Re: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources if the compressor supports batching.

[Barry Song <21cnbao@gmail.com>]

On Wed, Aug 27, 2025 at 12:06 PM Sridhar, Kanchana P
<kanchana.p.sridhar@intel.com> wrote:
>
>
> > -----Original Message-----
> > From: Barry Song <21cnbao@gmail.com>
> > Sent: Monday, August 25, 2025 10:17 PM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> > Cc: linux-kernel@vger.kernel.org; linux-mm@kvack.org;
> > hannes@cmpxchg.org; yosry.ahmed@linux.dev; nphamcs@gmail.com;
> > chengming.zhou@linux.dev; usamaarif642@gmail.com;
> > ryan.roberts@arm.com; ying.huang@linux.alibaba.com; akpm@linux-
> > foundation.org; senozhatsky@chromium.org; linux-crypto@vger.kernel.org;
> > herbert@gondor.apana.org.au; davem@davemloft.net;
> > clabbe@baylibre.com; ardb@kernel.org; ebiggers@google.com;
> > surenb@google.com; Accardi, Kristen C <kristen.c.accardi@intel.com>;
> > Gomes, Vinicius <vinicius.gomes@intel.com>; Feghali, Wajdi K
> > <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
> > Subject: Re: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources
> > if the compressor supports batching.
> >
> > > > > > [...]
> > > > > > >
> > > > > > > +       /*
> > > > > > > +        * Set the unit of compress batching for large folios, for quick
> > > > > > > +        * retrieval in the zswap_compress() fast path:
> > > > > > > +        * If the compressor is sequential (@pool->compr_batch_size is
> > 1),
> > > > > > > +        * large folios will be compressed in batches of
> > > > > > ZSWAP_MAX_BATCH_SIZE
> > > > > > > +        * pages, where each page in the batch is compressed
> > sequentially.
> > > > > > > +        * We see better performance by processing the folio in batches
> > of
> > > > > > > +        * ZSWAP_MAX_BATCH_SIZE, due to cache locality of working
> > set
> > > > > > > +        * structures.
> > > > > > > +        */
> > > > > > > +       pool->batch_size = (pool->compr_batch_size > 1) ?
> > > > > > > +                               pool->compr_batch_size :
> > ZSWAP_MAX_BATCH_SIZE;
> > > > > > > +
> > > > > > >         zswap_pool_debug("created", pool);
> > > > > > >
> > > > > > >         return pool;
> > > > > > >
> > > > > >
> > > > > > It’s hard to follow — you add batch_size and compr_batch_size in this
> > > > > > patch, but only use them in another. Could we merge the related
> > changes
> > > > > > into one patch instead of splitting them into several that don’t work
> > > > > > independently?
> > > > >
> > > > > Hi Barry,
> > > > >
> > > > > Thanks for reviewing the code and for your comments! Sure, I can merge
> > > > > this patch with the next one. I was trying to keep the changes
> > modularized
> > > > > to a) zswap_cpu_comp_prepare(), b) zswap_store() and c)
> > > > zswap_compress()
> > > > > so the changes are broken into smaller parts, but I can see how this can
> > > > > make the changes appear disjointed.
> > > > >
> > > > > One thing though: the commit logs for each of the patches will
> > > > > also probably need to be merged, since I have tried to explain the
> > > > > changes in detail.
> > > >
> > > > It’s fine to merge the changelog and present the story as a whole. Do we
> > >
> > > Sure.
> > >
> > > > really need both pool->batch_size and pool->compr_batch_size? I assume
> > > > pool->batch_size = pool->compr_batch_size if HW supports batch;
> > otherwise
> > > > pool->compr_batch_size = 1.
> > >
> > > Actually not exactly. We have found value in compressing in batches of
> > > ZSWAP_MAX_BATCH_SIZE even for software compressors. Latency benefits
> > > from cache locality of working-set data. Hence the approach that we have
> > > settled on is pool->batch_size = ZSWAP_MAX_BATCH_SIZE if
> > > the compressor does not support batching (i.e., if pool->compr_batch_size is
> > 1).
> > > If it does, then pool->batch_size = pool->compr_batch_size.
> >
> > I understand that even without a hardware batch, you can still
> > have some software batching that excludes compression.
> >
> > However, based on the code below, it looks like
> > pool->compr_batch_size is almost always either equal to
> > pool->batch_size or 1:
> >
> > +       pool->compr_batch_size = min(ZSWAP_MAX_BATCH_SIZE,
> > +                                    crypto_acomp_batch_size(acomp_ctx->acomp));
>
> I would like to explain some of the considerations in coming up with this
> approach:
>
> 1) The compression algorithm gets to decide an optimal batch-size.
>     For a hardware accelerator such as IAA, this value could be different
>     than ZSWAP_MAX_BATCH_SIZE.
>
> 2) ZSWAP_MAX_BATCH_SIZE acts as a limiting factor to the # of acomp_ctx
>     per-CPU resources that will be allocated in zswap_cpu_comp_prepare();
>     as per Yosry's suggestion. This helps limit the memory overhead for
>     batching algorithms.
>
> 3) If a batching algorithm works with a batch size "X" , where
>      1 < X < ZSWAP_MAX_BATCH_SIZE, two things need to happen:
>      a) We want to only allocate "X" per-CPU resources.
>      b) We want to process the folio in batches of "X", not ZSWAP_MAX_BATCH_SIZE
>           to avail of the benefits of hardware parallelism. This is the compression
>           step size you also mention.
>           In particular, we cannot treat batch_size as ZSWAP_MAX_BATCH_SIZE,
>           and send a batch of ZSWAP_MAX_BATCH_SIZE pages to zswap_compress()
>           in this case. For e.g., what if the compress step-size is 6, but the new
>           zswap_store_pages() introduced in patch 23 sends 8 pages to
>           zswap_compress() because ZSWAP_MAX_BATCH_SIZE is set to 8?
>           The code in zswap_compress() could get quite messy, which will impact
>           latency.

If ZSWAP_MAX_BATCH_SIZE is set to 8 and there is no hardware batching,
compression is done with a step size of 1. If the hardware step size is 4,
compression occurs in two steps. If the hardware step size is 6, the first
compression uses a step size of 6, and the second uses a step size of 2.
Do you think this will work?

I don’t quite understand why you want to save
ZSWAP_MAX_BATCH_SIZE - X resources, since even without hardware batching
you are still allocating all ZSWAP_MAX_BATCH_SIZE resources. This is the
case for all platforms except yours.

Thanks
Barry