Re: [PATCH 5/6] mm/zswap: only support zswap_exclusive_loads_enabled

[Chengming Zhou <zhouchengming@bytedance.com>]

On 2024/2/2 02:12, Johannes Weiner wrote:
> On Thu, Feb 01, 2024 at 03:49:05PM +0000, Chengming Zhou wrote:
>> The !zswap_exclusive_loads_enabled mode will leave compressed copy in
>> the zswap tree and lru list after the folio swapin.
>>
>> There are some disadvantages in this mode:
>> 1. It's a waste of memory since there are two copies of data, one is
>>    folio, the other one is compressed data in zswap. And it's unlikely
>>    the compressed data is useful in the near future.
>>
>> 2. If that folio is dirtied, the compressed data must be not useful,
>>    but we don't know and don't invalidate the trashy memory in zswap.
>>
>> 3. It's not reclaimable from zswap shrinker since zswap_writeback_entry()
>>    will always return -EEXIST and terminate the shrinking process.
>>
>> On the other hand, the only downside of zswap_exclusive_loads_enabled
>> is a little more cpu usage/latency when compression, and the same if
>> the folio is removed from swapcache or dirtied.
>>
>> Not sure if we should accept the above disadvantages in the case of
>> !zswap_exclusive_loads_enabled, so send this out for disscusion.
>>
>> Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
> 
> This is interesting.
> 
> First, I will say that I never liked this config option, because it's
> nearly impossible for a user to answer this question. Much better to
> just pick a reasonable default.

Agree.

> 
> What should the default be?
> 
> Caching "swapout work" is helpful when the system is thrashing. Then
> recently swapped in pages might get swapped out again very soon. It
> certainly makes sense with conventional swap, because keeping a clean
> copy on the disk saves IO work and doesn't cost any additional memory.
> 
> But with zswap, it's different. It saves some compression work on a
> thrashing page. But the act of keeping compressed memory contributes
> to a higher rate of thrashing. And that can cause IO in other places
> like zswap writeback and file memory.
> 
> It would be useful to have an A/B test to confirm that not caching is
> better. Can you run your test with and without keeping the cache, and
> in addition to the timings also compare the deltas for pgscan_anon,
> pgscan_file, workingset_refault_anon, workingset_refault_file?

I just A/B test kernel building in tmpfs directory, memory.max=2GB.
(zswap writeback enabled and shrinker_enabled, one 50GB swapfile)

From the below results, exclusive mode has fewer scan and refault.

                              zswap-invalidate-entry        zswap-invalidate-entry-exclusive
real                          63.80                         63.01                         
user                          1063.83                       1061.32                       
sys                           290.31                        266.15                        
                              zswap-invalidate-entry        zswap-invalidate-entry-exclusive
workingset_refault_anon       2383084.40                    1976397.40                    
workingset_refault_file       44134.00                      45689.40                      
workingset_activate_anon      837878.00                     728441.20                     
workingset_activate_file      4710.00                       4085.20                       
workingset_restore_anon       732622.60                     639428.40                     
workingset_restore_file       1007.00                       926.80                        
workingset_nodereclaim        0.00                          0.00                          
pgscan                        14343003.40                   12409570.20                   
pgscan_kswapd                 0.00                          0.00                          
pgscan_direct                 14343003.40                   12409570.20                   
pgscan_khugepaged             0.00                          0.00