Re: Free memory never fully used, swapping
From: Simon Kirby
Date: Wed Nov 24 2010 - 03:47:01 EST
On Tue, Nov 23, 2010 at 12:35:31AM -0800, Dave Hansen wrote:
> I wish. :) The best thing to do is to watch stuff like /proc/vmstat
> along with its friends like /proc/{buddy,meminfo,slabinfo}. Could you
> post some samples of those with some indication of where the bad
> behavior was seen?
>
> I've definitely seen swapping in the face of lots of free memory, but
> only in cases where I was being a bit unfair about the numbers of
> hugetlbfs pages I was trying to reserve.
So, Dave and I spent quite some time today figuring out was going on
here. Once load picked up during the day, kswapd actually never slept
until late in the afternoon. During the evening now, it's still waking
up in bursts, and still keeping way too much memory free:
http://0x.ca/sim/ref/2.6.36/memory_tonight.png
(NOTE: we did swapoff -a to keep /dev/sda from overloading)
We have a much better idea on what is happening here, but more questions.
This x86_64 box has 4 GB of RAM; zones are set up as follows:
[ 0.000000] Zone PFN ranges:
[ 0.000000] DMA 0x00000001 -> 0x00001000
[ 0.000000] DMA32 0x00001000 -> 0x00100000
[ 0.000000] Normal 0x00100000 -> 0x00130000
...
[ 0.000000] On node 0 totalpages: 1047279
[ 0.000000] DMA zone: 56 pages used for memmap
[ 0.000000] DMA zone: 0 pages reserved
[ 0.000000] DMA zone: 3943 pages, LIFO batch:0
[ 0.000000] DMA32 zone: 14280 pages used for memmap
[ 0.000000] DMA32 zone: 832392 pages, LIFO batch:31
[ 0.000000] Normal zone: 2688 pages used for memmap
[ 0.000000] Normal zone: 193920 pages, LIFO batch:31
So, "Normal" is relatively small, and DMA32 contains most of the RAM.
Watermarks from /proc/zoneinfo are:
Node 0, zone DMA
min 7
low 8
high 10
protection: (0, 3251, 4009, 4009)
Node 0, zone DMA32
min 1640
low 2050
high 2460
protection: (0, 0, 757, 757)
Node 0, zone Normal
min 382
low 477
high 573
protection: (0, 0, 0, 0)
This box has a couple bnx2 NICs, which do about 60 Mbps each. Jumbo
frames were disabled for now (to try to stop big order allocations), but
this did not stop atomic allocations of order 3 coming in, as found with:
perf record --event kmem:mm_page_alloc --filter 'order>=3' -a --call-graph -c 1 -a sleep 10
perf report
__alloc_pages_nodemask
alloc_pages_current
new_slab
__slab_alloc
__kmalloc_node_track_caller
__alloc_skb
__netdev_alloc_skb
bnx2_poll_work
>From my reading of this, it seems like __alloc_skb uses kmalloc(), and
kmalloc uses the kmalloc slab unless (unlikely(size > SLUB_MAX_SIZE)),
where SLUB_MAX_SIZE is 2 * PAGE_SIZE, in which case kmalloc_large is
called which allocates pages directly. This means that reception of
jumbo frames probably actually results in (consistent) smaller order
allocations! Anyway, these GFP_ATOMIC allocations don't seem to be
failing, BUT...
Right after kswapd goes to sleep, we're left with DMA32 with 421k or so
free pages, and Normal with 20k or so free pages (about 1.8 GB free).
Immediately, zone Normal starts being used until it reaches about 468
pages free in order 0, nothing else free. kswapd is not woken here,
but allocations just start coming from zone DMA32 instead. While this
happens, the occasional order=3 allocations coming in via the slab from
__alloc_skb seem to be picking away at the available order=3 chunks.
/proc/buddyinfo shows that there are 10k or so when it starts, so this
succeeds easily.
After a minute or so, available order-3 start reaching a lower number,
like 20 or so. order-4 then starts dropping as it is split into order-3,
until it reaches 20 or so as well. Then, order-3 hits 0, and kswapd is
woken. When this occurs, there are still a few order-5, order-6, etc.,
available. I presume the GFP_ATOMIC allocation can still split buddies
here, still making order-3 available without sleeping, because there is
no allocation failure message that I can see.
Here is a "while true; do sleep 1; grep -v 'DMA ' /proc/buddyinfo; done"
("DMA" zone is totally untouched, always, so excluded; white space
crushed to avoid wrapping), while it happens:
Node 0, zone DMA 2 1 1 2 1 1 1 0 1 1 3
Node 0, zone DMA32 25770 29441 14512 10426 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
...
