linux-kernel - Re: Excessive printks increase top mem usage?

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Message-ID: <b681c62b0702040310g766048ci1af89174c4cb9d07@mail.gmail.com>
Date:	Sun, 4 Feb 2007 16:40:09 +0530
From:	"yogeshwar sonawane" <yogyas@...il.com>
To:	davids@...master.com
Cc:	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: Excessive printks increase top mem usage?

cc to lkml

On 2/3/07, yogeshwar sonawane <yogyas@...il.com> wrote:
> Thanks for the explanation.
>
> On 1/26/07, David Schwartz <davids@...master.com> wrote:
> >
> > > On 1/25/07, yogeshwar sonawane <yogyas@...il.com> wrote:
> > > > Hi all,
> > > > I am running a user application which will just open/close my driver
> > > > (simple one, empty functions with only printks) infinitely.
> > > > A massive use of printk can slow down the system noticeably OR it can
> > > > affect some time calculations.
> > > > Apart from this, it was increasing top mem usage also. After closing
> > > > the application, the memory consumption was not coming down(not
> > > > freeing mem). Is this the expected behaviour? OR i am missing
> > > > something?
> > > >
> > > > Can anybody help me in guiding the reason for this? Any help/links plz.
> > > >
> > > > Thanks in advance,
> > > > Yogeshwar
> >
> > What does "increased top mem usage" mean? You have given no reason to
> > suggest that there's anything unusual about this.
> >
> > Here's one theory: Some program is writing these kernel messages to a file.
> > Because some other process might come and read that file later, the kernel
> > keeps copies of the file data in memory.
> >
> > The kernel sees no advantage in having memory free. Free memory is memory
> > that's not doing any good. Better to keep data in memory that might be
> > useful later. We can always free the memory later if (and only if) we have
> > something better to do with it.
>
> One more thing:-
> If i want to allocate physical memory using __get_free_pages, and
> there is no free mem available, then kernel will release mem used by
> file data & use it for my request. Is this correct?
> I just tried similar thing. My test machine has 4GB RAM. On my test
> machine, as explained in my previous mail, i was printing excessive
> messages. Due to this top was showing mem usage aroung 1.9GB. Then i
> tried allocation of 4MB phy mem(using __get_free_pages) iteratively
> till it fails. The allocation succeeded upto @ 4-1.9 GB. I thought
> that some mem from 1.9 GB will be released for phy mem allocation. But
> it didn't. Such runs were tried 3-4 times. But consistently it
> succeeded upto 4-1.9GB. So is this normal ?
>
> regards,
> Yogeshwar
>
> >
> > DS
> >
> >
> > -
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