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    SubjectRe: Latency writing to an mlocked ext4 mapping
    On Tue, Oct 25, 2011 at 5:26 AM, Jan Kara <> wrote:
    > On Wed 19-10-11 22:59:55, Andy Lutomirski wrote:
    >> On Wed, Oct 19, 2011 at 7:17 PM, Andy Lutomirski <> wrote:
    >> > On Wed, Oct 19, 2011 at 6:15 PM, Andy Lutomirski <> wrote:
    >> >> On Wed, Oct 19, 2011 at 6:02 PM, Andreas Dilger <> wrote:
    >> >>> 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.
    >> >>
    >> >> 3.0.6, with no relevant patches.  (I have a one-liner added to the tcp
    >> >> code that I'll submit sometime soon.)  Would this explain the latency
    >> >> in file_update_time or is that a separate issue?  file_update_time
    >> >> seems like a good thing to make fully asynchronous (especially if the
    >> >> file in question is a fifo, but I've already moved my fifos to tmpfs).
    >> >
    >> > On, I got one instance of:
    >> >
    >> > call_rwsem_down_read_failed ext4_map_blocks ext4_da_get_block_prep
    >> > __block_write_begin ext4_da_write_begin ext4_page_mkwrite do_wp_page
    >> > handle_pte_fault handle_mm_fault do_page_fault page_fault
    >> >
    >> > but I'm not seeing the large numbers of the ext4_page_mkwrite trace
    >> > that I get on 3.0.6.  file_update_time is now by far the dominant
    >> > cause of latency.
    >> The culprit seems to be do_wp_page -> file_update_time ->
    >> mark_inode_dirty_sync.  This surprises me for two reasons:
    >>  - Why the _sync?  Are we worried that data will be written out before
    >> the metadata?  If so, surely there's a better way than adding latency
    >> here.
    >  _sync just means that inode will become dirty for fsync(2) purposes but
    > not for fdatasync(2) purposes - i.e. it's just a timestamp update (or
    > it could be something similar).
    >>  - Why are we calling file_update_time at all?  Presumably we also
    >> update the time when the page is written back (if not, that sounds
    >> like a bug, since the contents may be changed after something saw the
    >> mtime update), and, if so, why bother updating it on the first write?
    >> Anything that relies on this behavior is, I think, unreliable, because
    >> the page could be made writable arbitrarily early by another program
    >> that changes nothing.
    >  We don't update timestamp when the page is written back. I believe this
    > is mostly because we don't know whether the data has been changed by a
    > write syscall, which already updated the timestamp, or by mmap. That is
    > also the reason why we update the timestamp at page fault time.
    >  The reason why file_update_time() blocks for you is probably that it
    > needs to get access to buffer where inode is stored on disk and because a
    > transaction including this buffer is committing at the moment, your thread
    > has to wait until the transaction commit finishes. This is mostly a problem
    > specific to how ext4 works so e.g. xfs shouldn't have it.
    >  Generally I believe the attempts to achieve any RT-like latencies when
    > writing to a filesystem are rather hopeless. How much hopeless depends on
    > the load of the filesystem (e.g., in your case of mostly idle filesystem I
    > can imagine some tweaks could reduce your latencies to an acceptable level
    > but once the disk gets loaded you'll be screwed). So I'd suggest that
    > having RT thread just store log in memory (or write to a pipe) and have
    > another non-RT thread write the data to disk would be a much more robust
    > design.

    Windows seems to do pretty well at this, and I think it should be fixable on
    Linux too. "All" that needs to be done is to remove the pte_wrprotect from
    page_mkclean_one. The fallout from that might be unpleasant, though, but
    it would probably speed up a number of workloads.

    Adding a whole separate process just to copy data from memory to disk sounds
    a bit like a hack -- that's what mmap + mlock would do if it worked better.
    Incidentally, pipes are no good. I haven't root-caused it yet, but both reading
    to and writing from pipes, even if O_NONBLOCK, can block. I haven't root-caused
    it yet.

    Anyway, I'll start sending patches to whittle away at the problem,
    starting right now :)

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      Last update: 2011-10-29 01:39    [W:0.027 / U:194.520 seconds]
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