Re: [PATCH RFC v3 0/4] mTHP-friendly compression in zsmalloc and zram based on multi-pages

[Sergey Senozhatsky <senozhatsky@chromium.org>]

On (24/11/27 09:31), Barry Song wrote:
> On Tue, Nov 26, 2024 at 11:53 PM Sergey Senozhatsky
> <senozhatsky@chromium.org> wrote:
> >
> > On (24/11/26 14:09), Sergey Senozhatsky wrote:
> > > > swap-out time(ms)       68711              49908
> > > > swap-in time(ms)        30687              20685
> > > > compression ratio       20.49%             16.9%
> >
> > I'm also sort of curious if you'd use zstd with pre-trained user
> > dictionary [1] (e.g. based on a dump of your swap-file under most
> > common workloads) would it give you desired compression ratio
> > improvements (on current zram, that does single page compression).
> >
> > [1] https://github.com/facebook/zstd?tab=readme-ov-file#the-case-for-small-data-compression
>
> Not yet, but it might be worth trying. A key difference between servers and
> Android phones is that phones have millions of different applications
> downloaded from the Google Play Store or other sources.

Maybe yes maybe not, I don't know.  It could be that that 99% of users
use the same 1% apps out of those millions.

> In this case, would using a dictionary be a feasible approach? Apologies
> if my question seems too naive.

It's a good question, and there is probably only one way to answer
it - through experiments, it's data dependent, so it's case-by-case.

> On the other hand, the advantage of a pre-trained user dictionary
> doesn't outweigh the benefits of large block compression? Can’t both
> be used together?

Well, so far the approach has many unmeasured unknowns and corner
cases, I don't think I personally even understand all of them to begin
with.  Not sure if I have a way to measure and analyze, that mTHP
swapout seems like a relatively new thing and it also seems that you
are still fixing some of its issues/shortcomings.