SCHED_ULE should not be the default

[O. Hartmann]

On 12/14/11 18:54, Tom Evans wrote:
> On Wed, Dec 14, 2011 at 11:06 AM, George Mitchell
> <george+freebsd at m5p.com> wrote:
>>
>> Dear Secret Masters of FreeBSD: Can we have a decision on whether to
>> change back to SCHED_4BSD while SCHED_ULE gets properly fixed?
>>
> 
> Please do not do this. This thread has shown that ULE performs poorly
> in very specific scenarios where the server is loaded with NCPU+1 CPU
> bound processes, and brought forward more complaints about
> interactivity in X (I've never noticed this, and use a FreeBSD desktop
> daily).

I would highly appreciate a decission against SCHED_ULE as the default
scheduler! SCHED_4BSD is considered a more mature entity and obviously
it seems that SCHED_ULE needs some refinements to achieve a better level
of quality.

> 
> On the other hand, we have very many benchmarks showing how poorly
> 4BSD scales on things like postgresql. We get much more load out of
> our 8.1 ULE DB and web servers than we do out of our 7.0 ones. It's
> easy to look at what you do and say "well, what suits my environment
> is clearly the best default", but I think there are probably more
> users typically running IO bound processes than CPU bound processes.

You compare SCHED_ULE on FBSD 8.1 with SCHED_4BSD on FBSD 7.0? Shouldn't
you compare SCHED_ULE and SCHED_4BSD on the very same platform?

Development of SCHED_ULE has been focused very much on DB like
PostgreSQL, no wonder the performance benefit. But this is also a very
specific scneario where SCHED_ULE shows a real benefit compared to
SCHED_4BSD.

> 
> I believe the correct thing to do is to put some extra documentation
> into the handbook about scheduler choice, noting the potential issues
> with loading NCPU+1 CPU bound processes. Perhaps making it easier to
> switch scheduler would also help?

Many people more experst in the issue than myself revealed some issues
in the code of both SCHED_ULE and even SCHED_4BSD. It would be a pitty
if all the discussions get flushed away like a "toilette-busisness" as
it has been done all the way in the past.


Well, I'd like to see a kind of "standardized" benchmark. Like on
openbenchmark.org or at phoronix.com. I know that Phoronix' way of
performing benchmarks is questionable and do not reveal much of the
issues, but it is better than nothing. I'm always surprised by the worse
performance of FreeBSD when it comes to threaded I/O. The differences
between Linux and FreeBSD of the same development maturity are
tremendous and scaring!

It is a long time since I saw a SPEC benchmark on a FreeBSD driven HPC
box. Most benchmark around for testing hardware are performed with Linux
and Linux seems to make the race in nearly every scenario. It would be
highly appreciable and interesting to see how Linux and FreeBSD would
perform in SPEC on the same hardware platform. This is only an idea.
Without a suitable benchmark with a codebase understood the discussion
is in many aspects pointless -both ways.


> 
> Cheers
> 
> Tom
> 
> References:
> 
> http://people.freebsd.org/~kris/scaling/mysql-freebsd.png
> http://suckit.blog.hu/2009/10/05/freebsd_8_is_it_worth_to_upgrade
> _______________________________________________

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