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From:   Yu Zhao <yuzhao@google.com>
Date:   Tue, 15 Mar 2022 23:44:38 -0600
Message-ID: <CAOUHufZV-f6-7T+ZFBVHUJjGn6HfeKoVdPxiOCG2PmxxXbWVZA@mail.gmail.com>
Subject: Re: [PATCH v7 04/12] mm: multigenerational LRU: groundwork
To:     Barry Song <21cnbao@gmail.com>
Cc:     Konstantin Kharlamov <Hi-Angel@yandex.ru>,
        Michael Larabel <Michael@michaellarabel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        "Aneesh Kumar K . V" <aneesh.kumar@linux.ibm.com>,
        Jens Axboe <axboe@kernel.dk>,
        Brian Geffon <bgeffon@google.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Donald Carr <d@chaos-reins.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Daniel Byrne <djbyrne@mtu.edu>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Hillf Danton <hdanton@sina.com>,
        Jan Alexander Steffens <heftig@archlinux.org>,
        =?UTF-8?Q?Holger_Hoffst=C3=A4tte?= <holger@applied-asynchrony.com>,
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        Michal Hocko <mhocko@kernel.org>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        Kernel Page Reclaim v2 <page-reclaim@google.com>,
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On Tue, Mar 15, 2022 at 10:37 PM Barry Song <21cnbao@gmail.com> wrote:
>
> On Wed, Mar 16, 2022 at 3:47 PM Yu Zhao <yuzhao@google.com> wrote:
> >
> > On Tue, Mar 15, 2022 at 4:29 AM Barry Song <21cnbao@gmail.com> wrote:
> >
> > <snipped>
> >
> > > > I guess the main cause of the regression for the previous sequence
> > > > with 16 entries is that the ebizzy has a new allocated copy in
> > > > search_mem(), which is mapped and used only once in each loop.
> > > > and the temp copy can push out those hot chunks.
> > > >
> > > > Anyway, I understand it is a trade-off between warmly embracing new
> > > > pages and holding old pages tightly. Real user cases from phone, server,
> > > > desktop will be judging this better.
> >
> > Thanks for all the details. I looked into them today and found no
> > regressions when running with your original program.
> >
> > After I explain why, I hope you'd be convinced that using programs
> > like this one is not a good way to measure things :)
> >
>
> Yep. I agree ebizzy might not be a good one to measure things.
> I chose it only because Kim's patchset which moved anon pages
> to inactive at the first detected access  was using it. Before kim's
> patchset, anon pages were placed in the active list from the first
> beginning:
> https://patchwork.kernel.org/project/linux-mm/cover/1581401993-20041-1-git-send-email-iamjoonsoo.kim@lge.com/
>
> in ebizzy, there is a used-once allocated memory in each
> search_mem(). I guess that is why Kim's patchset chose
> it.
>
> > Problems:
> > 1) Given the 2.5GB configuration and a sequence of cold/hot chunks, I
> > assume your program tries to simulate a handful of apps running on a
> > phone.  A short repeating sequence is closer to sequential access than
> > to real user behaviors, as I suggested last time. You could check out
> > how something similar is done here [1].
> > 2) Under the same assumption (phone), C programs are very different
> > from Android apps in terms of runtime memory behaviors, e.g., JVM GC
> > [2].
> > 3) Assuming you are interested in the runtime memory behavior of C/C++
> > programs, your program is still not very representative. All C/C++
> > programs I'm familiar with choose to link against TCmalloc, jemalloc
> > or implement their own allocators. GNU libc, IMO, has a small market
> > share nowadays.
> > 4) TCmalloc/jemalloc are not only optimized for multithreading, they
> > are also THP aware. THP is very important when benchmarking page
> > reclaim, e.g., two similarly warm THPs can comprise 511+1 or 1+511 of
> > warm+cold 4K pages. The LRU algorithm that chooses more of the former
> > is at the disadvantage. Unless it's recommended by the applications
> > you are trying to benchmark, THP should be disabled. (Android
> > generally doesn't use THP.)
> > 5) Swap devices are also important. Zram should NOT be used unless you
> > know your benchmark doesn't generate incompressible data. The LRU
> > algorithm that chooses more incompressible pages is at disadvantage.
> >
>
> Thanks for all the information above. very useful.
>
> > Here is my result: on the same Snapdragon 7c + 2.5GB RAM + 1.5GB
> > ramdisk swap, with your original program compiled against libc malloc
> > and TCMalloc, to 32-bit and 64-bit binaries:
>
> I noticed an important difference is that you are using ramdisk, so there
> is no cost on "i/o". I assume compression/decompression is the i/o cost to
> zRAM.

The cost is not the point; the fairness is:

1) Ramdisk is fair to both LRU algorithms.
2) Zram punishes the LRU algorithm that chooses incompressible pages.
IOW, this algorithm needs to compress more pages in order to save the
same amount of memory.

