What kernel are you using? A change to keep pages consistent during writeout was landed not too long ago (maybe Linux 3.0) in order to allow checksumming of the data. We discussed doing copy-on-write, but there are relatively few mmap users and it wasn't clear whether the complexity was worth it. Cheers, Andreas On 2011-10-19, at 6:39 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > I have a real-time program that has everything mlocked (i.e. > mlockall(MCL_CURRENT | MCL_FUTURE)). It has some log files opened for > writing. Those files are opened and memset to zero in another thread > to fault everything in. The system is under light I/O load with very > little memory pressure. > > Latencytop shows frequent latency in the real-time threads. The main > offenders are: > > schedule sleep_on_page wait_on_page_bit ext4_page_mkwrite do_wp_page > handle_pte_fault handle_mm_fault do_page_fault page_fault > > schedule do_get_write_access jbd2_journal_get_write_access > __ext4_journal_get_write_access ext4_reserve_inode_write > ext4_mark_inode_dirty ext4_dirty_inode __mark_inode_dirty > file_update_time do_wp_page handle_pte_fault handle_mm_fault > > > I imagine the problem is that the system is periodically writing out > my dirty pages and marking them clean (and hence write protected). > When I try to write to them, the kernel makes them writable again, > which causes latency either due to updating the inode mtime or because > the file is being written to disk when I try to write to it. > > Is there any way to prevent this? One possibility would be a way to > ask the kernel not to write the file out to disk. Another would be a > way to ask the kernel to make a copy of the file when it writes it > disk and leave the original mapping writable. > > Obviously I can fix this by mapping anonymous memory, but then I need > another thread to periodically write my logs out to disk, and if that > crashes, I lose data. > > -- > Andy Lutomirski > AMA Capital Management, LLC > Office: (310) 553-5322 > Mobile: (650) 906-0647 > -- > To unsubscribe from this list: send the line "unsubscribe linux-ext4" in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
