Re: Latency writing to an mlocked ext4 mapping

[Jan Kara]

On Mon 31-10-11 16:14:47, Andy Lutomirski wrote:
> On Mon, Oct 31, 2011 at 4:10 PM, Jan Kara <jack@xxxxxxx> wrote:
> > On Fri 28-10-11 16:37:03, Andy Lutomirski wrote:
> >> On Tue, Oct 25, 2011 at 5:26 AM, Jan Kara <jack@xxxxxxx> wrote:
> >> >>  - 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.
> >  Well, but Linux's mm pretty much depends the pte_wrprotect() so that's
> > unlikely to go away in a forseeable future. The reason is that we need to
> > reliably account the number of dirty pages so that we can throttle
> > processes that dirty too much of memory and also protect agaist system
> > going into out-of-memory problems when too many pages would be dirty (and
> > thus hard to reclaim). Thus we create clean pages as write-protected, when
> > they are first written to, we account them as dirtied and unprotect them.
> > When pages are cleaned by writeback, we decrement number of dirty pages
> > accordingly and write-protect them again.
> 
> What about skipping pte_wrprotect for mlocked pages and continuing to
> account them dirty even if they're actually clean?  This should be a
> straightforward patch except for the effect on stable pages for
> writeback.  (It would also have unfortunate side effects on
> ctime/mtime without my other patch to rearrange that code.)
  Well, doing proper dirty accounting would be a mess (you'd have to
unaccount dirty pages during munlock etc.) and I'm not sure what all would
break when page writes would not be coupled with page faults. So I don't
think it's really worth it.

Avoiding IO during a minor fault would be a decent thing which might be
worth pursuing. As you properly noted "stable pages during writeback"
requirement is one obstacle which won't be that trivial to avoid though...

								Honza
-- 
Jan Kara <jack@xxxxxxx>
SUSE Labs, CR
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