Re: Latency writing to an mlocked ext4 mapping

[Andreas Dilger <adilger@dilger.ca>]

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@amacapital.net> 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
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