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SubjectRe: [2.5] IRQ distribution in the 2.5.52 kernel
On Tuesday 07 January 2003 20:50, Kamble, Nitin A wrote:
> Hello All,
> We were looking at the performance impact of the IRQ routing from
> the 2.5.52 Linux kernel. This email includes some of our findings
> about the way the interrupts are getting moved in the 2.5.52 kernel.
> Also there is discussion and a patch for a new implementation. Let
> me know what you think at


I got a chance to run the NetBench benchmark with your patch on 2.5.54-mjb2
kernel. NetBench measures SMB/CIFS performance by using several SMB clients
(in this case 44 Windows 2000 systems), sending SMB requests to a Linux
server running Samba 2.2.3a+sendfile. Result is in throughput, Mbps.
Generally the network traffic on the server is 60% recv, 40% tx.

I believe we have very similar systems. Mine is a 4 x 1.6 GHz, 1 MB L3 P4
Xeon with 4 GB DDR memory (3.2 GB/sec I believe). The chipset is "Summit".
I also have more than one Intel e1000 adapters.

I decided to run a few configurations, first with just one adapter, with and
without HT support in the kernel (acpi=off), then add another adapter and
test again with/without HT.

Here are the results:

4P, no HT, 1 x e1000, no kirq: 1214 Mbps, 4% idle
4P, no HT, 1 x e1000, kirq: 1223 Mbps, 4% idle, +0.74%

I suppose we didn't see much of an improvement here because we never run into
the situation where more than one interrupt with a high rate is routed to a
single CPU on irq_balance.

4P, HT, 1 x e1000, no kirq: 1214 Mbps, 25% idle
4P, HT, 1 x e1000, kirq: 1220 Mbps, 30% idle, +0.49%

Again, not much of a difference just yet, but lots of idle time. We may have
reached the limit at which one logical CPU can process interrupts for an
e1000 adapter. There are other things I can probably do to help this, like
int delay, and NAPI, which I will get to eventually.

4P, HT, 2 x e1000, no kirq: 1269 Mbps, 23% idle
4P, HT, 2 x e1000, kirq: 1329 Mbps, 18% idle +4.7%

OK, almost 5% better! Probably has to do with a couple of things; the fact
that your code does not route two different interrupts to the same
core/different logical cpus (quite obvious by looking at /proc/interrupts),
and that more than one interrupt does not go to the same cpu if possible. I
suspect irq_balance did some of those [bad] things some of the time, and we
observed a bottleneck in int processing that was lower than with kirq.

I don't think all of the idle time is because of a int processing bottleneck.
I'm just not sure what it is yet :) Hopefully something will become obvious
to me...

Overall I like the way it works, and I believe it can be tweaked to work with
NUMA when necessary. I hope to have access to a specweb system on a NUMA box
soon, so we can verify that.

-Andrew Theurer

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