Re: [PATCH 11/12] vmscan: Write out dirty pages in batch

[Mel Gorman]

On Mon, Jun 14, 2010 at 09:15:15PM -0700, Andrew Morton wrote:
> On Tue, 15 Jun 2010 13:20:34 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> 
> > On Mon, Jun 14, 2010 at 06:39:57PM -0700, Andrew Morton wrote:
> > > On Tue, 15 Jun 2010 10:39:43 +1000 Dave Chinner <david@xxxxxxxxxxxxx> wrote:
> > > 
> > > > 
> > > > IOWs, IMO anywhere there is a context with significant queue of IO,
> > > > that's where we should be doing a better job of sorting before that
> > > > IO is dispatched to the lower layers. This is still no guarantee of
> > > > better IO (e.g. if the filesystem fragments the file) but it does
> > > > give the lower layers a far better chance at optimal allocation and
> > > > scheduling of IO...
> > > 
> > > None of what you said had much to do with what I said.
> > > 
> > > What you've described are implementation problems in the current block
> > > layer because it conflates "sorting" with "queueing".  I'm saying "fix
> > > that".
> > 
> > You can't sort until you've queued.
> 
> Yes you can.  That's exactly what you're recommending!  Only you're
> recommending doing it at the wrong level.  The fs-writeback radix-tree
> walks do it at the wrong level too.  Sorting should be done within, or
> in a layer above the block queues, not within the large number of
> individual callers.
> 
> > > And...  sorting at the block layer will always be superior to sorting
> > > at the pagecache layer because the block layer sorts at the physical
> > > block level and can handle not-well-laid-out files and can sort and merge
> > > pages from different address_spaces.
> > 
> > Yes it, can do that. And it still does that even if the higher
> > layers sort their I/O dispatch better,
> > 
> > Filesystems try very hard to allocate adjacent logical offsets in a
> > file in adjacent physical blocks on disk - that's the whole point of
> > extent-indexed filesystems. Hence with modern filesystems there is
> > generally a direct correlation between the page {mapping,index}
> > tuple and the physical location of the mapped block.
> > 
> > i.e. there is generally zero physical correlation between pages in
> > different mappings, but there is a high physical correlation
> > between the index of pages on the same mapping.
> 
> Nope.  Large-number-of-small-files is a pretty common case.  If the fs
> doesn't handle that well (ie: by placing them nearby on disk), it's
> borked.
> 
> > Hence by sorting
> > where we have a {mapping,index} context, we push out IO that is
> > much more likely to be in contiguous physical chunks that the
> > current random page shootdown.
> > 
> > We optimise applications to use these sorts of correlations all the
> > time to improve IO patterns. Why can't we make the same sort of
> > optmisations to the IO that the VM issues?
> 
> We can, but it shouldn't be specific to page reclaim.  Other places
> submit IO too, and want the same treatment.
> 
> > > Still, I suspect none of it will improve anything anyway.  Those pages
> > > are still dirty, possibly-locked and need to go to disk.  It doesn't
> > > matter from the MM POV whether they sit in some VM list or in the
> > > request queue.
> > 
> > Oh, but it does.....
> 
> The only difference is that pages which are in the queue (current
> implementation thereof) can't be shot down by truncate.
> 
> > > Possibly there may be some benefit to not putting so many of these
> > > unreclaimable pages into the queue all at the the same time.  But
> > > that's a shortcoming in the block code: we should be able to shove
> > > arbitrary numbers of dirty page (segments) into the queue and not gum
> > > the system up.  Don't try to work around that in the VM.
> > 
> > I think you know perfectly well why the system gums up when we
> > increase block layer queue depth: it's the fact that the _VM_ relies
> > on block layer queue congestion to limit the amount of dirty memory
> > in the system.
> 
> mm, a little bit still, I guess.  Mainly because dirty memory
> management isn't zone aware, so even though we limit dirty memory
> globally, a particular zone(set) can get excessively dirtied.
> 
> Most of this problem happen on the balance_dirty_pages() path, where we
> already sort the pages in ascending logical order.
> 
> > We've got a feedback loop between the block layer and the VM that
> > only works if block device queues are kept shallow. Keeping the
> > number of dirty pages under control is a VM responsibility, but it
> > is putting limitations on the block layer to ensure that the VM
> > works correctly.  If you want the block layer to have deep queues,
> > then someone needs to fix the VM not to require knowledge of the
> > internal operation of the block layer for correct operation.
> > 
> > Adding a few lines of code to sort a list in the VM is far, far
> > easier than redesigning the write throttling code....
> 
> It's a hack and a workaround.  And I suspect it won't make any
> difference, especially given Mel's measurements of the number of dirty
> pages he's seeing coming off the LRU.  Although those numbers may well
> be due to the new quite-low dirty memory thresholds.  
> 

