Date: Mon, 17 Jun 2024 14:48:26 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Barry Song <21cnbao@...il.com>
Cc: akpm@...ux-foundation.org,  chrisl@...nel.org,  baohua@...nel.org,
  kaleshsingh@...gle.com,  kasong@...cent.com,
  linux-kernel@...r.kernel.org,  linux-mm@...ck.org,  ryan.roberts@....com
Subject: Re: [PATCH v2 0/2] mm: swap: mTHP swap allocator base on swap
 cluster order

Hi, Barry,

Barry Song <21cnbao@...il.com> writes:

> On Sat, Jun 15, 2024 at 2:59 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>>
>> On Fri, 14 Jun 2024 19:51:11 -0700 Chris Li <chrisl@...nel.org> wrote:
>>
>> > > I'm having trouble understanding the overall impact of this on users.
>> > > We fail the mTHP swap allocation and fall back, but things continue to
>> > > operate OK?
>> >
>> > Continue to operate OK in the sense that the mTHP will have to split
>> > into 4K pages before the swap out, aka the fall back. The swap out and
>> > swap in can continue to work as 4K pages, not as the mTHP. Due to the
>> > fallback, the mTHP based zsmalloc compression with 64K buffer will not
>> > happen. That is the effect of the fallback. But mTHP swap out and swap
>> > in is relatively new, it is not really a regression.
>>
>> Sure, but it's pretty bad to merge a new feature only to have it
>> ineffective after a few hours use.
>>
>> > >
>> > > > There is some test number in the V1 thread of this series:
>> > > > https://lore.kernel.org/r/20240524-swap-allocator-v1-0-47861b423b26@kernel.org
>> > >
>> > > Well, please let's get the latest numbers into the latest patchset.
>> > > Along with a higher-level (and quantitative) description of the user impact.
>> >
>> > I will need Barray's help to collect the number. I don't have the
>> > setup to reproduce his test result.
>> > Maybe a follow up commit message amendment for the test number when I get it?
>
> Although the issue may seem complex at a systemic level, even a small program can
> demonstrate the problem and highlight how Chris's patch has improved the
> situation.
>
> To demonstrate this, I designed a basic test program that maximally allocates
> two memory blocks:
>
>  *   A memory block of up to 60MB, recommended for HUGEPAGE usage
>  *   A memory block of up to 1MB, recommended for NOHUGEPAGE usage
>
> In the system configuration, I enabled 64KB mTHP and 64MB zRAM, providing more than
> enough space for both the 60MB and 1MB allocations in the worst case. This setup
> allows us to assess two effects:
>
> 1.  When we don't enable mem2 (small folios), we consistently allocate and free
>     swap slots aligned with 64KB.  whether there is a risk of failure to obtain
>     swap slots even though the zRAM has sufficient free space?
> 2.  When we enable mem2 (small folios), the presence of small folios may lead
>     to fragmentation of clusters, potentially impacting the swapout process for
>     large folios negatively.
>
> (2) can be enabled by "-s", without -s, small folios are disabled.
>
> The script to configure zRAM and mTHP:
>
> echo lzo > /sys/block/zram0/comp_algorithm
> echo 64M > /sys/block/zram0/disksize
> echo never > /sys/kernel/mm/transparent_hugepage/hugepages-2048kB/enabled
> echo always > /sys/kernel/mm/transparent_hugepage/hugepages-64kB/enabled
> mkswap /dev/zram0
> swapon /dev/zram0
>
> The test program I made today after receiving Chris' patchset v2
>
> (Andrew, Please let me know if you want this small test program to
> be committed into kernel/tools/ folder. If yes, please let me know,
> and I will cleanup and prepare a patch):
>
> #define _GNU_SOURCE
> #include <stdio.h>
> #include <stdlib.h>
> #include <unistd.h>
> #include <string.h>
> #include <sys/mman.h>
> #include <errno.h>
> #include <time.h>
>
> #define MEMSIZE_MTHP (60 * 1024 * 1024)
> #define MEMSIZE_SMALLFOLIO (1 * 1024 * 1024)
> #define ALIGNMENT_MTHP (64 * 1024)
> #define ALIGNMENT_SMALLFOLIO (4 * 1024)
> #define TOTAL_DONTNEED_MTHP (16 * 1024 * 1024)
> #define TOTAL_DONTNEED_SMALLFOLIO (256 * 1024)
> #define MTHP_FOLIO_SIZE (64 * 1024)
>
> #define SWPOUT_PATH \
>     "/sys/kernel/mm/transparent_hugepage/hugepages-64kB/stats/swpout"
> #define SWPOUT_FALLBACK_PATH \
>     "/sys/kernel/mm/transparent_hugepage/hugepages-64kB/stats/swpout_fallback"
>
> static void *aligned_alloc_mem(size_t size, size_t alignment)
> {
>     void *mem = NULL;
>     if (posix_memalign(&mem, alignment, size) != 0) {
>         perror("posix_memalign");
>         return NULL;
>     }
