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

Yosry Ahmed
2025-03-06T21:21:04Z

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%.

>
> Thanks,
> Kanchana
>