really
> hurt you; neither linux nor the chipset will do this.
in fact i have two cards, and 4 CPUs, but i was interested in then answer
Robert gave, that only _some_ machines distribute interrupts in hardware.
software distribution is obviously not good. consider this scenario:
you have one machine with 4 cpus, and two ethernet cards with a lot of
traffic on them. if you bind every card to one cpu, two of them are not used,
so you really use only half the power. not let's say you have so much traffic
(with limiting enabled, htb or something), that the two CPUs are on 100% all
the time, but the other two are doing nothing.
now if you could balance that to all 4 cpus, you could use all the power AND
no cpu would be used 100%.
but i guess the machine has to do it in hardware, and that was the real
question. irqbalance has no use here, i think, only if the hardware supports it.
is there a way to find out (some easy way) for sure, if one particular
hardware supports this interrupt distribution?
> in fact i have two cards, and 4 CPUs, but i was interested in then answer
> Robert gave, that only _some_ machines distribute interrupts in hardware.
> software distribution is obviously not good. consider this scenario:
>
> you have one machine with 4 cpus, and two ethernet cards with a lot of
> traffic on them. if you bind every card to one cpu, two of them are not used,
> so you really use only half the power. not let's say you have so much traffic
> (with limiting enabled, htb or something), that the two CPUs are on 100% all
> the time, but the other two are doing nothing.
>
> now if you could balance that to all 4 cpus, you could use all the power AND
> no cpu would be used 100%.
actually this will give you worse performance than only using 2 cores.
The reason for this is twofold
1) If you rotate the irqs, TCP and IP packet fragments will arrive at
different CPUs. This in turn means that a VERY expensive reassembly path
gets taken, compared to local-cpu-only reassembly
2) If you rotate the irqs, you bounce cachelines between the caches ALL
THE TIME, which is also very expensive.
Both make it more likely that you'll be slower than just using only 2
cores...
--
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org
Arjan van de Ven wrote:
>> in fact i have two cards, and 4 CPUs, but i was interested in then answer
>> Robert gave, that only _some_ machines distribute interrupts in hardware.
>> software distribution is obviously not good. consider this scenario:
>>
>> you have one machine with 4 cpus, and two ethernet cards with a lot of
>> traffic on them. if you bind every card to one cpu, two of them are not used,
>> so you really use only half the power. not let's say you have so much traffic
>> (with limiting enabled, htb or something), that the two CPUs are on 100% all
>> the time, but the other two are doing nothing.
>>
>> now if you could balance that to all 4 cpus, you could use all the power AND
>> no cpu would be used 100%.
>
> actually this will give you worse performance than only using 2 cores.
> The reason for this is twofold
> 1) If you rotate the irqs, TCP and IP packet fragments will arrive at
> different CPUs. This in turn means that a VERY expensive reassembly path
> gets taken, compared to local-cpu-only reassembly
> 2) If you rotate the irqs, you bounce cachelines between the caches ALL
> THE TIME, which is also very expensive.
>
> Both make it more likely that you'll be slower than just using only 2
> cores...
and i guess it doesn't matter if the distribution is being done by the
hardware, from the point of view of the kernel, i would still get the
performance penalty.
and what if CPU0 and CPU1 is actually the same CPU, only duo core, and i'm
distributing interrupts to them, and with the other card to CPU2 and 3 (which
are part of the other physical CPU) ?
i'm just trying to figure it out, i have no real knowledge of the inner
kernel workings, so i dont know. but i really would like to use all 4 cores.
just how expensive is that reassembly path ?
>
> and i guess it doesn't matter if the distribution is being done by the
> hardware, from the point of view of the kernel, i would still get the
> performance penalty.
for the cache bouncing you save assuming you have an Intel cpu (eg one
where the cache is shared). You don't save on the cross-cpu reassembly,
that is an entire different algorithm path you hit there..
> i'm just trying to figure it out, i have no real knowledge of the inner
> kernel workings, so i dont know. but i really would like to use all 4 cores.
> just how expensive is that reassembly path ?
depends on your traffic to be honest, probably a question more suited
for net-dev list.
--
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
Test the interaction between Linux and your BIOS via http://www.linuxfirmwarekit.org
Arjan van de Ven wrote:
>> and i guess it doesn't matter if the distribution is being done by the
>> hardware, from the point of view of the kernel, i would still get the
>> performance penalty.
>
> for the cache bouncing you save assuming you have an Intel cpu (eg one
> where the cache is shared). You don't save on the cross-cpu reassembly,
> that is an entire different algorithm path you hit there..
>
>
>
>> i'm just trying to figure it out, i have no real knowledge of the inner
>> kernel workings, so i dont know. but i really would like to use all 4 cores.
>> just how expensive is that reassembly path ?
>
> depends on your traffic to be honest, probably a question more suited
> for net-dev list.
>
i'll look into it, thanks a lot for the comments.