LinuxQuestions.org - kswapd 100% CPU

- Linux - Server (https://www.linuxquestions.org/questions/linux-server-73/)

- - kswapd 100% CPU (https://www.linuxquestions.org/questions/linux-server-73/kswapd-100-cpu-687451/)

Hi All,
Description:
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 continuous 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 trys to balance order 10 of zone higmem ?

Details:
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)?

I'm wondering, does only one specific or more kernel versions exhibit this behaviour? Did you ever go on the LKML with this?

KSWAPD algorithm

First thank you for your reply :)
I encountered it on kernel 2.6.18, I don't know how to reach the people in LKML cause as I see it it might be a bug in the KSWAPD algorithm.

Quote:

Originally Posted by wassim_dagash (Post 3362649)

I encountered it on kernel 2.6.18

Better try a recent one first then?

Quote:

Originally Posted by wassim_dagash (Post 3362649)

I don't know how to reach the people in LKML

The LKML is at lkml.org. Maybe read http://www.kernel.org/pub/linux/docs/lkml/ before posting.

Is this problem fixed in later versions of Linux

Quote:

Originally Posted by wassim_dagash (Post 3360451)

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)?

I know this is years later, but I'm having what seems like a similar problem with Linux kernel version 2.6.18 although I don't have the capability to instrument the kernel to be sure.

There is a patch available titled "prevent kswapd from freeing excessive amounts of lowmem", which I think is in the 2.6.24 kernel.

wassim_dagash, did a later version of Linux fix your problem?

Ben Slade

Hey,
Version 2.6.18 does NOT include any patches regarding this issue, if I recall correctly it would be added on 2.6.32 or later.