Re: [RFC PATCH 0/3] Avoid the use of congestion_wait under zonepressure
From: Mel Gorman
Date: Mon Mar 22 2010 - 19:51:18 EST
On Mon, Mar 15, 2010 at 01:09:35PM -0700, Andrew Morton wrote:
> On Mon, 15 Mar 2010 13:34:50 +0100
> Christian Ehrhardt <ehrhardt@xxxxxxxxxxxxxxxxxx> wrote:
>
> > c) If direct reclaim did reasonable progress in try_to_free but did not
> > get a page, AND there is no write in flight at all then let it try again
> > to free up something.
> > This could be extended by some kind of max retry to avoid some weird
> > looping cases as well.
> >
> > d) Another way might be as easy as letting congestion_wait return
> > immediately if there are no outstanding writes - this would keep the
> > behavior for cases with write and avoid the "running always in full
> > timeout" issue without writes.
>
> They're pretty much equivalent and would work. But there are two
> things I still don't understand:
>
> 1: Why is direct reclaim calling congestion_wait() at all? If no
> writes are going on there's lots of clean pagecache around so reclaim
> should trivially succeed. What's preventing it from doing so?
>
> 2: This is, I think, new behaviour. A regression. What caused it?
>
120+ kernels and a lot of hurt later;
Short summary - The number of times kswapd and the page allocator have been
calling congestion_wait and the length of time it spends in there
has been increasing since 2.6.29. Oddly, it has little to do
with the page allocator itself.
Test scenario
=============
X86-64 machine 1 socket 4 cores
4 consumer-grade disks connected as RAID-0 - software raid. RAID controller
on-board and a piece of crap, and a decent RAID card could blow
the budget.
Booted mem=256 to ensure it is fully IO-bound and match closer to what
Christian was doing
At each test, the disks are partitioned, the raid arrays created and an
ext2 filesystem created. iozone sequential read/write tests are run with
increasing number of processes up to 64. Each test creates 8G of files. i.e.
1 process = 8G. 2 processes = 2x4G etc
iozone -s 8388608 -t 1 -r 64 -i 0 -i 1
iozone -s 4194304 -t 2 -r 64 -i 0 -i 1
etc.
Metrics
=======
Each kernel was instrumented to collected the following stats
pg-Stall Page allocator stalled calling congestion_wait
pg-Wait The amount of time spent in congestion_wait
pg-Rclm Pages reclaimed by direct reclaim
ksd-stall balance_pgdat() (ie kswapd) staled on congestion_wait
ksd-wait Time spend by balance_pgdat in congestion_wait
Large differences in this do not necessarily show up in iozone because the
disks are so slow that the stalls are a tiny percentage overall. However, in
the event that there are many disks, it might be a greater problem. I believe
Christian is hitting a corner case where small delays trigger a much larger
stall.
Why The Increases
=================
The big problem here is that there was no one change. Instead, it has been
a steady build-up of a number of problems. The ones I identified are in the
block IO, CFQ IO scheduler, tty and page reclaim. Some of these are fixed
but need backporting and others I expect are a major surprise. Whether they
are worth backporting or not heavily depends on whether Christian's problem
is resolved.
Some of the "fixes" below are obviously not fixes at all. Gathering this data
took a significant amount of time. It'd be nice if people more familiar with
the relevant problem patches could spring a theory or patch.
The Problems
============
1. Block layer congestion queue async/sync difficulty
fix title: asyncconfusion
fixed in mainline? yes, in 2.6.31
affects: 2.6.30
2.6.30 replaced congestion queues based on read/write with sync/async
in commit 1faa16d2. Problems were identified with this and fixed in
2.6.31 but not backported. Backporting 8aa7e847 and 373c0a7e brings
2.6.30 in line with 2.6.29 performance. It's not an issue for 2.6.31.
2. TTY using high order allocations more frequently
fix title: ttyfix
fixed in mainline? yes, in 2.6.34-rc2
affects: 2.6.31 to 2.6.34-rc1
2.6.31 made pty's use the same buffering logic as tty. Unfortunately,
it was also allowed to make high-order GFP_ATOMIC allocations. This
triggers some high-order reclaim and introduces some stalls. It's
fixed in 2.6.34-rc2 but needs back-porting.
3. Page reclaim evict-once logic from 56e49d21 hurts really badly
fix title: revertevict
fixed in mainline? no
affects: 2.6.31 to now
For reasons that are not immediately obvious, the evict-once patches
*really* hurt the time spent on congestion and the number of pages
reclaimed. Rik, I'm afaid I'm punting this to you for explanation
because clearly you tested this for AIM7 and might have some
theories. For the purposes of testing, I just reverted the changes.
4. CFQ scheduler fairness commit 718eee057 causes some hurt
fix title: none available
fixed in mainline? no
affects: 2.6.33 to now
A bisection finger printed this patch as being a problem introduced
between 2.6.32 and 2.6.33. It increases a small amount the number of
times the page allocator stalls but drastically increased the number
of pages reclaimed. It's not clear why the commit is such a problem.
