Date: 30 Jan 2006 07:00:11 -0500
Message-ID: <20060130120011.24515.qmail@science.horizon.com>
From: linux@horizon.com
To: pierre-olivier.gaillard@fr.thalesgroup.com
Subject: Re: Can on-demand loading of user-space executables be disabled ?
Cc: linux-kernel@vger.kernel.org
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1754
Lines: 39

> As far as I understand what happens when I start a Linux program, the
> executable file is mmaped into memory and the execution of the code
> itself prompts Linux to load the required pages of the program.
> 
> I expect that this could cause unwanted delays during program execution
> when a function that has never been used (nor loaded into memory) is
> called. This delay could be bigger than 10ms while the 2.6 kernel is
> usually quite predictable thanks to Ingo Molnar and others' work.
> 
> Is Linux really using on-demand loading ?

Yes.

> Is it very different from what I described in the first paragraph ?

No, not really (it's not different).  The pages are mapped, and brought
in via page faults.  There is read-ahead code, so if you start hitting
sequential pages, it'll read more than is immediately necessary in the
hope of keeping up.

> Can on-demand loading be disabled ? (This would seem convenient for my 
> applications since I generally start a program that is meant to run as 
> predictably as possible for months.)

Well, the easy way to disable on-demand loading is munmap().
Then the data won't be loaded and you'll get SIGSEGV.

However, I don't think that's what you mean.
If you want to keep code in memory, so it can't suffer a page
fault, either on initial load or by being swapped out to make
room for something else, use mlock() or mlockall().

If you just want to encourage (but not *insist*) code to be in memory,
I think Linux implements madvise(MADV_WILLNEED) sensibly.
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