Re: [PATCH] mm: swap: async free swap slot cache entries

[David Rientjes <rientjes@google.com>]

On Sat, 23 Dec 2023, Chris Li wrote:

> > How do you quantify the impact of the delayed swap_entry_free()?
> >
> > Since the free and memcg uncharge are now delayed, is there not the
> > possibility that we stay under memory pressure for longer?  (Assuming at
> > least some users are swapping because of memory pressure.)
> >
> > I would assume that since the free and uncharge itself is delayed that in
> > the pathological case we'd actually be swapping *more* until the async
> > worker can run.
> 
> Thanks for raising this interesting question.
> 
> First of all, the swap_entry_free() does not impact "memory.current".
> It reduces "memory.swap.current". Technically it is the swap pressure
> not memory pressure that suffers the extra delay.
> 
> Secondly, we are talking about delaying up to 64 swap entries for a
> few microseconds.

What guarantees that the async freeing happens within a few microseconds?

> Where the swap slot cache itself delays the freeing
> of the entries for an arbitrary amount of time. It is not freed until
> the cache is full of 64 entries. This delay can be seconds or even
> minutes. Adding a few microseconds of  extra delay to existing seconds
> delay really makes no difference from the swap pressure point of view.
> 
> Chris
>