linux-kernel - RE: [PATCH v11 22/24] mm: zswap: Allocate pool batching resources if the compressor supports batching.

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Message-ID: <PH7PR11MB8121B86E65349F5CF854CA57C939A@PH7PR11MB8121.namprd11.prod.outlook.com>
Date: Tue, 26 Aug 2025 04:56:36 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Barry Song <21cnbao@...il.com>
CC: "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "hannes@...xchg.org"
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	"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.


> -----Original Message-----
> From: Barry Song <21cnbao@...il.com>
> Sent: Monday, August 25, 2025 9:42 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.
> 
> On Tue, Aug 26, 2025 at 12:27 PM Sridhar, Kanchana P
> <kanchana.p.sridhar@...el.com> wrote:
> >
> >
> >
> > > -----Original Message-----
> > > From: Barry Song <21cnbao@...il.com>
> > > Sent: Monday, August 25, 2025 8:48 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.
> > >
> > > Hi Kanchana,
> > >
> > >
> > > [...]
> > > >
> > > > +       /*
> > > > +        * 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.

Besides this, pool->compr_batch_size helps to distinguish the number of
acomp_ctx resources during zswap_compress(); distinctly from the batch size.

> It seems pool->compr_batch_size should either
> be a bool or be dropped. If we drop it, you can still check whether HW
> supports batch via crypto_acomp_batch_size() when doing compression:
> 
> if (crypto_acomp_batch_size() > 1)
>     compress in steps of PAGE_SIZE;
> else
>     compress in steps of batch_size;
> 
> no ?

I could do this, but it will impact latency. As I was mentioning in an earlier
response to Nhat, this (keeping compr_batch_size and batch_size distinct)
was a small memory overhead per zswap_pool (one more u8 data member),
given that there are very few zswap pools. For this trade off, we are able to
minimize computes in zswap_compress()..

Thanks,
Kanchana

> 
> Thanks
> Barry