Re: [Bug 200105] High paging activity as soon as the swap is touched (with steps and code to reproduce it)

[Michal Hocko <mhocko@kernel.org>]

[I am really sorry to be slow on responding]

On Sat 21-07-18 10:39:05, Daniel Jordan wrote:
> John's issue only happens using a LUKS encrypted swap partition,
> unencrypted swap or swap encrypted without LUKS works fine.
> 
> In one test (out5.txt) where most system memory is taken by anon pages
> beforehand, the heavy direct reclaim that Michal noticed lasts for 24
> seconds, during which on average if I've crunched my numbers right,
> John's test program was allocating at 4MiB/s, the system overall
> (pgalloc_normal) was allocating at 235MiB/s, and the system was
> swapping out (pswpout) at 673MiB/s. pgalloc_normal and pswpout stay
> roughly the same each second, no big swings.
>
> Is the disparity between allocation and swapout rate expected?
> 
> John ran perf during another test right before the last test program
> was started (this doesn't include the initial large allocation
> bringing the system close to swapping).  The top five allocators
> (kmem:mm_page_alloc):
> 
> # Overhead      Pid:Command
> # ........  .......................
> #
>     48.45%     2005:memeater     # the test program
>     32.08%       73:kswapd0
>      3.16%     1957:perf_4.17
>      1.41%     1748:watch
>      1.16%     2043:free

Huh, kswapd allocating memory sounds really wrong here. Is it possible
that the swap device driver is double buffering and allocating a new
page for each one to swap out?
-- 
Michal Hocko
SUSE Labs