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SubjectRE: [2.5] IRQ distribution in the 2.5.52 kernel
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Hi Andrew,
Your benchmark results are very impressive. Thanks for trying it out.
I have some thoughts after seeing the results.

> Nitin,
>
> 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%
[NK] It is surprising to see single e1000 is giving bandwidth more than
1Gbps. What can be the reason for this extra bandwidth? ... Maybe
compression is happening somewhere.

>
> 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%
[NK] It can be a case that throughput is getting limited by the network
infrastructure or total load of clients. If we know the theoretical
desired maximum throughput then we will get a better idea about the
bottleneck. It would be interesting to see the results, after adding one
more e1000 card to the server.

>
> OK, almost 5% better!
[NK] It's a pretty good number!

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.
[NK] I also believe so.

I hope to have access to a specweb system on a NUMA
> box
> soon, so we can verify that.
>
> -Andrew Theurer
[NK]
Thanks & regards,
Nitin
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