Re: Free memory never fully used, swapping

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On Wed, 2010-11-24 at 16:46 +0800, Simon Kirby wrote:
> 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?
even kswapd is woken after order>3 is empty, the issue will occur since
the order > 3 pages will be used soon and kswapd still needs to reclaim
some pages. So the issue is there is high order page allocation and
lumpy reclaim wrongly reclaims some pages. maybe you should use slab
instead of slub to avoid high order allocation.

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