Re: Hardcore trashing without any swap

[LÃszlà Monda]

On Fri, Jun 11, 2010 at 3:47 PM, Ed Tomlinson <edt@xxxxxx> wrote:
> On Friday 11 June 2010 08:53:50 LÃszlà Monda wrote:
>> On Fri, Jun 11, 2010 at 2:16 PM, Alan Cox <alan@xxxxxxxxxxxxxxxxxxx> wrote:
>> > On Fri, 11 Jun 2010 02:10:33 +0200
>> > LÃszlà Monda <laci@xxxxxxxx> wrote:
>> >
>> >> Hi List,
>> >>
>> >> The problem I'm facing with is very simple, yet extremely irritating
>> >> in nature. ÂI have a laptop with 4G RAM and I don't use any swap.
>> >> Whenever the RAM is full my system keeps trashing. ÂThis makes X and
>> >> SSH completely unresponsive for about a hour then a bunch of processes
>> >> gets killed and it's usable again.
>> >>
>> >> How is possible that my system is trashing even though I don't use any swap?
>> >
>> > Because you don't have any swap. Its having to dump stuff it doesn't want
>> > to like bits of applications that it can retrieve back from disk.
>>
>> I can read what you wrote but cannot really understand it. ÂPlease
>> tell me where my logic fails:
>>
>> No swap -> no dedicated space on disk to dump stuff -> no disk I/O
>> should happen at all
>
> No. ÂThis is not the case. ÂIf the vm needs memory it will discard pages from that are
> backed by objects _not_ stored in swap - like executables. ÂOnly if there is nothing to
> discard will it start killing... ÂThat being said you need to read up on what Alan's
> suggestion below does - or add a swapfile (which works nearly as well as a swap partition
> now).

By reading your reply I think I understood what's going on.

Previously I thought that trashing can only happen because the kernel
discards pages to swap which I thought is the only reason why disk I/O
can happen in such a case.  It didn't made sense to me because I don't
use swap.

The other possibility that occured to me is the following scenario:
1) Process A grows really big and fills up the RAM.
2) The kernel discards code pages from process B in favor of process A.
3) Process A has some RAM and keeps growing further until it almost
fills up the RAM.
4) Process B gets scheduled and needs to be paged in which makes
process A paged out, hence disk I/O occurs.
5) Process A gets scheduled and needs to be paged in which makes
process B paged out, hence disk I/O occurs.
6) Go to 4)

This is not truly an infinite loop because the memory gradually gets
filled up in 4) and 5) but it happens really slowly because process
code has to be paged in upon every rescheduling.

So I've just realized that trashing cannot only happen due to paging
out to swap.  It can also happen due to simply discarding code pages
and paging them in later.

Is the above scenario valid in my case?

I suppose that the OOM killer could kill process A in step 3) right
away but the scheduler is fast and the 4)-5) loop keeps making disk
I/O for a very long time until process A exhausts the memory and the
OOM killer intervenes eventually.

I've also realized that it's probably impossible to truly reliably
foresee trashing so that's why there are no out of the box solutions
for my problem.  The best hack I can think of is a daemon that
monitors memory usage in about every 10 milisec and kills the biggest
process if something seems to go wrong before trashing can begin.

>> >> I'd expect the kernel to immediately kill the largest process without
>> >> any trashing so I could continue my work right after the event. ÂHow
>> >> is it possible to configure?
>
> See above. ÂIts not out of memory that it can discard without killing...
>
>> > It isn't.
>> >
>> > However if you want to avoid overcommit and thrashing play with
>> >
>> > /proc/sys/vm/overcommit*

Although I feel that my understanding about this topic is rather
vague, it seems to me that the best settings for me is policy 0 and 0
percentage.  Would this setting avoid trashing with no swap?

-- 
LÃszlà Monda <http://monda.hu>
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