KSWAPD Algorithm - 100% CPU

From: wassim dagash
Date: Wed Dec 03 2008 - 02:05:01 EST

Hi All,
I countered a weird problem with kswapd:
it runs in some infinite loop trying to swap until order 10 of zone
highmem is OK, While zone higmem (as I understand) has nothing to do
with contiguous memory (cause there is no 1-1 mapping) which means
kswapd will continue to try to balance order 10 of zone highmem
forever (or until someone release a very large chunk of highmem).
Can anyone please explain me the algorithm of kswapd and why it tries
to balance order 10 of zone higmem ?

I build an instrumented kernel with debug messages in
"zone_watermark_ok" function, and from the code and debug messages I
see that "zone_watermark_ok" returns 0 when kswapd invokes it (through
balance_pgdat) in order to decide if zone highmem is balanced or not,
which lead in some configurations to infinite loop of kswapd ( if no
large chunks of highmem released) . I added a condition to
"balance_pgdat" so it doesn't try to balance order higher than 1 in
zone highmem and this conditon solved the problem, what are the risks
with such solution? isn't it a bug that kswapd is looking for
continuous memory in zone highmem ( as I understand there is no 1-1
mapping in zone highmem which is meaningless in kswapd)?


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