On 19.02.2014 22:04, Adrian Chadd wrote:
On 19 February 2014 11:59, Alexander Motin <m...@freebsd.org> wrote:

So if we're moving towards supporting (among others) a pcbgroup / RSS
hash style work load distribution across CPUs to minimise
per-connection lock contention, we really don't want the scheduler to
decide it can schedule things on other CPUs under enough pressure.
That'll just make things worse.

True, though it is also not obvious that putting second thread on CPU run
queue is better then executing it right now on another core.

Well, it depends if you're trying to optimise for "run all runnable
tasks as quickly as possible" or "run all runnable tasks in contexts
that minimise lock contention."

The former sounds great as long as there's no real lock contention
going on. But as you add more chances for contention (something like
"100,000 concurrent TCP flows") then you may end up having your TCP
timer firing stuff interfere with more TXing or RXing on the same

100K TCP flows probably means 100K locks. That means that chance of lock collision on each of them is effectively zero. More realistic it could be to speak about cache coherency traffic, etc, but I still think that number of expired timeouts should be much lower then number of other flow data accesses.

Chasing this stuff down is a pain, because it only really shows up
when you're doing lots of concurrency.

I'm happy to make this a boot-time option and leave it off for the
time being. How's that?

I generally hate tunables like that. There are too few people who may even try to make grounded decision in that question. If you think it right -- just do it, otherwise -- don't do it. I am not really objecting, more like sounding concerns.

Alexander Motin
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