Matthew Wilcox <willy@xxxxxxxxxxxxx> writes: > On Fri, Jul 26, 2024 at 09:46:18PM +1200, Barry Song wrote: >> A user space interface can be implemented to select different swap-in >> order policies, similar to the mTHP allocation order policy. We need >> a distinct policy because the performance characteristics of memory >> allocation differ significantly from those of swap-in. For example, >> SSD read speeds can be much slower than memory allocation. With >> policy selection, I believe we can implement mTHP swap-in for >> non-SWAP_SYNCHRONOUS scenarios as well. However, users need to understand >> the implications of their choices. I think that it's better to start >> with at least always never. I believe that we will add auto in the >> future to tune automatically, which can be used as default finally. > > I strongly disagree. Use the same sysctl as the other anonymous memory > allocations. I still believe we have some reasons for this tunable. 1. As Ryan pointed out in [1], swap-in with large mTHP orders may cause long latency, which some users might want to avoid. [1] https://lore.kernel.org/lkml/f0c7f061-6284-4fe5-8cbf-93281070895b@xxxxxxx/ 2. We have readahead information available for swap-in, which is unavailable for anonymous page allocation. This enables us to build an automatic swap-in order policy similar to that for page cache order based on readahead. 3. Swap-out/swap-in cycles present an opportunity to identify hot pages. In many use cases, we can utilize mTHP for hot pages and order-0 page for cold pages, especially under memory pressure. When an mTHP has been swapped out, it indicates that it could be a cold page. Converting it to order-0 pages might be a beneficial policy. -- Best Regards, Huang, Ying
