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SubjectRe: [PATCH 0/6] [RFC] writeback: try to write older pages first
On Mon, Jul 26, 2010 at 03:18:03PM +0800, Wu Fengguang wrote:
> > On Thu, Jul 22, 2010 at 01:09:28PM +0800, Wu Fengguang wrote:
> > >
> > > The basic way of avoiding pageout() is to make the flusher sync inodes in the
> > > right order. Oldest dirty inodes contains oldest pages. The smaller inode it
> > > is, the more correlation between inode dirty time and its pages' dirty time.
> > > So for small dirty inodes, syncing in the order of inode dirty time is able to
> > > avoid pageout(). If pageout() is still triggered frequently in this case, the
> > > 30s dirty expire time may be too long and could be shrinked adaptively; or it
> > > may be a stressed memcg list whose dirty inodes/pages are more hard to track.
> > >
> >
> > Have you confirmed this theory with the trace points? It makes perfect
> > sense and is very rational but proof is a plus.
>
> The proof would be simple.
>
> On average, it takes longer time to dirty a large file than a small file.
>
> For example, when uploading files to a file server with 1MB/s
> throughput, it will take 10s for a 10MB file and 30s for a 30MB file.
> This is the common case.
>
> Another case is some fast dirtier. It may take 10ms to dirty a 100MB
> file and 10s to dirty a 1G file -- the latter is dirty throttled to
> the much lower IO throughput due to too many dirty pages. The opposite
> may happen, however this is more likely in possibility. If both are
> throttled, it degenerates to the above file server case.
>
> So large files tend to contain dirty pages of more varied age.
>

Ok.

> > I'm guessing you have
> > some decent writeback-related tests that might be of use. Mine have a
> > big mix of anon and file writeback so it's not as clear-cut.
>
> A neat trick is to run your test with `swapoff -a` :)
>

Good point.

> Seriously I have no scripts to monitor pageout() calls.
> I'll explore ways to test it.
>

I'll see about running tests with swapoff.

> > Monitoring it isn't hard. Mount debugfs, enable the vmscan tracepoints
> > and read the tracing_pipe. To reduce interference, I always pipe it
> > through gzip and do post-processing afterwards offline with the script
> > included in Documentation/
>
> Thanks for the tip!
>
> > Here is what I got from sysbench on x86-64 (other machines hours away)
> >
> >
> > SYSBENCH FTrace Reclaim Statistics
> > traceonly-v5r6 nodirect-v5r7 flusholdest-v5r7 flushforward-v5r7
> > Direct reclaims 683 785 670 938
> > Direct reclaim pages scanned 199776 161195 200400 166639
> > Direct reclaim write file async I/O 64802 0 0 0
> > Direct reclaim write anon async I/O 1009 419 1184 11390
> > Direct reclaim write file sync I/O 18 0 0 0
> > Direct reclaim write anon sync I/O 0 0 0 0
> > Wake kswapd requests 685360 697255 691009 864602
> > Kswapd wakeups 1596 1517 1517 1545
> > Kswapd pages scanned 17527865 16817554 16816510 15032525
> > Kswapd reclaim write file async I/O 888082 618123 649167 147903
> > Kswapd reclaim write anon async I/O 229724 229123 233639 243561
> > Kswapd reclaim write file sync I/O 0 0 0 0
> > Kswapd reclaim write anon sync I/O 0 0 0 0
>
> > Time stalled direct reclaim (ms) 32.79 22.47 19.75 6.34
> > Time kswapd awake (ms) 2192.03 2165.17 2112.73 2055.90
>
> I noticed that $total_direct_latency is divided by 1000 before
> printing the above lines, so the unit should be seconds?
>

Correct. That figure was generated by another post-processing script that
creates the table. It got the units wrong so the percentage time lines are
wrong and the time spent awake is saying ms when it should say seconds. Sorry
about that.

> > User/Sys Time Running Test (seconds) 663.3 656.37 664.14 654.63
> > Percentage Time Spent Direct Reclaim 0.00% 0.00% 0.00% 0.00%
> > Total Elapsed Time (seconds) 6703.22 6468.78 6472.69 6479.62
> > Percentage Time kswapd Awake 0.03% 0.00% 0.00% 0.00%
>
> I don't see the code for generating the "Percentage" lines. And the
> numbers seem too small to be true.
>

The code is in a table-generation script that had access to data on the
length of time the test ran. I was ignoring the percentage time line
since early on and I missed the error.

The percentage time spent on direct reclaim is

direct_reclaim*100/(user_time+sys_time+stalled_time)

A report based on a corrected scipt looks like

traceonly-v5r6 nodirect-v5r9 flusholdest-v5r9 flushforward-v5r9
Direct reclaims 683 528 808 943
Direct reclaim pages scanned 199776 298562 125991 83325
Direct reclaim write file async I/O 64802 0 0 0
Direct reclaim write anon async I/O 1009 3340 926 2227
Direct reclaim write file sync I/O 18 0 0 0
Direct reclaim write anon sync I/O 0 0 0 0
Wake kswapd requests 685360 522123 763448 827895
Kswapd wakeups 1596 1538 1452 1565
Kswapd pages scanned 17527865 17020235 16367809 15415022
Kswapd reclaim write file async I/O 888082 869540 536427 89004
Kswapd reclaim write anon async I/O 229724 262934 253396 215861
Kswapd reclaim write file sync I/O 0 0 0 0
Kswapd reclaim write anon sync I/O 0 0 0 0
Time stalled direct reclaim (seconds) 32.79 23.46 20.70 7.01
Time kswapd awake (seconds) 2192.03 2172.22 2117.82 2166.53

