Re: [PATCH] nptl: Disable THP on thread stack if it incurs in large RSS usage

[Wilco Dijkstra <Wilco.Dijkstra@arm.com>]

Hi Adhemerval,

> Right, I was using the wrong assumption then on how THP acts (mostly due the
> two threads discussion).  Running more tests I see your point and it seems
> what is happening in fact.  I will drop this patch since it really does not
> make much sense.
>
> So, do you think adding a pthread extension to control it would be a improvement
> or maybe work towards a heuristic to check if THP would be indeed valuable would
> be better?

Adding something to pthread to make it more configurable seems reasonable, but
like with tunables, I bet most applications won't ever use this. So adding heuristics
that work well for most applications would be best.

It seems Linux doesn't support aligning large mmaps, and as a result the code in
alloc_new_heap looks very messy and complex. We'd need something similar to
ensure deterministic alignment of the top of the stack for the cases where we
decide to use huge pages. We could reserve a number of default pages for the
initial stack so you avoid increasing RSS size if you use a small amount of stack.

However even switching off THP for all small stacks (eg. default size or smaller)
would avoid the random use of THP without needing to set the tunable.

Cheers,
Wilco