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.
--
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