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SubjectRe: [RFC PATCH 5/9] mm: zswap: remove zswap_same_filled_pages_enabled
On Fri, Mar 29, 2024 at 03:02:10PM +0100, Maciej S. Szmigiero wrote:
> On 29.03.2024 03:14, Yosry Ahmed wrote:
> > On Thu, Mar 28, 2024 at 1:06 PM Yosry Ahmed <yosryahmed@google.com> wrote:
> >>
> >> On Thu, Mar 28, 2024 at 12:11 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >>>
> >>> On Mon, Mar 25, 2024 at 11:50:13PM +0000, Yosry Ahmed wrote:
> >>>> There is no logical reason to refuse storing same-filled pages more
> >>>> efficiently and opt for compression. Remove the userspace knob.
> >>>>
> >>>> Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
> >>>
> >>> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
> >>>
> >>> I also think the non_same_filled_pages_enabled option should go
> >>> away. Both of these tunables are pretty bizarre.
> >>
> >> Happy to remove both in the next version :)
> >
> > I thought non_same_filled_pages_enabled was introduced with the
> > initial support for same-filled pages, but it was introduced
> > separately (and much more recently):
> > https://lore.kernel.org/all/7dbafa963e8bab43608189abbe2067f4b9287831.1641247624.git.maciej.szmigiero@oracle.com/
> >
> > I am CCing Maciej to hear more about the use case for this.
>
> Thanks for CCing me.
>
> I introduced "non_same_filled_pages_enabled" a few years ago to
> enable using zswap in a lightweight mode where it is only used for
> its ability to store same-filled pages effectively.

But all the pages it rejects go to disk swap instead, which is much
slower than compression...

> As far as I remember, there were some interactions between full
> zswap and the cgroup memory controller - like, it made it easier
> for an aggressive workload to exceed its cgroup memory.high limits.

Ok, that makes sense! A container fairness measure, rather than a
performance optimization.

Fair enough, but that's moot then with cgroup accounting of the
backing memory, f4840ccfca25 ("zswap: memcg accounting").

Thanks for prodiving context.

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