Hi Shaohua, On Sun, Jan 29, 2017 at 09:51:17PM -0800, Shaohua Li wrote: > We are trying to use MADV_FREE in jemalloc. Several issues are found. Without > solving the issues, jemalloc can't use the MADV_FREE feature. > - Doesn't support system without swap enabled. Because if swap is off, we can't > or can't efficiently age anonymous pages. And since MADV_FREE pages are mixed > with other anonymous pages, we can't reclaim MADV_FREE pages. In current > implementation, MADV_FREE will fallback to MADV_DONTNEED without swap enabled. > But in our environment, a lot of machines don't enable swap. This will prevent > our setup using MADV_FREE. > - Increases memory pressure. page reclaim bias file pages reclaim against > anonymous pages. This doesn't make sense for MADV_FREE pages, because those > pages could be freed easily and refilled with very slight penality. Even page > reclaim doesn't bias file pages, there is still an issue, because MADV_FREE > pages and other anonymous pages are mixed together. To reclaim a MADV_FREE > page, we probably must scan a lot of other anonymous pages, which is > inefficient. In our test, we usually see oom with MADV_FREE enabled and nothing > without it. Fully agreed, the anon LRU is a bad place for these pages. > For the first two issues, introducing a new LRU list for MADV_FREE pages could > solve the issues. We can directly reclaim MADV_FREE pages without writting them > out to swap, so the first issue could be fixed. If only MADV_FREE pages are in > the new list, page reclaim can easily reclaim such pages without interference > of file or anonymous pages. The memory pressure issue will disappear. Do we actually need a new page flag and a special LRU for them? These pages are basically like clean cache pages at that point. What do you think about clearing their PG_swapbacked flag on MADV_FREE and moving them to the inactive file list? The way isolate+putback works should not even need much modification, something like clear_page_mlock(). When the reclaim scanner finds anon && dirty && !swapbacked, it can again set PG_swapbacked and goto keep_locked to move the page back into the anon LRU to get reclaimed according to swapping rules. > For the third issue, we can add a separate RSS count for MADV_FREE pages. The > count will be increased in madvise syscall and decreased in page reclaim (eg, > unmap). One issue is activate_page(). A MADV_FREE page can be promoted to > active page there. But there isn't mm_struct context at that place. Iterating > vma there sounds too silly. The patchset don't fix this issue yet. Hopefully > somebody can share a hint how to fix this issue. This problem also goes away if we use the file LRUs. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
