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

[Adhemerval Zanella Netto <adhemerval.zanella@linaro.org>]

On 17/05/23 15:16, Wilco Dijkstra wrote:
> Hi Adhemerval,
> 
>> But the threads themselves do not end up using all the VMA region allocated
>> for them.  Using the test program you can see it:
> 
> Obviously that is due to there being a 2MB gap between each stack. After the
> huge page there is a default page for the guard and then empty space for the
> alignment till the next stack.
> 
>> So even if the stack is not aligned to default large page, THP will still
>> back up the thread allocation.
> 
> THP can only be used if the mmap covers an aligned huge page. If you
> allocate lots of 2MB stacks then you get threads * guardsize / 2MB number
> of huge pages since some stacks will become aligned.
> 
>> The issues is, if the mmap is also aligned 
>> to THP size, the guard setup will trigger the issue that will increase RSS.
> 
> No it won't. If it is aligned it will get a huge page, and the guard will be before
> the huge page (since we allocate 2MB + guardsize).
> 
>> This seems to be same conclusion OpenJVM and some kernel discussion has
>> reached as well [1] [2].
> 
> If that is what they claim, they are wrong. This is not how THP or our stack
> allocation works. You can see the allocation of the THP and guard pages
> if you print the smaps (build with -DPRINT_PROC_SMAPS).

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?