To: linux-kernel@vger.kernel.org
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From: Bill Davidsen <davidsen@tmr.com>
Newsgroups: mail.linux-kernel
Subject: Re: per-process shared information
Date: Tue, 19 Oct 2004 11:09:46 -0400
Organization: TMR Associates, Inc
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Andrea Arcangeli wrote:
> On Fri, Oct 15, 2004 at 09:19:13AM -0400, Albert Cahalan wrote:
> 
>>I don't see why it is such trouble to provide the old data.
> 
> 
> I agree with you w.r.t. binary compatibility, here it's even a "source
> compatibility" matter, a recompile wouldn't fix it.
> 
> However I wasn't exactly advocating to keep it 100% backwards
> compatible in this case: somebody already broke it from 2.5.x to
> 2.6.9-rc, and since there was a very good reason for that, we should
> probably declare it broken.  Here there has been a very strong technical
> reason to break statm, but they didn't break binary and source
> compatibility gratuitously like some solaris kernel developer seems to
> think in some blog.
> 
> the problem is that when ps xav wants to know the RSS it reads statm,
> so we just cannot hurt ps xav to show the "old shared" information that
> would be extremely slow to collect.
> 
> I was only not happy about dropping the old feature completely instead
> of providing it with a different new API. Now I think the solution Hugh
> just proposed with the anon_rss should mimic the old behaviour well
> enough and it's probably the right way to go, it's still not literally
> the same, but I doubt most people from userspace could notice the
> difference, and most important it provides useful information, which is
> the number of _physical_ pages mapped that aren't anonymous memory, this
> is very valuable info and it's basically the same info that people was
> getting from the old "shared". So I like it.

I think that's clearly the right solution. Going to significant effort 
to produce compatible but incorrect values and/or formats is not 
desirable. I've seen this with users and applications, too, complaining 
that the new output doesn't match the old, even when the old was clearly 
wrong.

-- 
    -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me
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