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SubjectRe: Free memory never fully used, swapping

(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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