linux-kernel - Re: [PATCH 0/8][for -mm] mem

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Date:	Mon, 21 Apr 2008 13:32:06 -0700
From:	"Tom May" <tom@...may.com>
To:	"KOSAKI Motohiro" <kosaki.motohiro@...fujitsu.com>
Cc:	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/8][for -mm] mem_notify v6

On Fri, Apr 18, 2008 at 3:07 AM, KOSAKI Motohiro
<kosaki.motohiro@...fujitsu.com> wrote:

>  I investigated again and found 2 problem in your test program.
>
>  1. text segment isn't locked.
>
>    if strong memory pressure happned, kernel may drop program text region.
>    then your test program suddenly slow down.
>
>    please use mlockall(MCL_CURRENT) before large buffer allocation.

Using mlock does enable the program to respond faster (and/or the
kernel doesn't have to find memory to fault the page in) and solves
the problem for this simple test program.  I think we're thinking of
the solution in two different ways: you want the program to react more
quickly or be "nicer", and I want the kernel to give notification
early enough to allow time for things that can (and do) happen when
things aren't so nice. I realize that in extreme circumstances oom may
be unavoidable, but a threshold-based notification, in addition to the
current /dev/mem_notify mechanism, would help avoid extreme
circumstances.  I'm going to look into doing this.

>  2. repeat open/close to /proc/meminfo.
>
>    in the fact, open(2) system call use a bit memory.
>    if call open(2) in strong memory pressure, doesn't return until
>    memory freed enough.
>    thus, it cause slow down your program sometimes.

This should be fine; I intentionally do the open/read/write/close
after freeing memory.

>  attached changed test program :)
>  it works well on my test environment.

I made your changes to my program (I'm using clone since I don't have
a pthreads library on my device, and I left PAGESIZE at 4K instead of
64K), and having memory locked does avoid oom in this case, but
unfortunately I don't think it's a general solution that will work
everywhere in my system.  (Although I'm going to try it.)

Thanks,
.tom
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