Re: Free memory never fully used, swapping

[Andrew Morton <akpm@linux-foundation.org>]

(cc linux-mm, where all the suckiness ends up)

On Mon, 15 Nov 2010 11:52:46 -0800
Simon Kirby <sim@hostway.ca> wrote:

> Hi!
> 
> We're seeing cases on a number of servers where cache never fully grows
> to use all available memory.  Sometimes we see servers with 4 GB of
> memory that never seem to have less than 1.5 GB free, even with a
> constantly-active VM.  In some cases, these servers also swap out while
> this happens, even though they are constantly reading the working set
> into memory.  We have been seeing this happening for a long time;
> I don't think it's anything recent, and it still happens on 2.6.36.
> 
> I noticed that CONFIG_NUMA seems to enable some more complicated
> reclaiming bits and figured it might help since most stock kernels seem
> to ship with it now.  This seems to have helped, but it may just be
> wishful thinking.  We still see this happening, though maybe to a lesser
> degree.  (The following observations are with CONFIG_NUMA enabled.)
> 
> I was eyeballing "vmstat 1" and "watch -n.2 -d cat /proc/vmstat" at the
> same time, and I can see distinctly that the page cache is growing nicely
> until a sudden event where 400 MB is freed within 1 second, leaving
> this particular box with 700 MB free again.  kswapd numbers increase in
> /proc/vmstat, which leads me to believe that __alloc_pages_slowpath() has
> been called, since it seems to be the thing that wakes up kswapd.
> 
> Previous patterns and watching of "vmstat 1" show that the swapping out
> also seems to occur during the times that memory is quickly freed.
> 
> These are all x86_64, and so there is no highmem garbage going on. 
> The only zones would be for DMA, right?  Is the combination of memory
> fragmentation and large-order allocations the only thing that would be
> causing this reclaim here?  Is there some easy bake knob for finding what
> is causing the free memory jumps each time this happens?
> 
> Kernel config and munin graph of free memory here:
> 
> http://0x.ca/sim/ref/2.6.36/
> 
> I notice CONFIG_COMPACTION is still "EXPERIMENTAL".  Would it be worth
> trying here?  It seems to enable defrag before reclaim, but that sounds
> kind of ...complicated...
> 
> Cheers,
> 
> Simon-
> 
> procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
>  r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
>  1  0  11496 401684  40364 2531844    0    0  4540   773 7890 16291 13  9 77  1
>  3  0  11492 400180  40372 2534204    0    0  5572   699 8544 14856 25  9 66  1
>  0  0  11492 394344  40372 2540796    0    0  5256   345 8239 16723 17  7 73  2
>  0  0  11492 388524  40372 2546236    0    0  5216   393 8687 17289 14  9 76  1
>  4  1  11684 716296  40244 2218612    0  220  6868  1837 11124 27368 28 20 51  0
>  1  0  11732 753992  40248 2181468    0  120  5240   647 9542 15609 38 11 50  1
>  1  0  11712 736864  40260 2197788    0    0  5872  9147 9838 16373 41 11 47  1
>  0  0  11712 738096  40260 2196984    0    0  4628   493 7980 15536 22 10 67  1
>  2  0  11712 733508  40260 2201756    0    0  4404   418 7265 16867 10  9 80  2

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