>     return mem;
> }
>
> static void random_madvise_dontneed(void *mem, size_t mem_size,
>                                      size_t align_size, size_t total_dontneed_size)
> {
>     size_t num_pages = total_dontneed_size / align_size;
>     size_t i;
>     size_t offset;
>     void *addr;
>
>     for (i = 0; i < num_pages; ++i) {
>         offset = (rand() % (mem_size / align_size)) * align_size;
>         addr = (char *)mem + offset;
>         if (madvise(addr, align_size, MADV_DONTNEED) != 0) {
>             perror("madvise dontneed");
>         }
>         memset(addr, 0x11, align_size);
>     }
> }
>
> static unsigned long read_stat(const char *path)
> {
>     FILE *file;
>     unsigned long value;
>
>     file = fopen(path, "r");
>     if (!file) {
>         perror("fopen");
>         return 0;
>     }
>
>     if (fscanf(file, "%lu", &value) != 1) {
>         perror("fscanf");
>         fclose(file);
>         return 0;
>     }
>
>     fclose(file);
>     return value;
> }
>
> int main(int argc, char *argv[])
> {
>     int use_small_folio = 0;
>     int i;
>     void *mem1 = aligned_alloc_mem(MEMSIZE_MTHP, ALIGNMENT_MTHP);
>     if (mem1 == NULL) {
>         fprintf(stderr, "Failed to allocate 60MB memory\n");
>         return EXIT_FAILURE;
>     }
>
>     if (madvise(mem1, MEMSIZE_MTHP, MADV_HUGEPAGE) != 0) {
>         perror("madvise hugepage for mem1");
>         free(mem1);
>         return EXIT_FAILURE;
>     }
>
>     for (i = 1; i < argc; ++i) {
>         if (strcmp(argv[i], "-s") == 0) {
>             use_small_folio = 1;
>         }
>     }
>
>     void *mem2 = NULL;
>     if (use_small_folio) {
>         mem2 = aligned_alloc_mem(MEMSIZE_SMALLFOLIO, ALIGNMENT_MTHP);
>         if (mem2 == NULL) {
>             fprintf(stderr, "Failed to allocate 1MB memory\n");
>             free(mem1);
>             return EXIT_FAILURE;
>         }
>
>         if (madvise(mem2, MEMSIZE_SMALLFOLIO, MADV_NOHUGEPAGE) != 0) {
>             perror("madvise nohugepage for mem2");
>             free(mem1);
>             free(mem2);
>             return EXIT_FAILURE;
>         }
>     }
>
>     for (i = 0; i < 100; ++i) {
>         unsigned long initial_swpout;
>         unsigned long initial_swpout_fallback;
>         unsigned long final_swpout;
>         unsigned long final_swpout_fallback;
>         unsigned long swpout_inc;
>         unsigned long swpout_fallback_inc;
>         double fallback_percentage;
>
>         initial_swpout = read_stat(SWPOUT_PATH);
>         initial_swpout_fallback = read_stat(SWPOUT_FALLBACK_PATH);
>
>         random_madvise_dontneed(mem1, MEMSIZE_MTHP, ALIGNMENT_MTHP,
>                                  TOTAL_DONTNEED_MTHP);
>
>         if (use_small_folio) {
>             random_madvise_dontneed(mem2, MEMSIZE_SMALLFOLIO,
>                                      ALIGNMENT_SMALLFOLIO,
>                                      TOTAL_DONTNEED_SMALLFOLIO);
>         }
>
>         if (madvise(mem1, MEMSIZE_MTHP, MADV_PAGEOUT) != 0) {
>             perror("madvise pageout for mem1");
>             free(mem1);
>             if (mem2 != NULL) {
>                 free(mem2);
>             }
>             return EXIT_FAILURE;
>         }
>
>         if (use_small_folio) {
>             if (madvise(mem2, MEMSIZE_SMALLFOLIO, MADV_PAGEOUT) != 0) {
>                 perror("madvise pageout for mem2");
>                 free(mem1);
>                 free(mem2);
>                 return EXIT_FAILURE;
>             }
>         }
>
>         final_swpout = read_stat(SWPOUT_PATH);
>         final_swpout_fallback = read_stat(SWPOUT_FALLBACK_PATH);
>
>         swpout_inc = final_swpout - initial_swpout;
>         swpout_fallback_inc = final_swpout_fallback - initial_swpout_fallback;
>
>         fallback_percentage = (double)swpout_fallback_inc /
>                               (swpout_fallback_inc + swpout_inc) * 100;
>
>         printf("Iteration %d: swpout inc: %lu, swpout fallback inc: %lu, Fallback percentage: %.2f%%\n",
>                i + 1, swpout_inc, swpout_fallback_inc, fallback_percentage);
>     }
>
>     free(mem1);
>     if (mem2 != NULL) {
>         free(mem2);
>     }
>
>     return EXIT_SUCCESS;
> }

Thank you very for your effort to write this test program.

TBH, personally, I thought that this test program isn't practical
enough.  Can we show performance difference with some normal workloads?

[snip]

--
Best Regards,
Huang, Ying

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