On Mon, Mar 03, 2025 at 09:34:04PM +0000, Sridhar, Kanchana P wrote: > > > -----Original Message----- > > From: Nhat Pham <nphamcs@xxxxxxxxx> > > Sent: Monday, March 3, 2025 10:22 AM > > To: lkp <lkp@xxxxxxxxx> > > Cc: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; linux- > > kernel@xxxxxxxxxxxxxxx; linux-mm@xxxxxxxxx; hannes@xxxxxxxxxxx; > > yosry.ahmed@xxxxxxxxx; chengming.zhou@xxxxxxxxx; > > usamaarif642@xxxxxxxxx; ryan.roberts@xxxxxxx; 21cnbao@xxxxxxxxx; > > ying.huang@xxxxxxxxxxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; linux- > > crypto@xxxxxxxxxxxxxxx; herbert@xxxxxxxxxxxxxxxxxxx; > > davem@xxxxxxxxxxxxx; clabbe@xxxxxxxxxxxx; ardb@xxxxxxxxxx; > > ebiggers@xxxxxxxxxx; surenb@xxxxxxxxxx; Accardi, Kristen C > > <kristen.c.accardi@xxxxxxxxx>; llvm@xxxxxxxxxxxxxxx; oe-kbuild- > > all@xxxxxxxxxxxxxxx; Feghali, Wajdi K <wajdi.k.feghali@xxxxxxxxx>; Gopal, > > Vinodh <vinodh.gopal@xxxxxxxxx> > > 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@xxxxxxxxx> 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 >
