Re: [RFC PATCH 6/9] mm: zswap: drop support for non-zero same-filled pages handling

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Mar 28, 2024 at 12:31 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Mon, Mar 25, 2024 at 11:50:14PM +0000, Yosry Ahmed wrote:
> > The current same-filled pages handling supports pages filled with any
> > repeated word-sized pattern. However, in practice, most of these should
> > be zero pages anyway. Other patterns should be nearly as common.
> >
> > Drop the support for non-zero same-filled pages, but keep the names of
> > knobs exposed to userspace as "same_filled", which isn't entirely
> > inaccurate.
> >
> > This yields some nice code simplification and enables a following patch
> > that eliminates the need to allocate struct zswap_entry for those pages
> > completely.
> >
> > There is also a very small performance improvement observed over 50 runs
> > of kernel build test (kernbench) comparing the mean build time on a
> > skylake machine when building the kernel in a cgroup v1 container with a
> > 3G limit:
> >
> >               base            patched         % diff
> > real          70.167          69.915          -0.359%
> > user          2953.068        2956.147        +0.104%
> > sys           2612.811        2594.718        -0.692%
> >
> > This probably comes from more optimized operations like memchr_inv() and
> > clear_highpage(). Note that the percentage of zero-filled pages during
> > this test was only around 1.5% on average, and was not affected by this
> > patch. Practical workloads could have a larger proportion of such pages
> > (e.g. Johannes observed around 10% [1]), so the performance improvement
> > should be larger.
> >
> > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@cmpxchg.org/
> >
> > Signed-off-by: Yosry Ahmed <yosryahmed@google.com>
>
> This is an interesting direction to pursue, but I actually thinkg it
> doesn't go far enough. Either way, I think it needs more data.
>
> 1) How frequent are non-zero-same-filled pages? Difficult to
>    generalize, but if you could gather some from your fleet, that
>    would be useful. If you can devise a portable strategy, I'd also be
>    more than happy to gather this on ours (although I think you have
>    more widespread zswap use, whereas we have more disk swap.)

I am trying to collect the data, but there are.. hurdles. It would
take some time, so I was hoping the data could be collected elsewhere
if possible.

The idea I had was to hook a BPF program to the entry of
zswap_fill_page() and create a histogram of the "value" argument. We
would get more coverage by hooking it to the return of
zswap_is_page_same_filled() and only updating the histogram if the
return value is true, as it includes pages in zswap that haven't been
swapped in.

However, with zswap_is_page_same_filled() the BPF program will run in
all zswap stores, whereas for zswap_fill_page() it will only run when
needed. Not sure if this makes a practical difference tbh.

>
> 2) The fact that we're doing any of this pattern analysis in zswap at
>    all strikes me as a bit misguided. Being efficient about repetitive
>    patterns is squarely in the domain of a compression algorithm. Do
>    we not trust e.g. zstd to handle this properly?

I thought about this briefly, but I didn't follow through. I could try
to collect some data by swapping out different patterns and observing
how different compression algorithms react. That would be interesting
for sure.

>
>    I'm guessing this goes back to inefficient packing from something
>    like zbud, which would waste half a page on one repeating byte.
>
>    But zsmalloc can do 32 byte objects. It's also a batching slab
>    allocator, where storing a series of small, same-sized objects is
>    quite fast.
>
>    Add to that the additional branches, the additional kmap, the extra
>    scanning of every single page for patterns - all in the fast path
>    of zswap, when we already know that the vast majority of incoming
>    pages will need to be properly compressed anyway.
>
>    Maybe it's time to get rid of the special handling entirely?

We would still be wasting some memory (~96 bytes between zswap_entry
and zsmalloc object), and wasting cycling allocating them. This could
be made up for by cycles saved by removing the handling. We will be
saving some branches for sure. I am not worried about kmap as I think
it's a noop in most cases.

I am interested to see how much we could save by removing scanning for
patterns. We may not save much if we abort after reading a few words
in most cases, but I guess we could also be scanning a considerable
amount before aborting. On the other hand, we would be reading the
page contents into cache anyway for compression, so maybe it doesn't
really matter?

I will try to collect some data about this. I will start by trying to
find out how the compression algorithms handle same-filled pages. If
they can compress it efficiently, then I will try to get more data on
the tradeoff from removing the handling.

Thanks for the insights.