> > # cat /sys/kernel/mm/lru_gen/enabled
> > 0x0003
> > # cat /sys/kernel/mm/transparent_hugepage/enabled
> > always madvise [never]
> >
> > # modprobe brd rd_nr=1 rd_size=1572864
> > # if=/dev/zero of=/dev/ram0 bs=1M
> > # mkswap /dev/ram0
> > # swapoff -a
> > # swapon /dev/ram0
> >
> > # ldd test_absl_32
> >         linux-vdso.so.1 (0xf6e7f000)
> >         libabsl_malloc.so.2103.0.1 =>
> > /usr/lib/libabsl_malloc.so.2103.0.1 (0xf6e23000)
> >         libpthread.so.0 => /lib/libpthread.so.0 (0xf6dff000)
> >         libc.so.6 => /lib/libc.so.6 (0xf6d07000)
> >         /lib/ld-linux-armhf.so.3 (0x09df0000)
> >         libabsl_base.so.2103.0.1 => /usr/lib/libabsl_base.so.2103.0.1
> > (0xf6ce5000)
> >         libabsl_raw_logging.so.2103.0.1 =>
> > /usr/lib/libabsl_raw_logging.so.2103.0.1 (0xf6cc4000)
> >         libabsl_spinlock_wait.so.2103.0.1 =>
> > /usr/lib/libabsl_spinlock_wait.so.2103.0.1 (0xf6ca3000)
> >         libc++.so.1 => /usr/lib/libc++.so.1 (0xf6c04000)
> >         libc++abi.so.1 => /usr/lib/libc++abi.so.1 (0xf6bcd000)
> > # file test_absl_64
> > test_absl_64: ELF 64-bit LSB executable, ARM aarch64, version 1
> > (SYSV), statically linked
> > # ldd test_gnu_32
> >         linux-vdso.so.1 (0xeabef000)
> >         libpthread.so.0 => /lib/libpthread.so.0 (0xeab92000)
> >         libc.so.6 => /lib/libc.so.6 (0xeaa9a000)
> >         /lib/ld-linux-armhf.so.3 (0x05690000)
> > # file test_gnu_64
> > test_gnu_64: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV),
> > statically linked
> >
> > ### baseline 5.17-rc8
> >
> > # perf record ./test_gnu_64 -t 4 -s $((200*1024*1024)) -S 6000000
> > 10 records/s
> > real 59.00 s
> > user 39.83 s
> > sys  174.18 s
> >
> >     18.51%  [.] memcpy
> >     15.98%  [k] __pi_clear_page
> >      5.59%  [k] rmqueue_pcplist
> >      5.19%  [k] do_raw_spin_lock
> >      5.09%  [k] memmove
> >      4.60%  [k] _raw_spin_unlock_irq
> >      3.62%  [k] _raw_spin_unlock_irqrestore
> >      3.61%  [k] free_unref_page_list
> >      3.29%  [k] zap_pte_range
> >      2.53%  [k] local_daif_restore
> >      2.50%  [k] down_read_trylock
> >      1.41%  [k] handle_mm_fault
> >      1.32%  [k] do_anonymous_page
> >      1.31%  [k] up_read
> >      1.03%  [k] free_swap_cache
> >
> > ### MGLRU v9
> >
> > # perf record ./test_gnu_64 -t 4 -s $((200*1024*1024)) -S 6000000
> > 11 records/s
> > real 57.00 s
> > user 39.39 s
> >
> >     19.36%  [.] memcpy
> >     16.50%  [k] __pi_clear_page
> >      6.21%  [k] memmove
> >      5.57%  [k] rmqueue_pcplist
> >      5.07%  [k] do_raw_spin_lock
> >      4.96%  [k] _raw_spin_unlock_irqrestore
> >      4.25%  [k] free_unref_page_list
> >      3.80%  [k] zap_pte_range
> >      3.69%  [k] _raw_spin_unlock_irq
> >      2.71%  [k] local_daif_restore
> >      2.10%  [k] down_read_trylock
> >      1.50%  [k] handle_mm_fault
> >      1.29%  [k] do_anonymous_page
> >      1.17%  [k] free_swap_cache
> >      1.08%  [k] up_read
> >
>
> I think your result is right. but if you take a look at the number of
> major faults, will you find mglru have more page faults?
> i ask this question because i can see mglru even wins with lower
> hit ratio in the previous report I sent.

Yes, I did see the elevated major faults:

# baseline total 11503878
majfault       4745116
pgsteal_kswapd 3056793
pgsteal_direct 3701969

# MGLRU total 11928659
pgmajfault     5762213
pgsteal_kswapd 2098253
pgsteal_direct 4068193