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Date: Wed, 27 Aug 2025 00:06:15 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Barry Song <21cnbao@...il.com>
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Subject: RE: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources if
 the compressor supports batching.


> -----Original Message-----
> From: Barry Song <21cnbao@...il.com>
> Sent: Monday, August 25, 2025 10:17 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: linux-kernel@...r.kernel.org; linux-mm@...ck.org;
> hannes@...xchg.org; yosry.ahmed@...ux.dev; nphamcs@...il.com;
> chengming.zhou@...ux.dev; usamaarif642@...il.com;
> ryan.roberts@....com; ying.huang@...ux.alibaba.com; akpm@...ux-
> foundation.org; senozhatsky@...omium.org; linux-crypto@...r.kernel.org;
> herbert@...dor.apana.org.au; davem@...emloft.net;
> clabbe@...libre.com; ardb@...nel.org; ebiggers@...gle.com;
> surenb@...gle.com; Accardi, Kristen C <kristen.c.accardi@...el.com>;
> Gomes, Vinicius <vinicius.gomes@...el.com>; Feghali, Wajdi K
> <wajdi.k.feghali@...el.com>; Gopal, Vinodh <vinodh.gopal@...el.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.
        

> 
> +       pool->batch_size = (pool->compr_batch_size > 1) ?
> +                               pool->compr_batch_size : ZSWAP_MAX_BATCH_SIZE;
> 
> 
> It seems one of these two variables may be redundant.
> For instance, no matter if pool->compr_batch_size > 1, could we always treat
> batch_size as ZSWAP_MAX_BATCH_SIZE?  if we remove
> pool->batch_size, could we just use pool->compr_batch_size as the
> step size for compression no matter if pool->compr_batch_size > 1.

To further explain the rationale for keeping these two distinct, we
statically compute the compress step-size by querying the algorithm
in zswap_cpu_comp_prepare() after the acomp_ctx->acomp has been
created. We store it in pool->compr_batch_size.

Next, in zswap_pool_create(), we do a one-time computation to determine
if the pool->batch_size needs to align with a non-1 batching acomp's
compr_batch_size; or, if it is a non-batching compressor, for the
pool->batch_size to be ZSWAP_MAX_BATCH_SIZE.

This enables further code simplifications/unification in zswap_compress();
and quick retrieval of the # of available acomp_ctx batching resources to
set up the SG lists per call to crypto_acomp_compress(). IOW, memo-ization
for latency gains and unified code paths.

I hope this clarifies things further. Thanks again, these are all good questions.

Best regards,
Kanchana

> 
> for example:
>        pool->compr_batch_size = min(ZSWAP_MAX_BATCH_SIZE,
>                                     crypto_acomp_batch_size(acomp_ctx->acomp));
> 
> Then batch the zswap store using ZSWAP_MAX_BATCH_SIZE, but set the
> compression step to pool->compr_batch_size.
> 
> Thanks
> Barry

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