From: Huang Ying <ying.huang@xxxxxxxxx> This patchset is to optimize the performance of Transparent Huge Page (THP) swap. Recently, the performance of the storage devices improved so fast that we cannot saturate the disk bandwidth with single logical CPU when do page swap out even on a high-end server machine. Because the performance of the storage device improved faster than that of single logical CPU. And it seems that the trend will not change in the near future. On the other hand, the THP becomes more and more popular because of increased memory size. So it becomes necessary to optimize THP swap performance. The advantages of the THP swap support include: - Batch the swap operations for the THP to reduce lock acquiring/releasing, including allocating/freeing the swap space, adding/deleting to/from the swap cache, and writing/reading the swap space, etc. This will help improve the performance of the THP swap. - The THP swap space read/write will be 2M sequential IO. It is particularly helpful for the swap read, which are usually 4k random IO. This will improve the performance of the THP swap too. - It will help the memory fragmentation, especially when the THP is heavily used by the applications. The 2M continuous pages will be free up after THP swapping out. - It will improve the THP utilization on the system with the swap turned on. Because the speed for khugepaged to collapse the normal pages into the THP is quite slow. After the THP is split during the swapping out, it will take quite long time for the normal pages to collapse back into the THP after being swapped in. The high THP utilization helps the efficiency of the page based memory management too. There are some concerns regarding THP swap in, mainly because possible enlarged read/write IO size (for swap in/out) may put more overhead on the storage device. To deal with that, the THP swap in should be turned on only when necessary. For example, it can be selected via "always/never/madvise" logic, to be turned on globally, turned off globally, or turned on only for VMA with MADV_HUGEPAGE, etc. This patchset is based on 04/13 head of mmotm/master. This patchset is the first step for the THP swap support. The plan is to delay splitting THP step by step, finally avoid splitting THP during the THP swapping out and swap out/in the THP as a whole. As the first step, in this patchset, the splitting huge page is delayed from almost the first step of swapping out to after allocating the swap space for the THP and adding the THP into the swap cache. This will reduce lock acquiring/releasing for the locks used for the swap cache management. With the patchset, the swap out throughput improves 15.5% (from about 3.73GB/s to about 4.31GB/s) in the vm-scalability swap-w-seq test case with 8 processes. The test is done on a Xeon E5 v3 system. The swap device used is a RAM simulated PMEM (persistent memory) device. To test the sequential swapping out, the test case creates 8 processes, which sequentially allocate and write to the anonymous pages until the RAM and part of the swap device is used up. Changelog: v9: - Rebased on latest -mm tree - Johannes done extensive cleanups and simplifications - Johannes resolved the code size increases - Update the test results v8: - Rebased on latest -mm tree - Reorganize the patchset per Johannes' comments - Merge add_to_swap_trans_huge() and add_to_swap() per Johannes' comments v7: - Rebased on latest -mm tree - Revise get_swap_pages() THP support per Tim's comments v6: - Rebased on latest -mm tree (cluster lock, etc). - Fix a potential uninitialized variable bug in __swap_entry_free() - Revise the swap read-ahead changes to avoid a potential race condition between swap off and swap out in theory. v5: - Per Hillf's comments, fix a locking bug in error path of __add_to_swap_cache(). And merge the code to calculate extra_pins into can_split_huge_page(). v4: - Per Johannes' comments, simplified swap cgroup array accessing code. - Per Kirill and Dave Hansen's comments, used HPAGE_PMD_NR instead of HPAGE_SIZE/PAGE_SIZE. - Per Anshuman's comments, used HPAGE_PMD_NR instead of 512 in patch description. v3: - Per Andrew's suggestion, used a more systematical way to determine whether to enable THP swap optimization - Per Andrew's comments, moved as much as possible code into #ifdef CONFIG_TRANSPARENT_HUGE_PAGE/#endif or "if (PageTransHuge())" - Fixed some coding style warning. v2: - Original [1/11] sent separately and merged - Use switch in 10/10 per Hiff's suggestion Best Regards, Huang, Ying -- 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>
