Re: [PATCH mm v3 3/3] mm: automatically penalize tasks with high swap use

[Jakub Kicinski <kuba@kernel.org>]

On Sun, 17 May 2020 06:44:52 -0700 Shakeel Butt wrote:
> > Use one counter for number of pages allocated under pressure
> > to save struct task space and avoid two separate hierarchy
> > walks on the hot path.
> 
> The above para seems out of place. It took some time to realize you
> are talking about current->memcg_nr_pages_over_high. IMO instead of
> this para, a comment in code would be much better.

Where would you like to see the comment? In struct task or where
counter is bumped?

> > Take the new high limit into account when determining if swap
> > is "full". Borrowing the explanation from Johannes:
> >
> >   The idea behind "swap full" is that as long as the workload has plenty
> >   of swap space available and it's not changing its memory contents, it
> >   makes sense to generously hold on to copies of data in the swap
> >   device, even after the swapin. A later reclaim cycle can drop the page
> >   without any IO. Trading disk space for IO.
> >
> >   But the only two ways to reclaim a swap slot is when they're faulted
> >   in and the references go away, or by scanning the virtual address space
> >   like swapoff does - which is very expensive (one could argue it's too
> >   expensive even for swapoff, it's often more practical to just reboot).
> >
> >   So at some point in the fill level, we have to start freeing up swap
> >   slots on fault/swapin.  
> 
> swap.high allows the user to force the kernel to start freeing swap
> slots before half-full heuristic, right?

I'd say that the definition of full is extended to include swap.high.