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
