Re: [PATCH 0/1] pagemap: swap location for shared pages

[Peter Xu]

Hi, Tiberiu,

On Fri, Jul 30, 2021 at 04:08:25PM +0000, Tiberiu A Georgescu wrote:
> This patch follows up on a previous RFC:
> 20210714152426.216217-1-tiberiu.georgescu@xxxxxxxxxxx
> 
> When a page allocated using the MAP_SHARED flag is swapped out, its pagemap
> entry is cleared. In many cases, there is no difference between swapped-out
> shared pages and newly allocated, non-dirty pages in the pagemap interface.
> 
> Example pagemap-test code (Tested on Kernel Version 5.14-rc3):
>     #define NPAGES (256)
>     /* map 1MiB shared memory */
>     size_t pagesize = getpagesize();
>     char *p = mmap(NULL, pagesize * NPAGES, PROT_READ | PROT_WRITE,
>     		   MAP_ANONYMOUS | MAP_SHARED, -1, 0);
>     /* Dirty new pages. */
>     for (i = 0; i < PAGES; i++)
>     	p[i * pagesize] = i;
> 
> Run the above program in a small cgroup, which causes swapping:
>     /* Initialise cgroup & run a program */
>     $ echo 512K > foo/memory.limit_in_bytes
>     $ echo 60 > foo/memory.swappiness
>     $ cgexec -g memory:foo ./pagemap-test
> 
> Check the pagemap report. Example of the current expected output:
>     $ dd if=/proc/$PID/pagemap ibs=8 skip=$(($VADDR / $PAGESIZE)) count=$COUNT | hexdump -C
>     00000000  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
>     *
>     00000710  e1 6b 06 00 00 00 00 a1  9e eb 06 00 00 00 00 a1  |.k..............|
>     00000720  6b ee 06 00 00 00 00 a1  a5 a4 05 00 00 00 00 a1  |k...............|
>     00000730  5c bf 06 00 00 00 00 a1  90 b6 06 00 00 00 00 a1  |\...............|
> 
> The first pagemap entries are reported as zeroes, indicating the pages have
> never been allocated while they have actually been swapped out.
> 
> This patch addresses the behaviour and modifies pte_to_pagemap_entry() to
> make use of the XArray associated with the virtual memory area struct
> passed as an argument. The XArray contains the location of virtual pages in
> the page cache, swap cache or on disk. If they are in either of the caches,
> then the original implementation still works. If not, then the missing
> information will be retrieved from the XArray.
> 
> Performance
> ============
> I measured the performance of the patch on a single socket Xeon E5-2620
> machine, with 128GiB of RAM and 128GiB of swap storage. These were the
> steps taken:
> 
>   1. Run example pagemap-test code on a cgroup
>     a. Set up cgroup with limit_in_bytes=4GiB and swappiness=60;
>     b. allocate 16GiB (about 4 million pages);
>     c. dirty 0,50 or 100% of pages;
>     d. do this for both private and shared memory.
>   2. Run `dd if=<PAGEMAP> ibs=8 skip=$(($VADDR / $PAGESIZE)) count=4194304`
>      for each possible configuration above
>     a.  3 times for warm up;
>     b. 10 times to measure performance.
>        Use `time` or another performance measuring tool.
> 
> Results (averaged over 10 iterations):
>                +--------+------------+------------+
>                | dirty% |  pre patch | post patch |
>                +--------+------------+------------+
>  private|anon  |     0% |      8.15s |      8.40s |
>                |    50% |     11.83s |     12.19s |
>                |   100% |     12.37s |     12.20s |
>                +--------+------------+------------+
>   shared|anon  |     0% |      8.17s |      8.18s |
>                |    50% | (*) 10.43s |     37.43s |
>                |   100% | (*) 10.20s |     38.59s |
>                +--------+------------+------------+
> 
> (*): reminder that pre-patch produces incorrect pagemap entries for swapped
>      out pages.
> 
> From run to run the above results are stable (mostly <1% stderr).
> 
> The amount of time it takes for a full read of the pagemap depends on the
> granularity used by dd to read the pagemap file. Even though the access is
> sequential, the script only reads 8 bytes at a time, running pagemap_read()
> COUNT times (one time for each page in a 16GiB area).
> 
> To reduce overhead, we can use batching for large amounts of sequential
> access. We can make dd read multiple page entries at a time,
> allowing the kernel to make optimisations and yield more throughput.
> 
> Performance in real time (seconds) of
> `dd if=<PAGEMAP> ibs=8*$BATCH skip=$(($VADDR / $PAGESIZE / $BATCH))
> count=$((4194304 / $BATCH))`:
> +---------------------------------+ +---------------------------------+
> |     Shared, Anon, 50% dirty     | |     Shared, Anon, 100% dirty    |
> +-------+------------+------------+ +-------+------------+------------+
> | Batch |  Pre-patch | Post-patch | | Batch |  Pre-patch | Post-patch |
> +-------+------------+------------+ +-------+------------+------------+
> |     1 | (*) 10.43s |     37.43s | |     1 | (*) 10.20s |     38.59s |
> |     2 | (*)  5.25s |     18.77s | |     2 | (*)  5.15s |     19.37s |
> |     4 | (*)  2.63s |      9.42s | |     4 | (*)  2.63s |      9.74s |
> |     8 | (*)  1.38s |      4.80s | |     8 | (*)  1.35s |      4.94s |
> |    16 | (*)  0.73s |      2.46s | |    16 | (*)  0.72s |      2.54s |
> |    32 | (*)  0.40s |      1.31s | |    32 | (*)  0.41s |      1.34s |
> |    64 | (*)  0.25s |      0.72s | |    64 | (*)  0.24s |      0.74s |
> |   128 | (*)  0.16s |      0.43s | |   128 | (*)  0.16s |      0.44s |
> |   256 | (*)  0.12s |      0.28s | |   256 | (*)  0.12s |      0.29s |
> |   512 | (*)  0.10s |      0.21s | |   512 | (*)  0.10s |      0.22s |
> |  1024 | (*)  0.10s |      0.20s | |  1024 | (*)  0.10s |      0.21s |
> +-------+------------+------------+ +-------+------------+------------+
> 
> To conclude, in order to make the most of the underlying mechanisms of
> pagemap and xarray, one should be using batching to achieve better
> performance.

So what I'm still a bit worried is whether it will regress some existing users.
Note that existing users can try to read pagemap in their own way; we can't
expect all the userspaces to change their behavior due to a kernel change.

Meanwhile, from the numbers, it seems to show a 4x speed down due to looking up
the page cache no matter the size of ibs=.  IOW I don't see a good way to avoid
that overhead, so no way to have the userspace run as fast as before.

Also note that it's not only affecting the PM_SWAP users; it potentially
affects all the /proc/pagemap users as long as there're file-backed memory on
the read region of pagemap, which is very sane to happen.

That's why I think if we want to persist it, we should still consider starting
from the pte marker idea.

I do plan to move the pte marker idea forward unless that'll be NACKed upstream
for some other reason, because that seems to be the only way for uffd-wp to
support file based memories; no matter with a new swp type or with special swap
pte.  I am even thinking about whether I should propose that with PM_SWAP first
because that seems to be a simpler scenario than uffd-wp (which will get the
rest uffd-wp patches involved then), then we can have a shared infrastructure.
But haven't thought deeper than that.

Thanks,

-- 
Peter Xu