Re: [PATCH mm v2 3/3] mm: automatically penalize tasks with high swap use

[Michal Hocko <mhocko@kernel.org>]

On Thu 14-05-20 16:21:30, Johannes Weiner wrote:
> On Thu, May 14, 2020 at 09:42:46AM +0200, Michal Hocko wrote:
> > On Wed 13-05-20 11:36:23, Jakub Kicinski wrote:
> > > On Wed, 13 May 2020 10:32:49 +0200 Michal Hocko wrote:
> > > > On Tue 12-05-20 10:55:36, Jakub Kicinski wrote:
> > > > > On Tue, 12 May 2020 09:26:34 +0200 Michal Hocko wrote:  
> > > > > > On Mon 11-05-20 15:55:16, Jakub Kicinski wrote:  
> > > > > > > Use swap.high when deciding if swap is full.    
> > > > > > 
> > > > > > Please be more specific why.  
> > > > > 
> > > > > How about:
> > > > > 
> > > > >     Use swap.high when deciding if swap is full to influence ongoing
> > > > >     swap reclaim in a best effort manner.  
> > > > 
> > > > This is still way too vague. The crux is why should we treat hard and
> > > > high swap limit the same for mem_cgroup_swap_full purpose. Please
> > > > note that I am not saying this is wrong. I am asking for a more
> > > > detailed explanation mostly because I would bet that somebody
> > > > stumbles over this sooner or later.
> > > 
> > > Stumbles in what way?
> > 
> > Reading the code and trying to understand why this particular decision
> > has been made. Because it might be surprising that the hard and high
> > limits are treated same here.
> 
> I don't quite understand the controversy.

I do not think there is any controversy. All I am asking for is a
clarification because this is non-intuitive.
 
> 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. Otherwise we could eventually run out of swap
> slots while they're filled with copies of data that is also in RAM.
> 
> We don't want to OOM a workload because its available swap space is
> filled with redundant cache.

Thanks this is a useful summary.
 
> That applies to physical swap limits, swap.max, and naturally also to
> swap.high which is a limit to implement userspace OOM for swap space
> exhaustion.
> 
> > > Isn't it expected for the kernel to take reasonable precautions to
> > > avoid hitting limits?
> > 
> > Isn't the throttling itself the precautious? How does the swap cache
> > and its control via mem_cgroup_swap_full interact here. See? This is
> > what I am asking to have explained in the changelog.
> 
> It sounds like we need better documentation of what vm_swap_full() and
> friends are there for. It should have been obvious why swap.high - a
> limit on available swap space - hooks into it.

Agreed. The primary source for a confusion is the naming here. Because
vm_swap_full doesn't really try to tell that the swap is full. It merely
tries to tell that it is getting full and so duplicated data should be
dropped.

-- 
Michal Hocko
SUSE Labs