Re: Some performance measurements on the FreeBSD network stack

On Fri, 20 Apr 2012, K. Macy wrote:

> On Fri, Apr 20, 2012 at 4:44 PM, Luigi Rizzo <rizzo@iet.unipi.it> wrote:

>> The small penalty when flowtable is disabled but compiled in is
>> probably because the net.flowtable.enable flag is checked
>> a bit deep in the code.
>>
>> The advantage with non-connect()ed sockets is huge. I don't
>> quite understand why disabling the flowtable still helps there.
>
> Do you mean having it compiled in but disabled still helps
> performance? Yes, that is extremely strange.

This reminds me that when I worked on this, I saw very large throughput
differences (in the 20-50% range) as a result of minor changes in
unrelated code.  I could get these changes intentionally by adding or
removing padding in unrelated unused text space, so the differences were
apparently related to text alignment.  I thought I had some significant
micro-optimizations, but it turned out that they were acting mainly by
changing the layout in related used text space where it is harder to
control.  Later, I suspected that the differences were more due to cache
misses for data than for text.  The CPU and its caching must affect this
significantly.  I tested on an AthlonXP and Athlon64, and the differences
were larger on the AthlonXP.  Both of these have a shared I/D cache so
pressure on the I part would affect the D part, but in this benchmark
the D part is much more active than the I part so it is unclear how
text layout could have such a large effect.

Anyway, the large differences made it impossible to trust the results
of benchmarking any single micro-benchmark.  Also, ministat is useless
for understanding the results.  (I note that luigi didn't provide any
standard deviations and neither would I. :-).  My results depended on
the cache behaviour but didn't change significantly when rerun, unless
the code was changed.

Bruce
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