I tested with a dirty ratio of 40 but I didn't see a major problem.
It's still a case with the tests I saw that direct reclaim of dirty pages
was a relatively rare event except when lumpy reclaim was involved. What
did change is the amount of scanning the direct reclaim and kswapd had to do
(both increased quite a bit) but the percentage of dirty pages encountered was
roughly the same (1-2% of scanned pages were dirty in the case of sysbench).

This is sysbench only rather than flooding with more data.

DIRTY RATIO == 20
FTrace Reclaim Statistics
                traceonly-v2r5  stackreduce-v2r5     nodirect-v2r5
Direct reclaims                               9843      13398      51651 
Direct reclaim pages scanned                871367    1008709    3080593 
Direct reclaim write async I/O               24883      30699          0 
Direct reclaim write sync I/O                    0          0          0 
Wake kswapd requests                       7070819    6961672   11268341 
Kswapd wakeups                                1578       1500        943 
Kswapd pages scanned                      22016558   21779455   17393431 
Kswapd reclaim write async I/O             1161346    1101641    1717759 
Kswapd reclaim write sync I/O                    0          0          0 
Time stalled direct reclaim (ms)             26.11      45.04       2.97 
Time kswapd awake (ms)                     5105.06    5135.93    6086.32 

User/Sys Time Running Test (seconds)        734.52    712.39     703.9
Percentage Time Spent Direct Reclaim         0.00%     0.00%     0.00%
Total Elapsed Time (seconds)               9710.02   9589.20   9334.45
Percentage Time kswapd Awake                 0.06%     0.00%     0.00%

DIRTY RATIO == 40
FTrace Reclaim Statistics
                traceonly-v2r5  stackreduce-v2r5     nodirect-v2r5
Direct reclaims                              29945      41887     163006 
Direct reclaim pages scanned               2853804    3075288   13142072 
Direct reclaim write async I/O               51498      63662          0 
Direct reclaim write sync I/O                    0          0          0 
Wake kswapd requests                      11899105   12466894   15645364 
Kswapd wakeups                                 945        891        522 
Kswapd pages scanned                      20401921   20674788   11319791 
Kswapd reclaim write async I/O             1381897    1332436    1711266 
Kswapd reclaim write sync I/O                    0          0          0 
Time stalled direct reclaim (ms)            131.78     165.08       5.47 
Time kswapd awake (ms)                     6321.11    6413.79    6687.67 

User/Sys Time Running Test (seconds)        709.91    718.39    664.28
Percentage Time Spent Direct Reclaim         0.00%     0.00%     0.00%
Total Elapsed Time (seconds)               9579.90   9700.42   9101.05
Percentage Time kswapd Awake                 0.07%     0.00%     0.00%

I guess what was really interesting was that even though raising the
dirty ratio allowed the test to complete faster, the percentage of time
spent in direct reclaim increased quite a lot. Again, just stopping
writeback in direct reclaim seemed to help.

> It would be interesting to code up a little test patch though, see if
> there's benefit to be had going down this path.
> 

I'll do this just to see what it looks like. To be frank, I lack taste when
it comes to how the block layer and filesystem should behave so am having
troube deciding if sorting the pages prior to submission is a good thing or
if it would just encourage bad or lax behaviour in the IO submission queueing.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab
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