Hi, 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. - Accounting. There are two accounting problems. We don't have a global accounting. If the system is abnormal, we don't know if it's a problem from MADV_FREE side. The other problem is RSS accounting. MADV_FREE pages are accounted as normal anon pages and reclaimed lazily, so application's RSS becomes bigger. This confuses our workloads. We have monitoring daemon running and if it finds applications' RSS becomes abnormal, the daemon will kill the applications even kernel can reclaim the memory easily. To address the first the two issues, we can either put MADV_FREE pages into a separate LRU list (Minchan's previous patches and V1 patches), or put them into LRU_INACTIVE_FILE list (suggested by Johannes). The patchset use the second idea. The reason is LRU_INACTIVE_FILE list is tiny nowadays and should be full of used once file pages. So we can still efficiently reclaim MADV_FREE pages there without interference with other anon and active file pages. Putting the pages into inactive file list also has an advantage which allows page reclaim to prioritize MADV_FREE pages and used once file pages. MADV_FREE pages are put into the lru list and clear SwapBacked flag, so PageAnon(page) && !PageSwapBacked(page) will indicate a MADV_FREE pages. These pages will directly freed without pageout if they are clean, otherwise normal swap will reclaim them. For the third issue, the previous post adds global accounting and a separate RSS count for MADV_FREE pages. The problem is we never get accurate accounting for MADV_FREE pages. The pages are mapped to userspace, can be dirtied without notice from kernel side. To get accurate accounting, we could write protect the page, but then there is extra page fault overhead, which people don't want to pay. Jemalloc guys have concerns about the inaccurate accounting, so this post drops the accounting patches temporarily. The info exported to /proc/pid/smaps for MADV_FREE pages are kept, which is the only place we can get accurate accounting right now. Thanks, Shaohua V3->V4: - rebase to latest -mm tree - Address several issues pointed out by Johannes and Minchan - Dropped vmstat and RSS accounting V2->V3: - rebase to latest -mm tree - Address severl issues pointed out by Minchan - Add more descriptions http://marc.info/?l=linux-mm&m=148710098701674&w=2 V1->V2: - Put MADV_FREE pages into LRU_INACTIVE_FILE list instead of adding a new lru list, suggested by Johannes - Add RSS support http://marc.info/?l=linux-mm&m=148616481928054&w=2 Minchan previous patches: http://marc.info/?l=linux-mm&m=144800657002763&w=2 ---------------------- Shaohua Li (6): mm: delete unnecessary TTU_* flags mm: don't assume anonymous pages have SwapBacked flag mm: move MADV_FREE pages into LRU_INACTIVE_FILE list mm: reclaim MADV_FREE pages mm: enable MADV_FREE for swapless system proc: show MADV_FREE pages info in smaps Documentation/filesystems/proc.txt | 4 +++ fs/proc/task_mmu.c | 8 +++++- include/linux/rmap.h | 4 +-- include/linux/swap.h | 2 +- include/linux/vm_event_item.h | 2 +- mm/huge_memory.c | 6 ++-- mm/khugepaged.c | 8 ++---- mm/madvise.c | 11 ++------ mm/memory-failure.c | 2 +- mm/migrate.c | 3 +- mm/rmap.c | 15 +++++++--- mm/swap.c | 56 +++++++++++++++++++++++--------------- mm/vmscan.c | 45 +++++++++++++++++------------- mm/vmstat.c | 1 + 14 files changed, 96 insertions(+), 71 deletions(-) -- 2.9.3 -- 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>
