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From Stas Bekman <s...@stason.org>
Subject Re: [RFC] Apache::SizeLimit on Linux
Date Wed, 21 Sep 2005 18:03:30 GMT
Torsten Foertsch wrote:
> Hi,
> 
> now that we have Apache2::SizeLimit using Linux::Smaps the old functionality 
> of accounting COW pages as shared is restored on linux 2.6. It should show 
> for linux 2.4 and 2.6 the same behavior (can someone using 2.4 and 2.6 check 
> this please? I have no 2.4 box available) since the algorithm computing the 
> figures is the same.
> 
> Hence, the problems are the same when paging occurs. And these problems make 
> it almost useless by default. The algorithm counting shared memory simply 
> checks the reference count of each page table entry. If it is at least 2 the 
> page is shared. If a page is not present (paged out) it is not counted at 
> all.
> 
> Now what happens if the master apache is mostly paged out? Then there are only 
> a few shared pages left. That makes SizeLimit terminate the process 
> practically after each check.
> 
> I have found 3 solutions:
> 
> 1. obviously: not to use swap space at all
> 
> 2. mlockall(2) while the master is initializing. This means the master apache 
> is always completely present in memory. The lock is not inherited by child 
> processes, thus, the workers can be paged out.
> 
> 3. patch linux to report shared swap space as well. I have not looked into 
> this any further.
> 
> Would it be right to patch the mlockall(2) into SizeLimit.pm with an option to 
> disable it?
> 
> This has the drawback that to mlockall a process you must be root. It could be 
> done in a PostConfig handler that checks if MIN_SHARED or MAX_UNSHARED are 
> set and executes the syscall only if yes. That would then also be the right 
> place to check for the presence of /proc/PID/smaps. If only MAX_PROCESS_SIZE 
> is set then checking /proc/PID/statm will also do and is a lot cheaper (on 
> 2.6).
> 
> I think a description of the behavior should be in the docs in any case.

I remember there was a long discussion on mlockall on the users list a few 
years ago. Please check the archives. I remember there were some issues 
with it, so we never adopted that approach.

Also shouldn't that functionality be abstracted into a module outside 
Apache2::? More people will be able to benefit from it and hopefully have 
a better testing/dev process.

I don't have 2.4 to test.

In any case, whatever the outcome of this thread is, I encourage you to 
document anything that you think needs to be documented. I haven't been 
using this tool for a while so I don't have much to say about the 
specifics of these changes.


-- 
__________________________________________________________________
Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker
http://stason.org/     mod_perl Guide ---> http://perl.apache.org
mailto:stas@stason.org http://use.perl.org http://apacheweek.com
http://modperlbook.org http://apache.org   http://mailchannels.com

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