RE: [PATCH v8 14/14] mm: zswap: Compress batching with request chaining in zswap_store() of large folios.

Sridhar, Kanchana P
2025-04-30T21:07:08Z

> -----Original Message-----
> From: Yosry Ahmed <yosry.ahmed@linux.dev>
> Sent: Thursday, March 6, 2025 1:21 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
> Cc: Nhat Pham <nphamcs@gmail.com>; lkp <lkp@intel.com>; linux-
> kernel@vger.kernel.org; linux-mm@kvack.org; hannes@cmpxchg.org;
> chengming.zhou@linux.dev; usamaarif642@gmail.com;
> ryan.roberts@arm.com; 21cnbao@gmail.com;
> ying.huang@linux.alibaba.com; akpm@linux-foundation.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>; llvm@lists.linux.dev; oe-kbuild-
> all@lists.linux.dev; Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal,
> Vinodh <vinodh.gopal@intel.com>
> Subject: Re: [PATCH v8 14/14] mm: zswap: Compress batching with request
> chaining in zswap_store() of large folios.
>
> On Mon, Mar 03, 2025 at 09:34:04PM +0000, Sridhar, Kanchana P wrote:
> >
> > > -----Original Message-----
> > > From: Nhat Pham <nphamcs@gmail.com>
> > > Sent: Monday, March 3, 2025 10:22 AM
> > > To: lkp <lkp@intel.com>
> > > Cc: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; linux-
> > > kernel@vger.kernel.org; linux-mm@kvack.org; hannes@cmpxchg.org;
> > > yosry.ahmed@linux.dev; chengming.zhou@linux.dev;
> > > usamaarif642@gmail.com; ryan.roberts@arm.com; 21cnbao@gmail.com;
> > > ying.huang@linux.alibaba.com; akpm@linux-foundation.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>; llvm@lists.linux.dev; oe-kbuild-
> > > all@lists.linux.dev; Feghali, Wajdi K <wajdi.k.feghali@intel.com>; Gopal,
> > > Vinodh <vinodh.gopal@intel.com>
> > > Subject: Re: [PATCH v8 14/14] mm: zswap: Compress batching with
> request
> > > chaining in zswap_store() of large folios.
> > >
> > > On Mon, Mar 3, 2025 at 3:07 AM kernel test robot <lkp@intel.com>
> wrote:
> > > >
> > > > Hi Kanchana,
> > > >
> > > > kernel test robot noticed the following build errors:
> > > >
> > > > > 1166 prefetchw(entries[j]);
> > > > --
> > >
> > > Why are we doing this anyway? Does it have a notable performance
> > > difference? At the very least, leave a comment explaining why we're
> > > prefetching this (although the build error suggests that we have to
> > > remove it anyway).
> >
> > Hi Nhat,
> >
> > Yes, it does. The use of prefetchw reduces sys time by ~1.5% because
> > it minimizes cache-miss latency by moving the zswap entry to the cache
> > before it is written to.
> >
> > This is data with kernel compilation test, v8 without prefetchw and v8 as-is:
> >
> > --------------------------------------------------------------------------------
> > Kernel compile v8 without v8 v8 without v8
> > allmodconfig prefetchw prefetchw
> > 2M folios
> > --------------------------------------------------------------------------------
> > zswap compressor deflate-iaa deflate-iaa zstd zstd
> > --------------------------------------------------------------------------------
> > real_sec 732.89 735.63 768.53 758.21
> > user_sec 15,708.37 15,699.84 15,702.64 15,678.73
> > sys_sec 4,632.58 4,563.70 5,735.06 5,635.69
> > --------------------------------------------------------------------------------
> > Max_Res_Set_Size_KB 1,874,672 1,867,516 1,874,684
> 1,872,888
> > --------------------------------------------------------------------------------
> > memcg_high 0 0 0 0
> > memcg_swap_fail 0 0 0 0
> > zswpout 114,742,930 112,836,725 92,904,961 89,596,085
> > zswpin 41,184,897 39,983,793 31,018,149 29,163,932
> > pswpout 625 1,069 558 1,059
> > pswpin 599 1,056 540 1,051
> > thp_swpout 1 2 1 2
> > thp_swpout_fallback 10,967 10,195 6,918 6,141
> > pgmajfault 42,588,331 41,349,069 31,931,882 30,006,422
> > ZSWPOUT-2048kB 7,661 8,710 6,799 7,480
> > SWPOUT-2048kB 1 2 1 2
> > --------------------------------------------------------------------------------
> >
> >
> > Sure, I will add a comment, and also "#include <linux/prefetch.h>" in
> zswap.c
> > that will resolve the build error. This is similar to how these files handle
> prefetchw:
> > mm/vmscan.c, kernel/locking/qspinlock.c, include/asm-generic/xor.h, etc.
>
> Please also explicitly mention that the prefetch and likely/unlikely
> annotations prevent regressions with software compression like zstd, and
> generally improve the performance with the batching code by ~1.5%.

Yes, I have mentioned this in the comments and commit log. So also the
mutex locking.

>
> >
> > Thanks,
> > Kanchana
> >