Node 0, zone DMA32 23343 29405 6062 6478 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 23187 29358 6047 5960 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 23000 29372 6047 5411 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 22714 29391 6076 4225 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 22354 29459 6059 3178 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 22202 29388 6035 2395 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21971 29411 6036 1032 1901 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21514 29388 6019 433 1796 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21334 29387 6019 240 1464 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21237 29421 6052 216 1336 123 4 0 0 0 0
Node 0, zone Normal 455 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20968 29378 6020 244 751 123 4 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20741 29383 6022 134 272 123 4 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20476 29370 6024 117 48 116 4 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 20343 29369 6020 110 23 10 2 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 21592 30477 4856 22 10 4 2 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 24388 33261 1985 6 10 4 2 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 25358 34080 1068 0 4 4 2 0 0 0 0
Node 0, zone Normal 453 1 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 75985 68954 5345 87 1 4 2 0 0 0 0
Node 0, zone Normal 18249 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81117 71630 19261 429 3 4 2 0 0 0 0
Node 0, zone Normal 17908 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81226 71299 21038 569 19 4 2 0 0 0 0
Node 0, zone Normal 18559 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81347 71278 21068 640 19 4 2 0 0 0 0
Node 0, zone Normal 17928 21 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81370 71237 21241 1073 29 4 2 0 0 0 0
Node 0, zone Normal 18187 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81401 71237 21314 1139 29 4 2 0 0 0 0
Node 0, zone Normal 16978 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81410 71239 21314 1145 29 4 2 0 0 0 0
Node 0, zone Normal 18156 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81419 71232 21317 1160 30 4 2 0 0 0 0
Node 0, zone Normal 17536 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81347 71144 21443 1160 31 4 2 0 0 0 0
Node 0, zone Normal 18483 7 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81300 71059 21556 1178 38 4 2 0 0 0 0
Node 0, zone Normal 18528 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81315 71042 21577 1180 39 4 2 0 0 0 0
Node 0, zone Normal 18431 2 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81301 71002 21702 1202 39 4 2 0 0 0 0
Node 0, zone Normal 18487 5 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81301 70998 21702 1202 39 4 2 0 0 0 0
Node 0, zone Normal 18311 0 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81296 71025 21711 1208 45 4 2 0 0 0 0
Node 0, zone Normal 17092 5 0 0 0 0 0 0 0 0 0
Node 0, zone DMA32 81299 71023 21716 1226 45 4 2 0 0 0 0
Node 0, zone Normal 18225 12 0 0 0 0 0 0 0 0 0
Running a perf record on the kswapd wakeup right when it happens shows:
perf record --event vmscan:mm_vmscan_wakeup_kswapd -a --call-graph -c 1 -a sleep 10
perf trace
swapper-0 [002] 1323136.979119: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
swapper-0 [002] 1323136.979131: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
lmtp-20593 [003] 1323136.984066: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
lmtp-20593 [003] 1323136.984079: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
swapper-0 [001] 1323136.985511: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
swapper-0 [001] 1323136.985515: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
lmtp-20593 [003] 1323136.985673: mm_vmscan_wakeup_kswapd: nid=0 zid=2 order=3
lmtp-20593 [003] 1323136.985675: mm_vmscan_wakeup_kswapd: nid=0 zid=1 order=3
This causes kswapd to throw out a bunch of stuff from Normal and from
DMA32, to try to get zone_watermark_ok() to be happy for order=3.
However, we have a heavy read load from all of the email stored on SSDs
on this box, and kswapd ends up fighting to try to keep reclaiming the
allocations (mostly order-0). During the whole day, it never wins -- the
allocations are faster. At night, it wins after a minute or two. The
fighting is happening in all of the lines after it awakes above.
slabs_scanned, kswapd_steal, kswapd_inodesteal (slowly),
kswapd_skip_congestion_wait, and pageoutrun go up in vmstat while kswapd
is running. With the box up for 15 days, you can see it struggling on
pgscan_kswapd_normal (from /proc/vmstat):
pgfree 3329793080
pgactivate 643476431
pgdeactivate 155182710
pgfault 2649106647
pgmajfault 58157157
pgrefill_dma 0
pgrefill_dma32 19688032
pgrefill_normal 7600864
pgrefill_movable 0
pgsteal_dma 0
pgsteal_dma32 465191578
pgsteal_normal 651178518
pgsteal_movable 0
pgscan_kswapd_dma 0
pgscan_kswapd_dma32 768300403
pgscan_kswapd_normal 34614572907
pgscan_kswapd_movable 0
pgscan_direct_dma 0
pgscan_direct_dma32 2853983
pgscan_direct_normal 885799
pgscan_direct_movable 0
pginodesteal 191895
pgrotated 27290463
So, here are my questions.
Why do we care about order > 0 watermarks at all in the Normal zone?
Wouldn't it make a lot more sense to just make the DMA32 zone the only
one we care about for larger-order allocations? Or is this required for
the hugepage stuff?
The fact that so much stuff is evicted just because order-3 hits 0 is
crazy, especially when larger order pages are still free. It seems like
we're trying to keep large orders free here. Why? Maybe things would be
better if kswapd does not reclaim at all unless the requested order is
empty _and_ all orders above are empty. This would require hugepage
users to use CONFIG_COMPACT, and have _compaction_ occur the way the
watermark checks work now, but people without CONFIG_HUGETLB_PAGE could
just actually use the memory. Would this work?
There is logic at the end of balance_pgdat() to give up balancing order>0
and just try another loop with order = 0 if sc.nr_reclaimed is <
SWAP_CLUSTER_MAX. However, when this order=0 pass returns, the caller of
balance_pgdat(), kswapd(), gets true from sleeping_prematurely() and just
calls right back to balance_pgdat() again. I think this is why this
logic doesn't seem to work here.
Is my assumption about GFP_ATOMIC order=3 working even when order 3 is
empty, but order>3 is not? Regardless, shouldn't kswapd be woken before
order 3 is 0 since it may have nothing above order 3 to split from, thus
actually causing an allocation failure? Does something else do this?
Ok, that's enough for now.. :)
Simon-
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/