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SubjectRe: init's children list is long and slows reaping children.
Jeff Garzik wrote:
> Linus Torvalds wrote:
>> On Fri, 6 Apr 2007, Jeff Garzik wrote:
>>> I would rather change the implementation under the hood to start per-CPU
>>> threads on demand, similar to a thread-pool implementation.
>>> Boxes with $BigNum CPUs probably won't ever use half of those threads.
>> The counter-argument is that boxes with $BigNum CPU's really don't
>> hurt from it either, and having per-process data structures is often
>> simpler and more efficient than trying to have some thread pool.
> Two points here:
> * A lot of the users in the current kernel tree don't rely on the
> per-CPU qualities. They just need multiple threads running.
> * Even with per-CPU data structures and code, you don't necessarily have
> to keep a thread alive and running for each CPU. Reap the ones that
> haven't been used in $TimeFrame, and add thread creation to the slow
> path that already exists in the bowels of schedule_work().
> Or if some kernel hacker is really motivated, all workqueue users in the
> kernel would benefit from a "thread audit", looking at working
> conditions to decide if the new kthread APIs are more appropriate.

spawn on demand would require heuristics and complexity though. And
I think there is barely any positive tradeoff to weigh it against.

>> IOW, once we get the processes off the global list, there just isn't
>> any downside from them. Sure, they use some memory, but people who buy
>> 1024-cpu machines won't care about a few kB per CPU..
>> So the *only* downside is literally the process list, and one
>> suggested patch already just removes kernel threads entirely from the
>> parenthood lists.
>> The other potential downside could be "ps is slow", but on the other
>> hand, having the things stick around and have things like CPU-time
>> accumulate is probably worth it - if there are some issues, they'd
>> show up properly accounted for in a way that process pools would have
>> a hard time doing.
> Regardless of how things are shuffled about internally, there will
> always be annoying overhead /somewhere/ when you have a metric ton of
> kernel threads. I think that people should also be working on ways to
> make the kernel threads a bit more manageable for the average human.

There are a few per CPU, but they should need no human management to
speak of.

Presumably if you have a 1024 CPU system, you'd generally want to be
running at least 1024 of your own processes there, so you already need
some tools to handle that magnitude of processes anyway.

>> So I really don't think this is worth changing things over, apart from
>> literally removing them from process lists, which I think everybody
>> agrees we should just do - it just never even came up before!
> I think there is a human downside. For an admin you have to wade
> through a ton of processes on your machine, if you are attempting to
> evaluate the overall state of the machine. Just google around for all
> the admins complaining about the explosion of kernel threads on
> production machines :)

User tools should be improved. It shouldn't be too hard to be able to
aggregate kernel thread stats into a single top entry, for example.

I'm not saying the number of threads couldn't be cut down, but there
is still be an order of magnitude problem there...

SUSE Labs, Novell Inc.
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