* Kristian Benoit <[email protected]> wrote:
> The numbers for PREEMPT_RT, however, have dramatically improved. All
> the 50%+ overhead we saw earlier has now gone away completely. The
> improvement is in fact nothing short of amazing. We were actually so
> surprised that we went around looking for any mistakes we may have
> done in our testing. We haven't found any though. So unless someone
> comes out with another set of numbers showing differently, we think
> that a warm round of applause should go to the PREEMPT_RT folks. If
> nothing else, it gives us satisfaction to know that these test rounds
> have helped make things better.
yeah, they definitely have helped, and thanks for this round of testing
too! I'll explain the recent changes to PREEMPT_RT that resulted in
these speedups in another mail.
Looking at your numbers i realized that the area where PREEMPT_RT is
still somewhat behind (the flood ping +~10% overhead), you might be
using an invalid test methodology:
> ping = on host: "sudo ping -f $TARGET_IP_ADDR"
i've done a couple of ping -f flood tests between various testboxes
myself, and one thing i found was that it's close to impossible to
create a stable, comparable packets per second workload! The pps rate
heavily fluctuated even within the same testrun. Another phenomenon i
noticed is that the PREEMPT_RT kernel has a tendency to handle _more_
ping packets per second, while the vanilla (and thus i suspect the
i-pipe) kernel throws away more packets.
Thus lmbench under PREEMPT_RT may perform 'slower', but in fact it was
just an unbalanced and thus unfair test. Once i created a stable packet
rate, PREEMPT_RT's IRQ overhead became acceptable.
(if your goal was to check how heavily external interrupts can influence
a PREEMPT_RT box, you should chrt the network IRQ thread to SCHED_OTHER
and renice it and softirq-net-rx and softirq-net-tx to nice +19.)
this phenomenon could be a speciality of my network setup, but still,
could you please verify the comparability of the ping -f workloads on
the vanilla and the PREEMPT_RT kernels? In particular, the interrupt
rate should be constant and comparable - but it might be better to look
at both the received and transmitted packets per second. (Since things
like iptraf are quite expensive when flood pinging is going on, the best
way i found to measure the packet rate was to process netstat -s output
via a simple script.)
Ingo
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
| |
 |