Re: [Qemu-devel] [PATCH] AIO: Reduce number of threads for 32bit hosts

[Kevin Wolf <kwolf@redhat.com>]

Am 14.01.2015 um 12:18 hat Paolo Bonzini geschrieben:
> 
> 
> On 14/01/2015 11:20, Kevin Wolf wrote:
> >> > The same problem applies to coroutine stacks, and those cannot be
> >> > throttled down as easily.  But I guess if you limit the number of
> >> > threads, the guest gets slowed down and doesn't create as many coroutines.
> > Shouldn't we rather try and decrease the stack sizes a bit? 1 MB per
> > coroutine is really a lot, and as I understand it, threads take even
> > more by default.
> 
> Yup, 2 MB.  Last time I proposed this, I think Markus was strongly in 
> the "better safe than sorry" camp. :)
> 
> But thread pool workers definitely don't need a big stack.

Right, I think we need to consider what kind of thread it is. For the
moment, I'm talking about the block layer only.

> >> > It would be nice to have a way to measure coroutine stack usage, similar
> >> > to what the kernel does.
> > The information which pages are mapped should be there somewhere...
> 
> Yes, there is mincore(2).  The complicated part is doing it fast, but
> perhaps it doesn't need to be fast.

Well, what do you want to use it for? I thought it would only be for a
one-time check where we usually end up rather than something that would
be enabled in production, but maybe I misunderstood.

> I tried gathering warning from GCC's -Wstack-usage=1023 option and the
> block layer does not seem to have functions with huge stacks in the I/O
> path.
> 
> So, assuming a maximum stack depth of 50 (already pretty generous since
> there shouldn't be any recursive calls) a 100K stack should be pretty
> much okay for coroutines and thread-pool threads.

The potential problem in the block layer is long backing file chains.
Perhaps we need to do something to solve that iteratively instead of
recursively.

> That said there are some offenders in the device models.  Other
> QemuThreads, especially VCPU threads, had better stay with a big stack.

Yes, that's not exactly surprising.

Kevin