Unfortunately, I could not test a revert of this patch. The CFQ and
block IO changes made in this window were extremely convulated and
overlapped heavily with a large number of patches altering the same
code as touched by commit 718eee057. I tried reverting everything
made on and after this commit but the results were unsatisfactory.
Hence, there is no fix in the results below
Results
=======
Here are the highlights of kernels tested. I'm omitting the bisection
results for obvious reasons. The metrics were gathered at two points;
after filesystem creation and after IOZone completed.
The lower the number for each metric, the better.
After Filesystem Setup After IOZone
pg-Stall pg-Wait pg-Rclm ksd-stall ksd-wait pg-Stall pg-Wait pg-Rclm ksd-stall ksd-wait
2.6.29 0 0 0 2 1 4 3 183 152 0
2.6.30 1 5 34 1 25 783 3752 31939 76 0
2.6.30-asyncconfusion 0 0 0 3 1 44 60 2656 893 0
2.6.30.10 0 0 0 2 43 777 3699 32661 74 0
2.6.30.10-asyncconfusion 0 0 0 2 1 36 88 1699 1114 0
asyncconfusion can be back-ported easily to 2.6.30.10. Performance is not
perfectly in line with 2.6.29 but it's better.
2.6.31 0 0 0 3 1 49175 245727 2730626 176344 0
2.6.31-revertevict 0 0 0 3 2 31 147 1887 114 0
2.6.31-ttyfix 0 0 0 2 2 46238 231000 2549462 170912 0
2.6.31-ttyfix-revertevict 0 0 0 3 0 7 35 448 121 0
2.6.31.12 0 0 0 2 0 68897 344268 4050646 183523 0
2.6.31.12-revertevict 0 0 0 3 1 18 87 1009 147 0
2.6.31.12-ttyfix 0 0 0 2 0 62797 313805 3786539 173398 0
2.6.31.12-ttyfix-revertevict 0 0 0 3 2 7 35 448 199 0
Applying the tty fixes from 2.6.34-rc2 and getting rid of the evict-once
patches bring things back in line with 2.6.29 again.
Rik, any theory on evict-once?
2.6.32 0 0 0 3 2 44437 221753 2760857 132517 0
2.6.32-revertevict 0 0 0 3 2 35 14 1570 460 0
2.6.32-ttyfix 0 0 0 2 0 60770 303206 3659254 166293 0
2.6.32-ttyfix-revertevict 0 0 0 3 0 55 62 2496 494 0
2.6.32.10 0 0 0 2 1 90769 447702 4251448 234868 0
2.6.32.10-revertevict 0 0 0 3 2 148 597 8642 478 0
2.6.32.10-ttyfix 0 0 0 3 0 91729 453337 4374070 238593 0
2.6.32.10-ttyfix-revertevict 0 0 0 3 1 65 146 3408 347 0
Again, fixing tty and reverting evict-once helps bring figures more in line
with 2.6.29.
2.6.33 0 0 0 3 0 152248 754226 4940952 267214 0
2.6.33-revertevict 0 0 0 3 0 883 4306 28918 507 0
2.6.33-ttyfix 0 0 0 3 0 157831 782473 5129011 237116 0
2.6.33-ttyfix-revertevict 0 0 0 2 0 1056 5235 34796 519 0
2.6.33.1 0 0 0 3 1 156422 776724 5078145 234938 0
2.6.33.1-revertevict 0 0 0 2 0 1095 5405 36058 477 0
2.6.33.1-ttyfix 0 0 0 3 1 136324 673148 4434461 236597 0
2.6.33.1-ttyfix-revertevict 0 0 0 1 1 1339 6624 43583 466 0
At this point, the CFQ commit "cfq-iosched: fairness for sync no-idle
queues" has lodged itself deep within CGQ and I couldn't tear it out or
see how to fix it. Fixing tty and reverting evict-once helps but the number
of stalls is significantly increased and a much larger number of pages get
reclaimed overall.
Corrado?
2.6.34-rc1 0 0 0 1 1 150629 746901 4895328 239233 0
2.6.34-rc1-revertevict 0 0 0 1 0 2595 12901 84988 622 0
2.6.34-rc1-ttyfix 0 0 0 1 1 159603 791056 5186082 223458 0
2.6.34-rc1-ttyfix-revertevict 0 0 0 0 0 1549 7641 50484 679 0
Again, ttyfix and revertevict help a lot but CFQ needs to be fixed to get
back to 2.6.29 performance.
Next Steps
==========
Jens, any problems with me backporting the async/sync fixes from 2.6.31 to
2.6.30.x (assuming that is still maintained, Greg?)?
Rik, any suggestions on what can be done with evict-once?
Corrado, any suggestions on what can be done with CFQ?
Christian, can you test the following amalgamated patch on 2.6.32.10 and
2.6.33 please? Note it's 2.6.32.10 because the patches below will not apply
cleanly to 2.6.32 but it will against 2.6.33. It's a combination of ttyfix
and revertevict. If your problem goes away, it implies that the stalls I
can measure are roughly correlated to the more significant problem you have.
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