Re: swapoff throttling and speedup?

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Wed, 03 Jun 2009 19:43:06 -0700
Joel Krauska <jkrauska@gmail.com> wrote:

> KAMEZAWA Hiroyuki wrote:
> >> 1. Has anyone tried making a nicer swapoff?
> >> Right now swapoff can be pretty aggressive if the system is otherwise
> >> heavily loaded.  On systems that I need to leave running other jobs,
> >> swapoff compounds the slowness of the system overall by burning up
> >> a single CPU and lots of IO
> >>
> >> I wrote a perl wrapper that briefly runs swapoff 
> >> and then kills it, but it would seem more reasonable to have a knob
> >> to make swapoff less aggressive. (max kb/s, etc)  
> >>
> >> It looked to me like the swapoff code was immediately hitting kernel 
> >> internals instead of doing more lifting itself (and making it 
> >> obvious where I could insert some sleeps)
> >>
> 
> I find I need a slower swapoff when a system that's already running very hot
> needs to be recovered from lots of swapping without overly impacting the other
> running processes.
> 
> The bulk of the work is still being done in normal RAM, and the overhead
> of consuming an entire CPU just for swapoff degrades my other running processes.
> 
> > How about throttling swapoff's cpu usage by cpu scheduler cgroup ?
> > No help ?
> 
> I think swapoff is all done as systemcalls, not in userspace, so I'm not
> sure that cgroups would apply here.  (granted I had never heard of control
> groups until just now...)
> 
IIUC, some "cond_resched()" , means "reschedule if necessary", are inserted to
swapoff's main loop. Then, limiting usage of cpu may have effects, I think.

Thanks,
-Kame

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