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SubjectRe: kswap spinning


On Sat, 18 Aug 2001, Mark Hemment wrote:

> Hi,
>
> Jumping from 2.4.7 to 2.4.9 has shown up a problem with the VM balancing
> code.
> Under load, I've seen kswapd become a CPU hog (on a 5GB box). Now that
> I've got a theory, I cannot reproduce the problem for confirmation, but
> here is the theory anyway.
>
> The tests in free_shortage() and inactive_shortage() assume that the
> memory state for a zone can be improved by calling refill_inactive_scan(),
> page_launder(), the inode/dcache shrinking functions, and the general
> slab-cache reaping func. Usually, some combination of the reaping
> functions will improve a zone's state - but not always.
>
> The problem is the DMA zone. As it is (relatively) small on some archs
> (IA-32 for example), it is possible that almost all pages from this zone
> are being used by a sub-systems which simply won't give them up. Examples
> of use are; vmalloc()ed page for modules, pre-allocated socket buffs
> for NICs, task-structs/kernel-stack - you get the idea.
>
> In the case I believe I'm seeing, both free_shortage() and
> inactive_shortage() are returning a shortage for the DMA zone which
> triggers all the work in do_try_to_free_pages(). But as there aren't any
> DMA pages left in the page-cache, being used as anonymous pages, or being
> used in the other places the code looks, do_try_to_free_pages() returns
> non-zero to kswapd() and off we go again. This also causes callers of
> try_to_free_pages() (from __alloc_pages()) to suck CPU cycles as well.
>
> On HIGHMEM boxes, it is possible for the NORMAL zone to get into the
> same state (although v unlikely).
>
> I'd rather not get into having specialist code in mm/vmscan.c (or arch
> specific code) to handle a small DMA zone - so what are the other
> solutions? (Assuming the above theory holds true.)
>
> One possible solution is not to give DMA memory out, except for;
> o explicit DMA allocations
> o page-cache, anonymous page, allocations
> assuming explicit DMA allocations are low, we'll at least know the
> remaining DMA pages are somewhere we can get at them - would need to be
> trigger by an arch specific flag for those that don't have a "tiny" zone.
> However, I don't like this, feels like fixing the wrong problem. :(

The DMA zone problem _can_ happen, but it should also happen with 2.4.7 I
think. (I'm not completly sure, though. Maybe Linus changes to
try_to_free_pages() are the reason...)

Lets first find out the actual problem.

Could you please boot with profile=2 and use readprofile to find out where
kswapd is spending its time?

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