Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

Eric Dumazet
2008-11-17T17:33:10Z

Ingo Molnar a écrit :
> * Ingo Molnar <mingo-X9Un+BFzKDI@public.gmane.org> wrote:
>
>>> 4% on my machine, but apparently my machine is sooooo special (see
>>> oprofile thread), so maybe its cpus have a hard time playing with
>>> a contended cache line.
>>>
>>> It definitly needs more testing on other machines.
>>>
>>> Maybe you'll discover patch is bad on your machines, this is why
>>> it's in net-next-2.6
>> ok, i'll try it on my testbox too, to check whether it has any effect
>> - find below the port to -git.
>
> it gives a small speedup of ~1% on my box:
>
> before: Throughput 3437.65 MB/sec 64 procs
> after: Throughput 3473.99 MB/sec 64 procs

Strange, I get 2350 MB/sec on my 8 cpus box. "tbench 8"

>
> ... although that's still a bit close to the natural tbench noise
> range so it's not conclusive and not like a smoking gun IMO.
>
> But i think this change might just be papering over the real
> scalability problem that this workload has in my opinion: that there's
> a single localhost route/dst/device that millions of packets are
> squeezed through every second:

Yes, this point was mentioned on netdev a while back.

>
> phoenix:~> ifconfig lo
> lo Link encap:Local Loopback
> inet addr:127.0.0.1 Mask:255.0.0.0
> UP LOOPBACK RUNNING MTU:16436 Metric:1
> RX packets:258001524 errors:0 dropped:0 overruns:0 frame:0
> TX packets:258001524 errors:0 dropped:0 overruns:0 carrier:0
> collisions:0 txqueuelen:0
> RX bytes:679809512144 (633.1 GiB) TX bytes:679809512144 (633.1 GiB)
>
> There does not seem to be any per CPU ness in localhost networking -
> it has a globally single-threaded rx/tx queue AFAICS even if both the
> client and server task is on the same CPU - how is that supposed to
> perform well? (but i might be missing something)

Stephen had a patch for this one too, but we got tbench noise too with this patch

http://kerneltrap.org/mailarchive/linux-netdev/2008/11/5/3926034

>
> What kind of test-system do you have - one with P4 style Xeon CPUs
> perhaps where dirty-cacheline cachemisses to DRAM were particularly
> expensive?

Its a HP BL460c g1

Dual quad-core cpus Intel E5450 @3.00GHz

So 8 logical cpus. My bench was "tbench 8"