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

[Daniel Jordan <lkmldmj@gmail.com>]

On 7/23/18 9:02 AM, Michal Hocko wrote:
> [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?
>