Re: [patch 1/9] Conditional Calls - Architecture Independent Code

Adrian Bunk
2007-06-13T21:51:06Z

On Wed, Jun 13, 2007 at 11:57:24AM -0400, Mathieu Desnoyers wrote:
> Hi Adrian,

Hi Mathieu,

>...
> > 2. What is the real-life performance improvement?
> > That micro benchmarks comparing cache hits with cache misses give great
> > looking numbers is obvious.
> > But what will be the performance improvement in real workloads after the
> > functions you plan to make conditional according to question 1 have been
> > made conditional?
>
> Hrm, I am trying to get interesting numbers out of lmbench: I just ran a
> test on a kernel sprinkled with about 50 markers at important sites
> (LTTng markers: system call entry/exit, traps, interrupt handlers, ...).
> The markers are compiled-in, but in "disabled state". Since the markers
> re-use the cond_call infrastructure, each marker has its own cond_call.
>...
> The results are that we really cannot tell that one is faster/slower
> than the other; the standard deviation is much higher than the
> difference between the two situations.
>
> Note that lmbench is a workload that will not trigger much L1 cache
> stress, since it repeats the same tests many times. Do you have any
> suggestion of a test that would be more representative of a real
> diversified (in term of in-kernel locality of reference) workload ?

Please correct me if I'm wrong, but I think 50 markers couldn't ever
result in a visible change:

You need a change that is big enough that it has a measurable influence
on the cache hit ratio.

I don't think you could get any measurable influence unless you get into
areas where > 10% of all code are conditional. And that's a percentage
I wouldn't consider being realistically.

And one big disadvantage of your implementation is the dependency on
MODULES. If you build all driver statically into the kernel, switching
from CONFIG_MODULES=y to CONFIG_MODULES=n already gives you for free a
functionally equivalent kernel that is smaller by at about 8% (depending
on the .config).

My impression is that your patches would add an infrastructure for a
nice sounding idea that will never have any real life effect.

> Thanks,
>
> Mathieu

cu
Adrian

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