Re: [PATCH] makedumpfile: change the wrong code to calculate bufsize_cyclic for elf dump

From: Vivek Goyal <vgoyal@redhat.com>
To: "bhe@redhat.com" <bhe@redhat.com>
Cc: "kexec@lists.infradead.org" <kexec@lists.infradead.org>,
	"d.hatayama@jp.fujitsu.com" <d.hatayama@jp.fujitsu.com>,
	Atsushi Kumagai <kumagai-atsushi@mxc.nes.nec.co.jp>,
	"zzou@redhat.com" <zzou@redhat.com>
Subject: Re: [PATCH] makedumpfile: change the wrong code to calculate bufsize_cyclic for elf dump
Date: Mon, 21 Apr 2014 11:14:30 -0400	[thread overview]
Message-ID: <20140421151429.GC4367@redhat.com> (raw)
In-Reply-To: <20140418142912.GA2023@dhcp-17-102.nay.redhat.com>

On Fri, Apr 18, 2014 at 10:29:12PM +0800, bhe@redhat.com wrote:
> 
> > >> It definitely will cause OOM. On my test machine, it has 100G memory. So
> > >> per old code, its needed_size is 3200K*2 == 6.4M, if currently free
> > >> memory is only 15M left, the free_size will be 15M*0.4 which is 6M. So
> > >> info->bufsize_cyclic is assigned to be 6M. and only 3M is left for other
> > >> use, e.g page cache, dynamic allocation. OOM will happen.
> > >>
> > >
> > >BTW, in our case, there's about 30M free memory when we started saving
> > >dump. It should be caused by my coarse estimation above.
> > 
> > Thanks for your description, I understand that situation and
> > the nature of the problem.
> > 
> > That is, the assumption that 20% of free memory is enough for
> > makedumpfile can be broken if free memory is too small.
> > If your machine has 200GB memory, OOM will happen even after fix
> > the too allocation bug.
> 
> Well, we have done some experiments to try to get the statistical memory
> range which kdump really need. Then a final reservation will be
> calculated automatically as (base_value + linear growth of total memory). 
> If one machine has 200GB memory, its reservation will grow too. Since
> except of the bitmap cost, other memory cost is almost fixed. 
> 
> Per this scheme things should be go well, if memory always goes to the
> edge of OOM, an adjust of base_value is needed. So a constant value as
> you said may not be needed.

That logic is old and we probably should get rid of at some point. We
don't want makedumpfile's memory usage to go up because system has
more physical RAM. That's why cyclic mode was introduced.

> 
> Instead, I am wondering how the 80% comes from, and why 20% of free
> memory must be safe.

I had come up with this 80% number randomly. So you think that's the
problem?

I am still scratching my head that why 30MB is not sufficient for
makdumpfile.

Thanks
Vivek

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