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

[Eric Dumazet]

Ingo Molnar a écrit :
> * Ingo Molnar <mingo@xxxxxxx> 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"


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/