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

[Sridhar, Kanchana P]

> -----Original Message-----
> From: Nhat Pham <nphamcs@xxxxxxxxx>
> Sent: Monday, March 3, 2025 10:22 AM
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> Cc: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>; linux-
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> 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.

Thanks,
Kanchana