On Mon 02-03-20 22:12:53, Huang, Ying wrote: > Michal Hocko <mhocko@xxxxxxxxxx> writes: [...] > > And MADV_FREE pages are a kind of cache as well. If the target node is > > short on memory then those will be reclaimed as a cache so a > > pro-active freeing sounds counter productive as you do not have any > > idea whether that cache is going to be used in future. In other words > > you are not going to free a clean page cache if you want to use that > > memory as a migration target right? So you should make a clear case > > about why MADV_FREE cache is less important than the clean page cache > > and ideally have a good justification backed by real workloads. > > Clean page cache still have valid contents, while clean MADV_FREE pages > has no valid contents. So penalty of discarding the clean page cache is > reading from disk, while the penalty of discarding clean MADV_FREE pages > is just page allocation and zeroing. And "just page allocation and zeroing" overhead is the primary motivation to keep the page in memory. It is a decision of the workload to use MADV_FREE because chances are that this will speed things up. All that with a contract that the memory goes away under memory pressure so with a good workload/memory sizing you do not really lose that optimization. Now you want to make decision on behalf of the consumer of the MADV_FREE memory. > I understand that MADV_FREE is another kind of cache and has its value. > But in the original implementation, during migration, we have already > freed the original "cache", then reallocate the cache elsewhere and > copy. This appears more like all pages are populated in mmap() always. > I know there's value to populate all pages in mmap(), but does that need > to be done always by default? It is not. You have to explicitly request MAP_POPULATE to initialize mmap. -- Michal Hocko SUSE Labs