User/Sys Time Running Test (seconds) 663.3 644.43 637.34 680.53
Percentage Time Spent Direct Reclaim 4.71% 3.51% 3.15% 1.02%
Total Elapsed Time (seconds) 6703.22 6477.95 6503.39 6781.90
Percentage Time kswapd Awake 32.70% 33.53% 32.56% 31.95%

> > Flush oldest actually increased the number of pages written back by
> > kswapd but the anon writeback is also high as swap is involved. Kicking
> > flusher threads also helps a lot. It helps less than previous released
> > because I noticed I was kicking flusher threads for both anon and file
> > dirty pages which is cheating. It's now only waking the threads for
> > file. It's still a reduction of 84% overall so nothing to sneeze at.
> >
> > What the patch did do was reduce time stalled in direct reclaim and time
> > kswapd spent awake so it still might be going the right direction. I
> > don't have a feeling for how much the writeback figures change between
> > runs because they take so long to run.
> >
> > STRESS-HIGHALLOC FTrace Reclaim Statistics
> > stress-highalloc stress-highalloc stress-highalloc stress-highalloc
> > traceonly-v5r6 nodirect-v5r7 flusholdest-v5r7 flushforward-v5r7
> > Direct reclaims 1221 1284 1127 1252
> > Direct reclaim pages scanned 146220 186156 142075 140617
> > Direct reclaim write file async I/O 3433 0 0 0
> > Direct reclaim write anon async I/O 25238 28758 23940 23247
> > Direct reclaim write file sync I/O 3095 0 0 0
> > Direct reclaim write anon sync I/O 10911 305579 281824 246251
> > Wake kswapd requests 1193 1196 1088 1209
> > Kswapd wakeups 805 824 758 804
> > Kswapd pages scanned 30953364 52621368 42722498 30945547
> > Kswapd reclaim write file async I/O 898087 241135 570467 54319
> > Kswapd reclaim write anon async I/O 2278607 2201894 1885741 1949170
> > Kswapd reclaim write file sync I/O 0 0 0 0
> > Kswapd reclaim write anon sync I/O 0 0 0 0
> > Time stalled direct reclaim (ms) 8567.29 6628.83 6520.39 6947.23
> > Time kswapd awake (ms) 5847.60 3589.43 3900.74 15837.59
> >
> > User/Sys Time Running Test (seconds) 2824.76 2833.05 2833.26 2830.46
> > Percentage Time Spent Direct Reclaim 0.25% 0.00% 0.00% 0.00%
> > Total Elapsed Time (seconds) 10920.14 9021.17 8872.06 9301.86
> > Percentage Time kswapd Awake 0.15% 0.00% 0.00% 0.00%
> >
> > Same here, the number of pages written back by kswapd increased but
> > again anon writeback was a big factor. Kicking threads when dirty pages
> > are encountered still helps a lot with a 94% reduction of pages written
> > back overall..
>
> That is impressive! So it definitely helps to reduce total number of
> dirty pages under memory pressure.
>

Yes.

> > Also, your patch really helped the time spent stalled by direct reclaim
> > and kswapd was awake a lot less less with tests completing far faster.
>
> Thanks. So it does improve the dirty page layout in the LRU lists.
>

It would appear to.

> > Overally, I still think your series if a big help (although I don't know if
> > the patches in linux-next are also making a difference) but it's not actually
> > reducing the pages encountered by direct reclaim. Maybe that is because
> > the tests were making more forward progress and so scanning faster. The
> > sysbench performance results are too varied to draw conclusions from but it
> > did slightly improve the success rate of high-order allocations.
> >
> > The flush-forward patches would appear to be a requirement. Christoph
> > first described them as a band-aid but he didn't chuck rocks at me when
> > the patch was actually released. Right now, I'm leaning towards pushing
> > it and judge by the Swear Meter how good/bad others think it is. So far
> > it's, me pro, Rik pro, Christoph maybe.
>
> Sorry for the delay, I'll help review it.
>

Don't be sorry, I still haven't reviewed the writeback patches.

> > > For a large dirty inode, it may flush lots of newly dirtied pages _after_
> > > syncing the expired pages. This is the normal case for a single-stream
> > > sequential dirtier, where older pages are in lower offsets. In this case we
> > > shall not insist on syncing the whole large dirty inode before considering the
> > > other small dirty inodes. This risks wasting time syncing 1GB freshly dirtied
> > > pages before syncing the other N*1MB expired dirty pages who are approaching
> > > the end of the LRU list and hence pageout().
> > >
> >
> > Intuitively, this makes a lot of sense.
> >
> > > For a large dirty inode, it may also flush lots of newly dirtied pages _before_
> > > hitting the desired old ones, in which case it helps for pageout() to do some
> > > clustered writeback, and/or set mapping->writeback_index to help the flusher
> > > focus on old pages.
> > >
> >
> > Will put this idea on the maybe pile.
> >
> > > For a large dirty inode, it may also have intermixed old and new dirty pages.
> > > In this case we need to make sure the inode is queued for IO before some of
> > > its pages hit pageout(). Adaptive dirty expire time helps here.
> > >
> > > OK, end of the vapour ideas. As for this patchset, it fixes the current
> > > kupdate/background writeback priority:
> > >
> > > - the kupdate/background writeback shall include newly expired inodes at each
> > > queue_io() time, as the large inodes left over from previous writeback rounds
> > > are likely to have less density of old pages.
> > >
> > > - the background writeback shall consider expired inodes first, just like the
> > > kupdate writeback
> > >
> >
> > I haven't actually reviewed these. I got testing kicked off first
> > because it didn't require brains :)
>
> Thanks all the same!
>

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


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