Re: [RFC 0/3] mm: Discard lazily freed pages when migrating

[Huang\, Ying]

Michal Hocko <mhocko@xxxxxxxxxx> writes:

> On Wed 04-03-20 19:15:20, Huang, Ying wrote:
>> Michal Hocko <mhocko@xxxxxxxxxx> writes:
>> 
>> > On Tue 03-03-20 19:49:53, Huang, Ying wrote:
> [...]
>> >> Because the penalty difference is so large, I
>> >> think it may be a good idea to always put clean MADV_FREE pages at the
>> >> tail of the inactive file LRU list?
>> >
>> > You are again making assumptions without giving any actual real
>> > examples. Reconstructing MADV_FREE pages cost can differ a lot.
>> 
>> In which situation the cost to reconstruct MADV_FREE pages can be higher
>> than the cost to allocate file cache page and read from disk?  Heavy
>> contention on mmap_sem?
>
> Allocating a page might be really costly but even without that.

File cache page needs to be allocated too.

> Reconstructnig the content of the page can be really high and actually
> much larger than an IO to get a page from the storage. Consider
> decompression or some other transformations to get the content.
> Userspace has means to detect that the content is still up-to-date as
> already has been mentioned so all those steps can be avoided.

Although I don't believe someone has really done this.  It's possible at
least in theory.

Best Regards,
Huang, Ying